are we alone in the universe? the search for another earthjoann eisberg lecturer of the year 2014...

Are We Alone in the Universe?The Search for Another Earth

Joann EisbergProfessor, Astronomy

Cover: First Image of the Earth from Space, 1968. Credit: NASA, Apollo 8 crew.http://commons.wikimedia.org/wiki/File:As08-16-2593.jpg

Chaffey College5885 Haven Avenue

Rancho Cucamonga, California

Are We Alone in the Universe?The Search for Another Earth

Joann EisbergProfessor, Astronomy

Faculty Lecturer of the Year2013-2014

April 16, 2014

Are We Alone in the Universe? The Search for Another Earth. i

Joann EisbergLecturer of the Year 2014

Joann Eisberg grew up in afamily of scientists. Her par-ents, Bob and Lila, were bothphysicists, so the beauty andprocess of the natural universewere staples of family conver-sation. Bob and Lila were alsofond of pointing out connec-tions between scientific theoryand technical skill, so Joannunderstood that if you knowhow the material worldworks, you can figure out howto make, fix, plant, cook, orsew nearly anything.

The Eisbergs were enthusiastic travelers. Bob’s sabbaticals from UC SantaBarbara took the family to England, Switzerland and Australia, giving Joannthe opportunity of learning about these countries by living in them and byattending local schools.

Getting to these destinations could be an adventure. Joann’s first trip toEngland was by freighter through the Panama Canal, while another time thefamily went to Europe the long way: via Asia, India and the Middle East.The trip to Australia almost ended in a hurricane-swept mid-Pacific ship-wreck from which they and 53 others were saved by days and nights of fran-tic bailing, a lucky break between storms, and Bob’s short-wave expertise.

It was a horrible experience, but made a good story, and it attracted enoughattention as a college application essay to snag Joann admission to Harvard,where she planned to major in physics.

It was exhilarating to be in a college environment where a nerd was nor-mal. But college physics didn’t appeal. In part, Joann’s preparation wasweaker than she’d hoped (but most students experience that) and in part, shehad mixed feelings about a career in an overwhelmingly male field.(Physical science in the 1980s was about 4% female.) But most important

Faculty Lecture of the Year29

Photo: Ardon Alger

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth. 28ii

was the nature of instruction. Joann’s high school science had used a cur-riculum rich in conceptual thinking and historical examples, more com-pelling than the non-interacting masses and frictionless surfaces of her fresh-man mechanics course.

Cruising the course catalog, Joann discovered that the history of science wasa real field of study, and that entire courses were actually offered in“Newton’s Principia,“ or “Popular Science in Renaissance England.” There,she found the motivation she needed to continue with modern science.Physicists, astronomers, geologists and biologists—every scientist today seesa world colored by the science of our ancestors. Those boring frictionlesssurfaces? They come to us via Galileo, who saw them as a step between theintractable complexity of Earth-bound motion, and the logical simplicity ofheavenly motion.

Joann sees her interest in history of science as a form of mental travel. If thepast is a foreign country, then the most foreign travel of all is to a paststripped of modern technology, and even of our modern conceptions ofnature.

In her undergraduate, graduate and postdoctoral education, Joann workedwith Owen Gingerich (Harvard University), Jim Bennett (CambridgeUniversity), Sam Schweber (Brandies Univeristy), and Margaret Rossiter(then of the National Science Foundation) on topics in history of astrono-my and the history of women in the sciences, from 17th to 20th centuries.

On her way to a PhD in the history of science, Joann picked up an educa-tion in astronomy, which proved practical, because she finished graduateschool in the tough academic economy of the early 1990s, when there wereno tenure-track jobs for historians of science.

Falling back on visiting and adjunct positions, as well as her ability to teachastronomy, women’s studies, and history, Joann began a career that led her allover academia, from the Smithsonian Institution’s National Air and SpaceMuseum in DC, to the University of Wisconsin Madison, to UC SantaBarbara, Citrus College, Caltech and Harvey Mudd, until she joined Chaffeyin 2003.

First at Citrus, and later, at Chaffey, Joann has developed successful distanceeducation courses in astronomy, including online courses and courses inChaffey’s program at the California Institution for Women. The greatestchallenges in distance courses are student engagement, active learning andfeedback. Joann is currently working on a sabbatical project to address these

27http://www.nasa.gov/mission_pages/kepler/news/kepler-47.html

28http://www.nasa.gov/mission_pages/kepler/news/41-new-transiting-planets.html#.UyDSbxbeObU

29http://www.nasa.gov/ames/kepler/digital-press-kit-kepler-planet-bonanza/#.UyDS8RbeObU

30http://keplerscience.arc.nasa.gov/K2/

31See sessions “Earth Analogues and Super-Earths,” and “Characterizing TransitingPlanets,” Kepler Science Conference II, Nov. 4-8, 2013. Archived video of presen-tations at http://kepler.nasa.gov/Science/ForScientists/keplerconference/

32http://tess.gsfc.nasa.gov/index.html and http://jwst.nasa.gov/index.html

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth. iii27

by creating audio- and image-rich lecture PDF files with embedded activi-ties and feedback that will give distant students an experience similar to thatin an active learning classroom.

A few of her other teaching interests are Chaffey’s Fast Track classes, flippedinstruction, and fostering critical thinking for general education science stu-dents.

Joann is married to Dave Kary, who teaches astronomy at Citrus College.Just about all of their teaching projects are collaborations. Without Dave’sencouragement and help, Joann wonders if she’d have dared to transformherself from historian to astronomer.

Dave and Joann have a daughter, Annie, and they hope to give Annie achance to see the world, as Joann’s parents gave her. Annie is graciously tol-erant of her parents’ insistence on pilgrimages to spots of scientific interest,from historic observatories and the homes of famous scientists (Herschel,Darwin) to glaciers and lava tubes. Some of the highlights of their recenttravel have been a trip to observe the total solar eclipse of 2012 from theSouth Pacific, and a trip to the Galapagos Islands.

Annie’s own interests include modern dance and playing euphonium in theClaremont High School Marching Band. She is thinking of studying archi-tectural engineering.

The things Joann most appreciates about Chaffey are: our wonderful stu-dents (especially every student who has entered astronomy class reluctantly,and finished thinking that science is cool after all) and Chaffey’s faculty andstaff. You are a joy to work with!

For their help and support of this presentation, Joann thanks Dave Kary,Annie Kary, Ardon Alger, Marie Boyd, Denise Kaisler, Linda Lamp, JackLissauer, Michael O’Bannon, Gary Reinschmidt,� Sean Stratton, CherieVentola, Donna Walker, the Chaffey Faculty Senate and the Chaffey CollegeGoverning Board.

Thank you all for your interest in hearing this Faculty Lecture of 2014.

11David Latham, Tsevi Mazeh, Robert Stefanik, Michel Mayor, Gilbert Burki, Theunseen companion of HD114762: a probable brown dwarf, Nature, 1989 339(6219): 38–40.

12A. Wolszczan, and D. Frail, A planetary system around the millisecond pulsar PSR1257 + 12, Nature, 1992 355 (6356): 145-147.

13Michel Mayor and Didier Queloz, A Jupiter-mass companion to a solar-type star,Nature, 1995 378 (6555): 355–359.

14http://astro.berkeley.edu/people/faculty/marcy.html

15Link to conference materials at http://spider.ipac.caltech.edu/staff/vgm/ppiv/

16A trove of Kepler information for everybody—scientists, students, educators,kids, the media and the public—can be found at these two websites:http://kepler.nasa.gov andhttp://www.nasa.gov/mission_pages/kepler/main/index.html

17http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=324and http://www.nasa.gov/ames/kepler/digital-press-kit-kepler-planet-bonan-za/#.UyH3aRbeObU

18Possible false positives include eclipsing binary stars where there is companionstar, not a planet, or effects from planets hosted by a star different than the star youintend to be observing. The summary in this and the next paragraphs comesmainly from Kepler Science Conference II, Nov. 4-8, 2013. Archived video of pre-sentations at http://kepler.nasa.gov/Science/ForScientists/keplerconference/

19http://www.nasa.gov/home/hqnews/2011/jan/HQ_11-007_Kepler_Rocky_Planet.html

20http://www.nasa.gov/home/hqnews/2013/apr/HQ_13-112_Kepler_62_find-ing.html#.UyC9IxbeObU

21http://www.nasa.gov/mission_pages/kepler/news/kepler-37b.html

22http://www.nasa.gov/centers/ames/events/2011/kepscicon-presskit.html

23http://www.nasa.gov/centers/ames/events/2013/kepler-62-and-69-presskit.htmland http://arxiv.org/abs/1305.2933

24http://www.nasa.gov/centers/ames/events/2011/kepler-20-presskit.html

25http://www.nasa.gov/centers/ames/events/2011/kepler-16b-presskit.html

26http://www.nasa.gov/mission_pages/kepler/news/kepler-34-35.html#.UyDRlhbeObU

Faculty Lecture of the Year1 Are We Alone in the Universe? The Search for Another Earth. 26

Are We Alone in the Universe? The Search for Another Earth

Are we alone, or do we live in a universe rich with other life?What question in science could tell us more?

Planets MatterWe live on Earth, and Earth is the only place in the universe for which wehave actual evidence of life. More than that: when the serious speculators listthe most general criteria for life, their lists require the kind of environmentfound only on a planet, or possibly a moon of a planet. Life that metabo-lizes, changes and reproduces demands complex chemistry and a density ofresources that aren’t present in the emptiness of space, the intense heat of astar, or the cold and instability of asteroids and comets. We’ll talk moreabout the details later, but a reasonable starting point for today is: Life needsplanets, so if we want to know whether there is life beyond the Earth, plan-ets are the place to look. Therefore:

The recent “explosion” in the number of extrasolar planets, or exo-planets, is perhaps the most exciting phenomenon in all of science.

- Chris Impey1

Chris Impey is an astronomer, so you may think his perspective biased, butI share it, and I hope to convince you!

Early History and a LittlePhilosophy of ScienceThe conviction that life might existbeyond the Earth has centuries of history;it goes back far longer than our modernunderstanding either of life, or of planets.

In the decades after Nicolas Copernicusdeclared Earth a planet—a body thatorbited the Sun—the Dominican friarGiordano Bruno speculated that the uni-verse contained an infinite number of

Nicolas Copernicushttp://commons.wikimedia.org/wiki/

File:Copernicus.jpg

References1Chris Impey, The First Thousand Exoplanets: Twenty Years of Excitement andDiscovery, Astrobiology, History, and Society, Advances in Astrobiology and Biogeophysics,Springer, 2013: 201.

2The relation between plentitude and Murphy’s Law is pointed out by Wikipedia.http://en.wikipedia.org/wiki/Plenitude_principle retrieved 1/15/14.

3Bruno was probably motivated by a more optimistic and theological version ofplentitude: God does not neglect to create any possible good thing. There arenewer sources on Bruno, but they pretty much all assume you start by reading theclassic book by Francis Yates, Giordano Bruno and the Hermetic Tradition, Routledge,1964.

4William Herschel, On the Nature and Construction of the Sun and Fixed Stars,Philosophical Transactions of the Royal Society of London, 1795 85: 46-72.At http://rstl.royalsocietypublishing.org/content/85/46.full.pdf+html

5If you would like to make a model with a scale of your choice, calculators areavailable online, for example, http://www.exploratorium.edu/ronh/solar_system/

6A good short history of SETI and updated analysis of the Drake Equation can befound at G. Schilling and A.M. MacRobert, The Chance of Finding Aliens, Sky &Telescope, 2013. Available online atskyandtelescope.com/resources/seti/3304541.html

7In 1959, physicists Giuseppi Cocconi and Philip Morrison published an articlearguing that radio telescopes might actually be able to detect a radio transmissionbroadcast by an extraterrestrial civilization. Searching for InterstellarCommunications, Nature, 1959 184 (4690): 844-846.

8In spite of the failure to find a signal, SETI projects are not fringe activities.They’ve attracted well-informed, creative scientists and increasingly sophisticatedtechnology. They remain some of the parts of astronomy that attract the most pub-lic attention. If you are interested, you can participate by donating your computersdowntime to analyze data from SETI@home project. http://setiathome.ssl.berke-ley.edu

9Other methods are summarized at http://planetquest.jpl.nasa.gov/page/methods

10Doppler shift is the increase or decrease of the pitch of sound, or the wavelengthof light, as the source moves towards or away from the observer. This is much moreeasily demonstrated than explained, so if the concept is unfamiliar, please followthis link to excellent animations on Wikipedia.http://en.wikipedia.org/wiki/Doppler_effect

Are We Alone in the Universe? The Search for Another Earth. 2Faculty Lecture of the Year25

stars, orbited by an infinite number of planets, and that these planets wereinhabited by intelligent life. Obviously, he hadn’t observed this; the ideaprobably springs from a philosophical belief in the plenitude, or complete-ness of the universe—if it can exist, it will. Lest this sound obscurely meta-physical, consider how many of us moderns subscribe to Murphy’s law, theevil twin of plentitude: if it can go wrong it will.2

Bruno’s idea, though rooted in theHermetic philosophy of the earlyRenaissance,3 also has modern appeal.It’s related to a belief that still motivatescosmology, an idea that we callCopernican: our vantage point is NOTspecial. The universe—or at least thephysical laws governing the universe—should look the same viewed from any-where. The Copernican idea is powerful,even necessary, for us to do science at all.If the laws of nature are different in otherplaces, how could we figure out whathappens there?

Bruno was burned as a heretic. A fewyears later Galileo was threatened withtorture for his advocacy ofCopernicanism, but the idea of the Earthas one planet among many has seducedand convinced thinkers in the centuriessince. The idea of our Sun as a star like

many others also has a long history. Descartes and Newton, the most signif-icant natural philosophers of the 17th Century, disagreed on much, but theyboth thought of the universe as an infinite distribution of Sun-like stars.William Herschel, whose superior telescopes earned him fame as the dis-coverer of the planet Uranus, also keenly examined sunspots and (cleverlybut wrongly) convinced himself that under hot exterior layers, the Sun hasa surface cool enough to be inhabitable. And at that point, he leapt to theconclusion that the Sun itself, as well as every planet and moon in the solarsystem, supported intelligent life.4

I’d like to introduce a few important reflections at this point. 1. These people had a very limited understanding of the scale of the uni-verse. While some (including Bruno, Descartes, Newton) did think ofother stars as comparable to our Sun, astronomers did not then imag-

Rene Descartes. Every star (S, L, C, O, K) is the center of its ownsolar system. Principia Philosophiae,

1644. http://www.loc.gov/exhibits/world/

heavens.html

biological source. Or perhaps we might detect a weird counter-naturalenergy distribution suggestive of a civilization harvesting solar power.

ConclusionThe proximity of TESS and the JWST mean that another Earth—a planetof the right size and composition, at the right distance from a Sun-likestar—is a discovery to expect in the next 5 to 10 years. Quite possibly, it’salready there awaiting discovery in Kepler data we already have. When wefind it, there’s a good chance we will be able to tell if it is inhabited. Forthe first time in human history, we may be able to answer a question humanshave asked for centuries. Are we alone, or do we live in a universe rich withother life?

Any answer will be the start of new, bigger, and better questions.

Faculty Lecture of the Year3 Are We Alone in the Universe? The Search for Another Earth. 24

ine the solar system as one tiny element of a much larger galaxy, theMilky Way, itself one of an uncounted number of comparable galaxiesextending to unknown distances in a vast universe. The unprecedent-ed size and quality of Herschel’s telescopes, and his incredible persist-ence in using them to make an exhaustive census of visible stars, madehim the first to gain even an inkling of the extent of our galaxy—andhis was a vast underestimate. Soon we will have to give some thoughtto modern ideas of astronomical scale.

2. Science is the interplay of theory and observation. In astronomy thismeans that telescopes are the technology that expands your grasp.

3. But observation is never the whole story. Science always seeks tounderstand process, because process is science’s approach to causality,to the WHY question. Process is more than you can see directly.Science understands what it sees by fitting observation into theory, theframework of ideas that allow us to explain and to predict. If we are toclaim an understanding of planets beyond Earth, it will be by makingtheories about the processes that govern their formation and exis-tence, and by testing predictions about the planetary parameters ourtheories predict.

Scale. Process. Observation. Theory. Prediction. The maturing of these ideasis key to making science modern. Let’s see how these play out in the searchfor life and exoplanets in more recent times.

We’ve long been confident that exo-Jupiters (mass bigger than 10 Earths)are worlds like our jovians. Whatever their interior, their gravity must haveaccreted deep, hydrogen-rich gas atmospheres as their outer layers. Thesmallest worlds (radius less than 1.5 Earth) present more possibilities. Theymight be rocky worlds with little to no atmosphere, like Earth and our otherterrestrial planets, or planets that share Earth’s chemistry but are so hot theirsurfaces are molten, or planets with chemistry somewhat different fromEarth’s. In between are the sub-Neptunes, which don’t much match any-thing in our system and may be quite exotic: scaled-up terrestrials madelargely of rock, but with deep, opaque, hot, possibly steam-drenched atmos-pheres and possibly surface oceans. Or sub-Neptunes may be water worlds,with most of the volume of the planet taken up by oceans thousands of kilo-meters deep, making their ocean floor effectively unreachable.

Data on the atmospheric chemistry of these worlds (which will requireother instruments than Kepler) should help distinguish these possibilities,which should tell us about their histories. Did these ubiquitous but ambigu-ous worlds form near their current locations, or are they “stripped downJupiters” that have migrated inward from cooler, more distant orbits andevaporated as they warmed?31

Habitability: the Last Word and the Next MissionsAtmospheric information is what we really need to tell us whether a plan-et is truly habitable. That is a goal of Kepler’s successor missions, theTransiting Exoplanet Survey Satellite (TESS), to launch in 2017, and theJames Web Space Telescope (JWST), to launch in 2018.32 TESS, which isin many ways like a more flexible and powerful version of Kepler, will makean all-sky transit survey and should detect a very large number of planetsaround bright, nearby stars. This population will make good targets forJWST’s infrared spectroscopes, instruments that measure the temperatureand chemistry of atmospheres by decoding information from the colors oflight they emit.

This information should answer some of Kepler’s most tantalizing questions:which exoplanets are rocky, watery or gaseous? Have they cooling clouds orheating greenhouse effects?

The last, most exciting information we might get from exoplanet atmos-pheres would be a biosignature. If, for example, we detected an atmosphererich in free oxygen, we’d guess that the planet was home to organisms witha process something like photosynthesis, because oxygen is so reactive weknow of no physical way to preserve it in an atmosphere without a copious

By counting stars distributed along the Milky Way, William Herschel concluded that ourgalaxy is shaped like the irregular shape in the middle.

Our Sun is the larger star in the middle. “On The Construction of the Heavens,”

Philosophical Transactions of the Royal Society of London, 1785.http://commons.wikimedia.org/wiki/File:Herschel-galaxy.jpg

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth.23 4

The 20th Century - Looking for Alien Life in the Solar SystemBy the turn of the 20th century, enthusiasm for the idea of extraterrestriallife was high for several reasons. Growing acceptance of Darwinian evolu-tion inclined people to see life as the result of natural processes. Advances inastrophysics confirmed our Sun as one of many stars—presumably makingits planetary system also one among many—and ever-improving telescopesinvited astronomers to search the planets in our solar system for signs of life.

The Italian, Giovanni Schiaparelli,and the American, PercivalLowell, had interpreted markingson the surface of Mars as channelsor canals, and Lowell went on toimagine a planet-wide infrastruc-ture program mounted by aMartian civilization, bringingwater from polar ice caps to equa-torial cities, sustaining a dyingpopulace on a drying planet.

Lowell’s career probably marks ahigh point. When he began,Martian canals were widelyaccepted, and speculation onMartian inhabitants was

respectable in scientific circles. But soon, professional astronomers rejectedMartian canals as optical illusions that vanished in better telescopes. As pro-fessional telescopes grew far beyond what could be afforded by even wealthyamateurs, and even more importantly, as graduate education became a near-universal qualification for research scien-tists, a dichotomy emerged between pop-ular & scientific perceptions of extrater-restrial life. Intelligent, heroic aliens likeLowell’s lived on in science fiction.Midcentury scientists and engineersfocused their attention on alien microbes.

The first chance to test the microbes ofanother planet came in the 1970s, whenthe Viking landers on Mars looked for biosignatures, chemicals whose releasefrom Martian soil might indicate organic

• Systems Confirmed by Transit Timing Variation (TTV).Whenthe Kepler mission was planned, astronomers assumed that exoplanetswould generally be confirmed via individual follow-up radial velocitymeasurements made by ground-based telescopes. The large number ofplanet candidate discovered by Kepler makes the follow-up observingprogram the significant bottleneck, and astronomers have been eagerfor alternate means of verification. A first approach was Transit TimingVariation. In systems of multiples, the planets disturb each other’s orbits,making their transits appear slightly late or early in patterns so distinc-tive as to be acceptable confirmation of planets. A large batch of sys-tems, Kepler-48 through Kepler-60, was confirmed in 2012 usingTTV.28

• Systems Confirmed by Multiplicity.More powerful then TTV wasthe realization that multiplicity, in and of itself, is good evidence that acandidate system is planetary. Systems of three or more stars that mightmimic multiple planetary systems are too gravitationally unstable tooccur with any frequency. Multiplicity is therefore the basis of a statis-tical technique that has led to a bonanza of planets, with the announce-ment of 715 new confirmed planets on February 26, 2014, more thantripling the previous number of Kepler confirmations.29

The Kepler2 MissionWith these discoveries, Kepler has been phenomenally successful, but alltechnology has a finite life. In May of 2013, after 4 years of data collection,Kepler lost its ability to point stably at its original field of view, due to thefailure of a second of its four reaction wheels. An extended mission is beingplanned, using Kepler’s optics to view a sequence of more accessible fields.The new mission, K2, should continue to find new planets around brightand nearby stars, and it is expected to contribute to our understanding ofplanet formation processes, young stars, stellar activity, stellar structure andevolution, and extragalactic science. Though its performance has declined,K2 still offers precision at least five times better than is achievable from theground.30

What are exoplanets made of?We’ve been talking about two ways to detect and measure planets, radialvelocity (related to mass) and transit (related to size). Putting mass and sizetogether gives density, which helps us answer new questions. What are exo-planets made of? What environment might they provide?

Percival Lowell’s map of Mars’ fictitious “canals,”Popular Science Monthly, 1916.http://commons.wikimedia.org/wiki/

File:PSM_V88_D217_Mars_regions_and_canals_mapped_by_lowell_team.png

Viking 2 Lander on the Surface ofMars, 1976. Credit: NASA.

http://commons.wikimedia.org/wiki/File:Viking_2_Image_of_Mars_Utopian_Pl

ain_-_GPN-2000-000426.jpg


metabolism. Neither Viking nor themany subsequent missions to Marsfound life, but they have found a histo-ry of radical climate change. Mars nowis a dusty, frozen, almost airless world,but the early Mars was warmer, wetter,and more Earth-like. It’s hard not to seethe early Mars as more hospitable.Current Mars missions explore thatpossibility.

Mars is not the only potential abode oflife in the solar system. Under the icysurface of Jupiter’s moon, Europa, liehuge unexplored oceans that may berich in hydrocarbons. Could there belife in Europa’s oceans as there isbeneath the icy surface of Earth’s arctic?Microbial life? Macroscopic life? Wewon’t know until we look.

So to summarize what we know,guess, and hope of life in oursolar system:

1. We’ve observed the sur-faces well enough to beconfident there is no othersurface life like us: big,technological, obvious.

2. The surfaces all look prettyhostile: frozen, broiled, toxic, or dry.

3. If terrestrial life is any guide, we do best to search for life that mayexist in hospitable niches. (Underground? Under ice?) Perhaps wewill find evidence of past life in eras of more moderate temperaturesor different atmospheric chemistry.

4. Extraterrestrial life in our solar system is more likely microbial.

If we hope to find the aliens of science fiction, it seems we must turn ourattention beyond the solar system. But now we meet one of the funda-mental problems of astronomy: enormous distances.

• Systems of multiple planets with solid and gaseous planets inthe “wrong” place. As we saw, our theory of solar system formationpredicts that rock and metal planets form inside the frost line, wherewater and ices would melt and evaporate, while gas planets form out-side. Systems that violate this pattern support the idea that planetsmigrate after formation, giving us a clue as to how we might extendand modify our formation theory. This is the theoretical problem thatmade the hot Jupiters so surprising. But the most problematic exam-ple yet discovered may be the Kepler-20 system, whose five planetsalternate large, gaseous and small, rocky worlds, all crammed into aregion smaller than the orbit of Mercury. The alternation of planet typessuggests that worlds do not migrate simply, but that they exchangeplaces in a dynamically complicated orbital dance.24

• Circumbinary systems, in which a pair of stars orbits eachother, and one or more planets orbit the stars. Or course someof the appeal here is sheer weirdness: wouldn’t you like to admire adouble sunset from the vantage point of such a planet? More seriously,as we have come to understand that as many as half of all stars are bina-ries, we appreciate the importance of knowing whether binaries canhost planets. Decades ago astronomers were skeptical, suspecting thatbinaries, with gravitational tugs towards more than one center, provid-ed too unstable an environment to form and hold planets. However, thediscoveries of Kepler-16b25, Kepler-34b, Kepler-35b26 and theKepler-47b and 47c27 system show that some binaries are indeedhosts. Perhaps a Luke Skywalker really is out there gazing at the dou-ble suns of a Tatooine.

This distributary fan is evidence of longwet episodes in Mars’s climate history.

Credit: NASA, JPL, Malin Space Science Systems.

http://www.msss.com/mars_images/moc/2003/11/13/index.html

Jupiter’s moon Europa has a cracked, icy sur-face, probably over deep oceans. Credit: NASA.

http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=6403

Artist’s Depiction of Kepler-16b, First Known Circumbinary Planet. Credit: NASA, Caltech, T. Pyle.

http://www.nasa.gov/mission_pages/kepler/multimedia/images/Kepler-16_planet-pov-art.html

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth. 621

Scale of the Solar System and Distance to the Nearest StarsDistances in astronomy are so large that nothing else in human experienceprepares us to imagine them directly. Even setting up a scale model is chal-lenging, because it is hard to choose a scale that bridges the gap between ourexperience and astronomical space.

A 12-inch Earth globe is so small that the 350 miles from Los Angeles toSan Francisco, which takes about an hour to fly, or six hours to drive, is onlyabout a half inch. On this scale, our Sun would be as tall as a 10-story build-ing, about 2 miles away.5 Jupiter, the largest planet, would be a cottage inSan Bernadino, and Neptune, the most distant planet, would be a 4-footbeach ball, on the coast near Oceanside. Now our model breaks down,because the next step we would like to take is to a star outside our solar sys-tem. On this scale, the next star would be another building-size object, threetimes as far away as the moon. This is beyond our experience, so we mustchoose a much smaller scale.

Imagine shrinking our model again, so that our Sun is represented by a 3-inch orange. On this scale, the Earth would be smaller than a poppy seed.Jupiter, the largest planet, could be represented by a blueberry. The distancebetween the Sun and the Earth would be 27 feet, and other planets wouldbe spread out over the Chaffey campus. On this preposterous scale—withour entire Earth represented by a speck I can barely resolve without read-ing glasses—the next star could be another orange, in Vancouver, Canada.

Beyond our solar system we find ourselves in the Milky Way Galaxy, animmense disk-shaped distribution of 100 billion stars. They range in diam-eter from 1/10 that of our Sun to 10 solar diameters. (Here we are consid-ering only “main sequence” stars—typical stars in their stable middle age,not the rarer and more exotic end stages of stellar existence: giants, whitedwarfs, neutron stars.) On the scale we are imagining, think of stars as blue-berries to beach balls, typically scattered at distances of thousands of miles.The galaxy, on this scale, would be almost 40 million miles across—half wayfrom here to the Sun.

SETI - The Search for Extraterrestrial IntelligenceOur investigation of astronomical scale tells us that the kind of close inspec-tion we’ve made of Mars is impossible for any object outside our solar sys-tem. We will have to take a different approach: searching for evidence thatcan travel to us over the vast distances of interstellar space. This motivates anew strategy that came in about 1960, to be called SETI—The Search forExtraterrestrial Intelligence.6

• Smallest Planet in the Habitable Zone of a Sun-like Star. SinceEarth-like planets are Kepler’s mission goal, location is as important assize, and here the goal is to move from small, overheated orbits to larg-er, more moderate ones. But this is a challenge for two reasons: first,the transit alignment itself favors planets close to their stars, and second,the briefer the orbit, the sooner the required three transits can beobserved. In 2011, Kepler-22b became the first planet confirmed inthe habitable zone of a Sun-like star. The planet is 2.4 times the size ofEarth. When it was confirmed as a planet, it was also the smallest foundin such an orbit. Though its density is not known accurately enough tobe certain, it might be a water world, with very deep oceans or possi-bly a thick atmosphere over a small, rocky core, so it is a promisingprospect for life. It became one of Kepler’s best-known (and most fre-quently illustrated) planets. The accompanying artist’s conception showswhat it might look like if it had a cloudy atmosphere above an oceanlayer.22 An even smaller candidate circling a Sun-like star is Kepler-69c, at 1.7 times the size of Earth. At its announcement in April 2013,it was thought to be in the habitable zone, but recent analysis suggestsits 242 day orbit is too close and too hot, making the planet more likeVenus.23

Figure 22. Artist’s Depiction of Kepler-22b, Small Exoplanet in the Habitable Zone of a Sun-like Star. Credit: NASA, Caltech.

http://kepler.nasa.gov/multimedia/artwork/artistsconcepts/?ImageID=184


In some ways, SETI resembles that sixties icon, Star Trek: galactic in scopeand focused on extraterrestrial life that is essentially like us—intelligent.Because this is astronomy, the definition of intelligence is very specific:broadcasting radio signals. The motivation is as practical as it is chauvinistic.Unless it broadcast, how could we find it? For unlike Star Trek, SETI bold-ly goes nowhere; SETI waits for the signal to come to us.

By 1960, the technology was in place. Radio is the efficient waveband, andastronomers calculated that if technologically advanced alien neighborsemitted radio signals, our telescopes could detect them.7 Frank Drake inau-gurated the search at the National Radio Astronomy Observatory at GreenBank, West Virginia. Drake’s was the first of a series of SETI projects con-tinuing to this day. To date, none have detected a signal bearing clear marksof extraterrestrial intelligence. Does the silence mean there are no aliens? Wedon’t know. We don’t know if we are listening to the right channel. Wedon’t know whether we’d recognize an alien signal if it arrived.8 Has SETIbeen of use at all?

The frequency of hosts, coupled with the frequency of systems, means thatthere are probably more planets than stars.

Putting these numbers together, we can be reasonably confident that thenearest planet with a solid surface at habitable temperatures lies less than 70light-years away, though it probably orbits one of the small, dim stars that aremore common than our sun.

Kepler’s Most Exciting PlanetsThere are so many different ways an exoplanet discovery can be excitingthat we can address only a few of them here:

• Smallest Exoplanet. Since large planets are easier to find, smallestplanets are the most newsworthy, and Kepler has discovered a series ofever-smaller planets. Kepler-10b, discovered in the first 8 months ofKepler data and announced in January 2011, is only 1.4 times the sizeof Earth. At its discovery it was the smallest exoplanet found. Its dis-covery was confirmed with radial velocity measurements from theKeck 10m telescope in Hawaii, which allowed astronomers to find itsmass (4.6 Earth masses) and density (8.8 g/cc), so it is also the first exo-planet we know to be a solid planet, with a rocky surface, rather than agas or water world. It orbits so close to its host star that it is tidallylocked, and sunlight melts the rock of its day-side surface.19 Subsequentrecord-holders for the smallest exoplanet have included Kepler-62c(about the size of Mars)20 and Kepler-37b, announced in February2013, which is the smallest exoplanet currently known, and is onlyslightly larger than our moon.21

Frank Drake and radio telescope. Credit: National Radio Astronomy Observatory.http://www.gb.nrao.edu/astrobiology/

Artist’s Depiction of Kepler-10b, First Known RockyExoplanet. Credit: NOAA.

http://sos.noaa.gov/Datasets/dataset.php?id=409

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth. 819

The Drake Equation - The Theoretical Side of SETIBack at the beginning of SETI, Drake set out to list the factors influencinghow many civilizations there might be in our galaxy whose signals we mightdetect. Some factors are astronomical:

How many suitable stars are there? (We’ll call this N*)What fraction of those stars have habitable planets? (fp)

Others are broadly biological, and I will define them but comment no further:On what fraction of those planets does life evolve? (flife)How often does life develop “intelligence,” aka broadcast using technology we could detect? (ftech)

Since stars, planets, and life forms have finite lifespans and light has a finitespeed, timing also matters:

What fraction are broadcasting from a point in space and time so that we could detect them now? (fnow)

If we multiply all the factors together we get:

N* × fp × flife × ftech × fnow = Number of civilizations we could detect

This is the Drake Equation (in streamlined form), and it’s the most usefuloutcome of SETI, not because it allows us to calculate an answer, butbecause it’s a powerful tool to help us to think about uncertainty.

For most of the half century since Drake started work, N* was the onlyterm better than a complete guess. But since there probably are five or tenbillion Sun-like stars in our galaxy, the other numbers can be small fractionsand the number of potentially detectable civilizations would still be verylarge. (Specifying “Sun-like” limits us to considering stars with the longevi-ty and brightness of our Sun. If dimmer, longer-lived stars could also hosthabitable planets, N* could be a hundred billion.)

Sustainability is at the heart of fnow. If the typical lifespan of a technologicalcivilization is short, there won’t be many civilizations hanging around forconversation. Conversely, if the universe is full of intelligent life, it tells usthat civilizations can learn to endure.

The rest of this talk is about fp. Our criteria for planet habitability are basedon the only example we know—terrestrial life. Because all life on Earthrequires liquid water, we define a habitable zone as a region around a star inwhich planets’ surfaces could lie between the freezing and boiling point forwater. For our Sun, the habitable zone includes the orbits of Earth and Mars

When we allow for systems that are not aligned in the direction that per-mits detection, we get even more dramatic numbers. At least 70% of mainsequence stars have a planet orbiting in their inner solar system. (Planets inlong, slow, outer system orbits would need observation times longer thanKepler’s mission. They may be there, but as yet they are unobserved.) So forthe first time in the history of astronomy, we can say that planets are verycommon—the rule, not the exception.

Equally astonishing are the kinds of planets discovered. While older search-es were overwhelmingly more sensitive to the biggest planets, the Jupiters,Kepler’s discoveries suggest that sub-Neptunes (2 to 6 times the size ofEarth) are the most common. This comes as a surprise, because our solar sys-tem has no planet in this size range. There are also large numbers of plan-ets small enough to call Earth-like or terrestrial (less than 2 times the size ofEarth). The Jupiters are rare. About a hundred Kepler planets are in theHabitable Zone, including about 20 of the terrestrials. Twenty-two percentof Kepler host stars have been found to have more than one candidate plan-et—or from the planet’s perspective, about 40% of Kepler planets haveneighbors. Multi-planet systems like ours are not unusual.

Known Exoplanets by Size as of 2/26/2014. Credit: NASA.http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=324


(depending on the strength of the planet’s greenhouse effect). For a small-er, dimmer star, the habitable zone would be closer in. But temperaturealone doesn’t make a planet habitable. To offer living conditions in any waylike Earth, a planet would also have to be approximately Earth-size. To seewhy, let’s look a little closer at the categories of planets we recognize in oursolar system.

Planets in Our Solar System, and How they Formed

In our solar system, astronomers recognize two kinds of planets:

• Terrestrial (Earth-like) planets are small, solid and close in to the Sun.• Jovian (Jupiter-like) planets are gas giants, far out from the Sun.

The types of planets differ in chemical composition, with inner planetsmade of rock and metal, and outer planets made of hydrogen, helium, water,ammonia and methane.

How did our planets get that way? The answer emerges from our forma-tion theories.

Stars, including our Sun, form from diffuse clouds made mostly of hydrogenand helium gas, with smaller amounts of rocky and metallic dust grains, aswell as icy crystals of water, ammonia and methane. (I’ll call this unwieldymouthful WAM.) These clouds are enormous, and their great mass exertsstrong gravity, pulling the matter together. Some matter falls to the centerforming the star, while other matter orbits the center in a flattened disk orpancake-shaped distribution that spins as it contracts. We can see examples

Also, the exoplanet’s orbit must be lined up with the observer’s line of sight;otherwise the planet will seem to us to pass just above or below the starrather than transiting. For an Earth-size planet orbiting at an Earth-like dis-tance around a Sun-like star, only about .5% will be correctly aligned, so ourfinal estimate of the frequency of planets has to be scaled up accordingly.

When three transits have been observed, an object can be declared a planetcandidate, but some form of additional evidence (for example, radial veloc-ity measurements from other instruments) are needed to confirm an objectas a planet. (More on this in a little.)

Prior to Kepler, a modest number of planets had been discovered using tran-sits, but Kepler used transit techniques to become the most important dis-coverer of exoplanets. As of February 26, 2014, Kepler had found 3845 plan-et candidates, 961 of which have been confirmed as planets.17 This num-ber alone is phenomenal; it means that the Kepler mission alone has discov-ered over half the planets known to humanity. Considerable effort has goneinto studying sources of false positives, and the consensus is that over 90%of the planet candidates are likely to be confirmed, leading to the astonish-ing figure of well over 3000 probable Kepler planets.18

Our Sun and planets, scaled to size, but not distancehttp://commons.wikimedia.org/wiki/File:Planets2013.jpg

Exoplanet Discoveries Timeline as of 2/26/2014. Credit: NASA.http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=324

Faculty Lecture of the Year Are We Alone in the Universe? The Search for Another Earth.

of such disks aroundyoung stars with sufficientfrequency and clarity tobe quite confident of thegeneral outlines of thistheory.

Planets form from clumpsof matter in the disks. Thislink between chemistryand location in our solarsystem suggests that ourplanets formed more orless in their current orbits.Close to the Sun where itwas warmer, WAM evapo-rated and couldn’t beincorporated into theclumps of solid rock andmetal that accreted tomake Mercury, Venus,

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Instead of looking around in different directions, Kepler has steadilyobserved one single field containing about a quarter of a million observablestars. A hundred thousand of these are good, stable targets for Kepler’s mis-sion: to look for moments when starlight dims as a planet transits, or passesbetween us and the star. The effect is tiny; about 1% of a Sun-like star’s lightwould be cut off by a giant planet, like Jupiter, and only 1/100 of 1% for aplanet like Earth.

To be sure that a planet’s transit really is the cause of dimming, astronomersmust observe a succession of three matching transits (same length, period,and depth) indicating that the same planet has made just over two completeorbits, passing three times in front of its star. A planet in an Earth-like orbitaround a Sun-like star would have to be observed for just over two years.

Artist’s conception of a protoplanetary disk surrounding a young star. The star’s heat evaporates nearby ices, so inner planets form from rock and metal dust, and are small. Fartherout, plentiful ice joins rock and metal, accreting into the large cores of outer planets. Credit: NASA.

http://www.nasa.gov/topics/universe/features/keck-life-zone.html

Real protoplanetary disks imaged by the Hubble SpaceTelescope. False color: the infrared luminosity measurestemperature. Credit: NASA, Hubble Space Telescope.

http://hubblesite.org/newscenter/archive/releases/1995/45/image/b/format/web_print/

Schematic. Transit of an exoplanet between causes a dip in the light curve of a star. Credit: NASA.

http://tess.gsfc.nasa.gov/science.html

Data. Light Curve for Transit of Kepler-23c. Note the vertical scale; the effect is subtle—about1 part in 5000—but clearly observable. Credit: NASA.http://kepler.nasa.gov/Mission/discoveries/kepler23c/

Are We Alone in the Universe? The Search for Another Earth.

Earth and Mars. Farther out where it was colder, WAM was icy, and joinedthe rock and metal making planetary cores. The addition of WAM increasedavailable solids, so outer solar system planets grew huge, with Uranus andNeptune becoming 15 and 17 times the mass of Earth. The gravity of thebiggest cores was sufficient to attract hydrogen and helium gas, raisingSaturn and Jupiter to 95 and 318 times the mass of Earth. With their greatinternal pressure and heat, the jovians have melted the ices of their forma-tion, and have turbulent, cloudy atmospheres above liquid interiors.

While the terrestrials offer stable platforms with high concentrations of thechemical components we think life needs, the jovians probably don’t. Weimagine that jovian planets are pretty unlikely abodes for life. Jovian moonsmay be a different story. In our system all the jovians have extensive moonsystems that share enough terrestrial properties at least to be speculative longshots for habitability.

Generalizing Formation TheoriesThis brief explanation of how our planets formed is part of a more generaltheory of the formation of planetary systems. It is problematic, however,because the defining idea of science is that theories must be tested. But youcan’t test a theory by comparing it to the same data set it was developed toexplain. We need more examples of other systems, so we can compare theproperties the theory predicts to the parameters of those systems.

Here are a few of the important questions that we might answer if we hada reasonable number of other solar systems to observe: • How much variety might there be between systems? Would differentstarting conditions result in systems very different from our own, eventhough the same basic physical processes take place?

• How stable are solar systems (including our own)? The stability we seein the current layout of our system might not be true of other systems,and might not even describe the many-billion year history of ours. Doplanets remain in the orbits in which they formed, or do they migrateinward? Until they crash into their Sun? Outward? Until they are lostfrom their system?

Variety and long-term stability have serious implications for our search for life.

Varieties of StarsBefore we consider planets in other solar systems, we need to think aboutone last thing: the types of stars they orbit. The variation among stars is evenmore profound than the size differences we have already discussed. • Mass is the determining parameter. This is because mass causes gravity,which compresses the interior, creating the conditions for the nuclear

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many others), that July afternoon was when we put together two sides tothis story. Excitement resonated through the auditorium as we understoodthat the observers and the theorists each had the answer to the other group’squestion. Never before or since have I so acutely felt that I was seeing sci-ence in the making.

KeplerFinding hot Jupiters and modifying solar system formation theories toaccount for them were the first big accomplishments of exosystem astrono-my, and through the early 2000s, excitement remained high as ground-basedradial velocity measurements gave us smaller and smaller planets in largerand larger orbits. But it left us pretty far from the goal of finding otherEarths.

All that changed in in 2009, with the launch of the Kepler space telescope.16The first of its kind, Kepler was optimized for the specific purpose of find-ing Earth-like planets. This makes it very different, even from other spacetelescopes. Kepler has an extraordinarily wide field of view: about 100 deg2or about 1/400 of the whole sky. This is about the area covered by your ownhand’s width at arm’s distance, and it’s about 100,000 times bigger than thefield of view of that famous and incredible imager, the Hubble SpaceTelescope.

Faculty Lecture of the Year

Artist’s Depiction of Kepler’s 3000 Ly Target Region in the Milky Way. Credit: NASA, Jon Lomberg.

http://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler-target-in-the-milkyway.html

For the theorist, however, hot Jupiters were a real puzzle: recall that our the-ory of solar system formation required ices from the cold and distant outersolar system to supply sufficient solids to accrete the great mass of a giantplanet. No selection effect should make it possible to discover an object thatcan’t form in the first place! Did the existence of hot Jupiters mean that ourformation theory was wrong? And if other systems had so many of theseobjects, and ours has none, is our system unusual? Unique?

Science in the Making, 1998Through the late 1990s continued discovery of more hot Jupiters madewidespread news, but within the astronomy community, the theory side ofthe story was quietly developing. In July of 1998, at a conference on pro-tostars and planets at UC Santa Barbara, the two sides came together.15 Iwas an adjunct at UC Santa Barbara at the time, turning myself from a his-torian of science into an astronomy teacher, and I slipped into the confer-ence not because it was precisely my area, but because I’d been tipped offthat significant new observations would be announced.

That day, I heard the most exciting scientific session of my career.

The morning session reviewed the exoplanets, mostly hot Jupiters, discov-ered to that time. There were ten, including two that were newly announcedat the meeting. Two new planets in one day sounds tiny now, but at thetime, was amazing. By lunchtime, I know I’d witnessed a historic announce-ment. But the best was yet to come.

In the afternoon, the theorists spoke. I find theory talks more challenging,but the message of these was clear: the theorists had an explanation thatmade hot Jupiters sound almost inevitable. If the disk of matter surroundinga young star is thick, planets forming in the disk should feel drag from thedisk material. Drag would slow planets down, which would cause them tospiral in towards the star. In a thick disk, migration would happen fast: inmillions of years, which is very short compared to the time it takes stars toform. So perhaps the question is NOT “How could those hot Jupiters havegotten so close to their stars?” Instead, it may be, “Why haven’t they fallenin?” Perhaps the fact that our Jupiter managed to stay so far away tells us thatour system is the unusual one.

Scientific publications have a way of sounding very orderly and restrained,almost as though theory and observations march hand in hand towards for-gone conclusions. But the reality is very different. Better-informed insid-ers probably knew what was coming, but certainly for me (and I think for


reactions that make stars shine. The smallest stars are about a tenth themass of our Sun, just big enough to generate nuclear energy. Thesmallest stars have surface temperatures of a few thousand degrees,about half the temperature of our Sun’s, so a planet must orbit closerfor Earth-like heating. The biggest stars are a hundred solar masses, sobig the energy they generate almost blows them apart. They have sur-faces at tens of thousands of degrees and fry their neighborhood withso much radiation that any planets around them would be sterilized.

• The smallest stars last for trillions of years—far longer than the age ofthe universe—while the big ones last only millions of years—compar-atively, just the blink of an eye. Our Sun’s lifespan will be intermediate,about 10 billion total years, and we are now near the midpoint of that.Since we are interested here in stars that might be long-term stablehosts for planetary systems, we can ignore starbirth and stars’ exotic endstages, focusing instead on their stable middle ages or “main sequence”lives.

• The smallest stars are overwhelmingly more common.

The Search for the FirstExoplanetsBy the 1980s, the search for exoplanets wason in earnest. But how did astronomershope to find them? Planets are dim, andthey are close to stars, which are bright.Photographing an exoplanet directly iscomparable to photographing a firefly,buzzing around a searchlight, a continentaway. So astronomers took a differentapproach: looking for ways that the planetmight modify the starlight, which is muchmore detectable. There are several ways,but we’ll focus on the two that have pro-duced the majority of the detections: radi-al velocity and transits.9

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The four planets that orbit the starHR 8799 are among the few planetsto have been directly imaged. Thelight from the star must be blockedso that the planets are detectable.

Credit: Ben Zuckerman.http://en.wikipedia.org/wiki/File:Benjamin_Zuckerman_HR_8799_planets_

image_Dec._2010.jpg

The radial velocity (approach and recession) of the host star can be

detected in the redshift and blueshift ofits light, revealing the existence of the

expoplanet. Credit: European Southern Observatoryhttp://commons.wikimedia.org/wiki/

File:ESO__The_Radial_Velocity_Method_(by).jpg


Radial velocity measurements depend on the physics of orbits. A planetorbiting a star gives the star a small wobble, and if that wobble moves thestar towards and away from Earth, we can observe a Doppler shift: subtlechanges in the wavelength of the star’s light.10 With technology of the1980s, this effect was barely detectable, and early results were met with skep-ticism.

This story is a little bittersweet for me, because in 1984 I wrangled an invi-tation to join the observing group of David Latham, one of the first to workseriously on this technique. But I backed out, because the project seemed sounlikely and—this sounds odd in light of the near ubiquity of cars andphones among our students today—because I lacked the communicationand transportation that would have gotten me out of town to an observingsession on clear nights that suddenly needed a backup observer. At the time,it seemed like a stretch: hardly any student I knew in those years had a car,answering machine, pager, or email address. In retrospect, I regret not hav-ing been perceptive enough to make the opportunity work somehow.

In 1989, Latham and his group discovered an object orbiting the star HD11476211. At discovery, Latham’s group was cautious, describing it as prob-ably a brown dwarf (the smallest kind of star), or possibly giant planet, at least11 times the mass of Jupiter or much more massive, depending on the anglefrom which we view the system. Certainly the discovery of Latham’s plan-et was proof of concept, and the planet is a candidate for the first exoplan-et discovered.

The first unambiguous exoplanet discovery was tantalizingly offbeat. In1992, an exosystem of multiple planets was discovered.12 This system,however, orbited the pulsar PSR 1257+12—the spinning relic left after asupernova has blown apart a massive, dying star. Astronomers had detectedDoppler shifts in the pulsar timing indicating a radial velocity due to anorbiting planet. This remains an oddity, and we’re still somewhat puzzledthat planets could either form or endure in so hostile an environment. Lifeon such planets seems extremely likely.

The discoveries that seemed like real progress towards the planetary systemswith potentially Earth-like worlds began in 1995. Michel Mayor (who hasbeen one of Latham’s co-authors) and Didier Queloz (Mayor’s student at theUniversity of Geneva) discovered 51 Pegasi b, a .5 Jupiter-mass planet in aclose orbit around 51 Pegasi, a Sun-like star lying only 50 light-years away.13

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After this, the pace of exoplanet discovery surged, dominated by the workof two groups. Using radial velocity methods, Mayor went on to becomeone of the most prolific discoverers of exoplanets. Among the approximate-ly 200 exoplanets discovered by Mayor and his collaborators are 51 Pegasib, and Gliese 581 c, the first exoplanet in a star's habitable zone. Even moreprolific was the Californian, Geoff Marcy, who with his collaborators hasdiscovered more than 250 exoplanets, including the first multiple planet sys-tem (Upsilon Andromedae), the first Saturn-sized planet, and the firstNeptune-sized planet.14

Finding Hot JupitersThe variety of “firsts” in the above list signals how rich of the issue of exo-planets had become, with discoveries of planets of different sizes, orbiting atdifferent distances, around stars of different masses. All the early exoplanetdiscoveries were “hot Jupiters”—much larger than Earth and even closer totheir host stars than Mercury, our innermost planet, is to our Sun. From theobserver’s point of view, this was unsurprising; large mass and short distancemake for a stronger attraction and give host stars larger, more easily observedradial velocities.

Artist’s Depiction of Planet HAT-P-7b, a “Hot Jupiter.” Credit: NASA, ESA, STSci, G. Bacon.

http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=139

are we alone in the universe? the search for another earthjoann eisberg lecturer of the year 2014...

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