Is there anybody out there? Assuming complex life occurs incredibly rarely in the universe, what is the best method to find planets which could host it?

Oliver Dudgeon

d’Overbroeck’s

Is there anybody out there?Assuming complex life occurs incredibly rarely in the universe, what is the best method to find planets which could host it?

Oliver Dudgeon9/5/2016

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ContentsDoes life exist elsewhere in the Universe? 3...................................................................................

The Rare Earth Hypothesis: Which factors have the greatest effect on the development of life? .5

Type of star and the distance the planet lies from it. 5................................................................

The mass, size and the type of a planet 7...................................................................................A system of bodies 7....................................................................................................................

Atmosphere and oceans 8...........................................................................................................

The planet’s position in its galaxy 9..............................................................................................

Which factors have the greatest effect? 9....................................................................................

Which exoplanet detection method works best for finding planets with these factors? 10..............

Direct imaging 11..........................................................................................................................

Transit events 15..........................................................................................................................

Radial velocity and astrometric measurements 17.......................................................................

Gravitational microlensing 19.......................................................................................................

Which method will work best for detecting hospitable planets? 20..................................................

Appendix: Glossary 20.....................................................................................................................

Bibliography 21................................................................................................................................

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Does life exist elsewhere in the Universe?Go travel far from any city at night and look up on a clear night. It’s astonishing to see how many stars are in the sky. This has lead humans to ask the question: does another place like Earth exist? It’s only in recent years that we really do think we are getting close to an answer.

In the past few decades, we have found that statistically speaking each one of those stars you see should have at least one planet in orbit . That’s an inconceivable number of planets. 1

Thousands of exoplanet candidates have been validated in the past few years.

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Figure 1: Graph showing the number of exoplanets discovered versus time.2

This leads to a question. If there are so many planets out there; thus so many opportunities for life to develop, then why have we not observed any signs to this day?

“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

—Arthur C. Clarke

Sara Seager, ‘The Search for Planets beyond Our Solar System’, 2015 <https://www.ted.com/talks/1sara_seager_the_search_for_planets_beyond_our_solar_system?language=en#t-35355> [accessed 8 July 2016].

‘Briefing Materials: 1,284 Newly Validated Kepler Planets’, NASA, 2016 <http://www.nasa.gov/feature/ames/kepler/2briefingmaterials160510> [accessed 8 July 2016].

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This is known as the Fermi Paradox, the contradiction between the number of chances for life to develop and the perceived absence of life. Given the time span for this to all occur (a large portion of the age of the universe) then you would expect to at least see evidence for life.

In recent years the research into exoplanets – planets like our neighbours orbiting foreign stars – has expanded massively. The Kepler mission has shown the planets are simply, common.

Many solutions to the Fermi Paradox have been proposed since Enrico Fermi asked the question giving the paradox its name. An example is an idea that humans are like zoo animals to more advanced civilisations. Another is that most civilisations are smart enough to not broadcast their whereabouts to not bring attention to a predator civilisation . 3

However, this report will focus on another: the Rare Earth hypothesis. It argues that complex life does not exist elsewhere due to a set of factors that are necessary for complex life to develop. It does not, however, go as far to say that simple bacterial life does not exist . 4

This hypothesis was given its name when a book by Peter Ward and Donald Brownlee was published titled ‘Rare Earth’. Ward and Brownlee, the two most notable proponents of the hypothesis, put forward several factors which they believed prevented life from developing elsewhere.

One important idea to understand when reading through ideas around alien life is that we simply do not know all the ways life could exist. We only know what we have seen so far on Earth. Therefore, there are two ways looking for life can be done. On the on hand we can look for planets that are very similar to Earth with a limited set of search criteria. On the other hand, with a much broader set of search criteria, we can have a much larger range of possibilities, albeit, some may end up being inhospitable.

Philipp Dettmer, The Fermi Paradox — Where Are All The Aliens? (Germany: Kurzgesagt, 2015) <https://www.youtube.com/watch?3v=sNhhvQGsMEc, http://kurzgesagt.org/> [accessed 8 July 2016].

James F. Kasting, ‘Essay Review: Peter Ward and Donald Brownlee’s “Rare Earth”’, Perspectives in Biology and Medicine, 44.1 4(2001), 117–31 (p. 117).

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The Rare Earth Hypothesis: Which factors have the greatest effect on the development of life? In ‘Rare Earth’ Ward and Brownlee list a set of factors which have an effect on whether a planet will develop life.

Table 1: Rare Earth factors as proposed by Ward and Brownlee 5

Firstly, some of these factors will not be possible to detect today (highlighted in red). Plate tectonics, planet tilt and the presence of a moon are currently beyond our technology. These ideas are too small to observe either directly or indirectly. The other factors highlighted in red are ideas that cannot be observed by looking through a telescope. Rather ideas about things that might need to occur for a planet to develop life. Each factor will have an effect on the development of life. However, it is debated how much of an effect they will have.

Type of star and the distance the planet lies from it. Astronomers have a classification system for stars. A star is placed under a group from the set O, B, A, F, G, K, M where M is the coolest and O is the hottest . Warmer stars tend to produce 6

more ionising ultraviolet light and cooler stars heat the planet less, so they can be less 7

hospitable to life.

Right distance from star Habitat for complex life. Liquid water near surface. Far enough to avoid tidal lock.

Right planetary mass Retain atmosphere and ocean. Enough heat for plate tectonics. Solid/Molten core.

Plate tectonics CO2 - silicate thermostat. Build up land mass. Enhance biotic diversity. Enable magnetic field.

Right mass of star Long enough lifetime. Not too much ultraviolet.

Jupiter-like neighbour Clear out comets and asteroids. Not too close, not too far.

A mars Small neighbour as possible life source to seed an Earth-like planet.

Ocean Not too much. Not too little.

Large Moon Right distance. Stabilizes tilt.

The right tilt Seasons not too severe.

Giant impacts Few giant impacts. No global sterilizing impacts after an initial period.

The right amount of carbon Enough for life. Not enough for runaway greenhouse.

Atmospheric properties Maintenance of adequate temperature, composition and pressure for plants and animals.

Biological evolution Successful evolutionary pathway to complex plants and animals.

Evolution of oxygen Invention of photosynthesis. Not too much or too little. Evolves at the right time.

Right kind of galaxy Enough heavy elements. Not small, elliptical or irregular.

Right position in galaxy Not in centre, edge or halo.

Wild cards Snowball Earth. Cambrian explosion. Inertial interchange event.

Donald Brownlee Peter Ward, Rare Earth: Why Complex Life Is Uncommon in the Universe (Springer, 2009).5

‘Classifying Stars - the Hertzsprung-Russell Diagram’, Australia Telescope National Facility <http://www.atnf.csiro.au/outreach/6education/senior/cosmicengine/stars_hrdiagram.html> [accessed 29 October 2016].

‘How Are Wavelength and Temperature Related?’, Hubble Site <http://hubblesite.org/reference_desk/faq/7answer.php.id=74&cat=light> [accessed 29 October 2016].

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The greater the distance from the star the smaller amount of thermal energy is absorbed by the planet. If a planet orbits too close to a star it absorbs too much radiation and becomes too hot for life. If it orbits too far from the star then it absorbs too little, so becomes too cold for life.

Furthermore, if a planet orbits too close to a star it can become tidally locked. This means its orbital period is about equal to it rotational period. This results in a single side of the planet always facing the star. The distance for a planet to become tidally locked is predicted to be proportional to for a star of mass, . 8

All life we know of on Earth requires liquid water. If a planet is too hot it becomes vapour and if it is too cold it becomes ice. Therefore, there will be a region around stars where water can be a liquid. This region is called the habitable zone . 9

However, it has also been argued that this region is too limiting. On Earth, life is found in and thrives in extreme environments. Such organisms are called extremophiles. This suggests that life can thrive in a much larger range of environments. It’s also argued that in the Solar System, life could develop on the moons of the gas giants. Most notably is Europa which is believed to have a water-ice ocean beneath the ocean and for the life to get its energy from sources other than the sun . Therefore, it’s possible that life could form outside of the habitable zone. 10

Giovanna Tinetti, Thérèse Encrenaz and Athena Coustenis, ‘Spectroscopy of Planetary Atmospheres in Our Galaxy’, Astronomy 8and Astrophysics Review, 21.1 (2013) <https://doi.org/10.1007/s00159-013-0063-6>.

Sara Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, 2009, p. 22 <http://seagerexoplanets.mit.edu/9ProfSeagerEbook.pdf> [accessed 8 July 2016].

Louis N. Irwin Dirk Schulze-Makuch, ‘Alternative Energy Sources Could Support Life on Europa’, Eos, Transactions, American 10Geophysical Union , 82.13 (2001), 150.

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The mass, size and the type of a planet The mass affects the gravity of a planet through the equation:

Where g is the acceleration due to gravity; G is the gravitational constant; ; M is the mass of the planet and r is the distance between an object and the centre of mass of the planet.

On Earth, the value of g is about . On a planet with a larger mass, the value of g is greater. This results in two important things. The planet is able to hold onto a larger atmosphere and objects weigh more; these are two factors that affect life.

If a planet has a denser atmosphere of the right gases it will hold onto more thermal energy. Too dense an atmosphere and this is a problem as it gets too hot for the necessary reactions to take place. If the atmosphere is too thin then the temperature will be too variable for life . 11

If objects weigh more then they need to be stronger to be able to move around. If a human were to stand of Jupiter they would weigh 2.6 times more as the acceleration due to gravity is 2.6 times greater than that on Earth. You wouldn’t be able to stand up.

If you can think of a planet that obeys the laws of physics it probably exists. In the Solar System alone, we have several types of planets: terrestrial, gas giants and ice giants. It has been argued that life requires a surface to form so gas giants wouldn’t be suitable as they have no distinct surface . 12

A system of bodiesWard and Brownlee argue that life on Earth is partly thanks to the arrangement of planets and moons in the Solar System. Firstly, a Jovian neighbour is said to reduce the number of comet collisions by long period comets, (LPCs), on Earth thus reducing the rate that mass extinctions can occur. It’s proposed that due to Jupiter’s large mass, it attracts comets and ejects them from the solar system by means of a gravitational slingshot. This is based on research by Gorge Wetherill . 13

However, since Wetherill’s report, further research has been made into Jupiter’s effect on collisions with asteroids from the asteroid belt. It was found that Jupiter is just as a good as a shield as a far less massive planet would be, for an asteroidal population. Furthermore, we have found that Jupiter mass planets are not rare. This shows that a Jupiter like planet in orbit along with a possible hospitable planet is not as important as Ward and Brownlee point out.

The Moon-Earth mass ratio is large compared to other moon-planet ratios. For example, Mars has two moons which are very small in comparison compared itself whereas, the Moon and Earth are more comparable. Mars’ moons are believed to be just captured asteroids but the Moon is believed to be created from a more dramatic event. A planet called Theia allegedly collided with the primordial Earth to from the Moon.

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 22.11

Peter Ward.12

Peter Ward, chap. 10.13

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Figure 2: Impact origin of the Moon as modelled by Cameron and Cannup, 199814

For this argument, Ward and Brownlee use research where a group showed, mathematically, the Earth’s tilt (obliquity) would vary chaotically from 0 to 85° without the Moon, where today it varies from 22° to 24.5° . Obliquities approaching 90° would result in extreme seasonal 15

cycles . 16

Whether these seasonal extremes would make Earth uninhabitable for complex life is debatable. For continents located near the equator, life would experience two summers and two winters each year but they would not experience extremes of temperatures like with higher latitudes. Temperatures could remain in the 0°C to 30°C range which is certainly habitable.

However, a planet’s Obliquity depends on the initial obliquity, the planet’s spin rate, and the masses and spins of other planets. Earth could be moonless and have the same obliquity as with today if it had a rotational period of 12 hours. The reason Earth is spinning today is due to the impact event. There is no method today to find what the Earth’s spin would be if the event had not occurred but there is no reason to believe it would be as slow as today .17

Atmosphere and oceansThe compounds that form the atmosphere around a planet are important, not just for reaction to form complex life but for the conditions on the planet. The composition of the atmosphere on a

Peter Ward, p. 231.14

Kasting, p. 120.15

Peter Ward, p. 223.16

Kasting, p. 210.17

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planet has a large part in determining the temperature of the atmosphere. If an atmosphere contains too much carbon dioxide or water vapour it can result in a runaway greenhouse effect. The temperature gets too hot for life and becomes uninhabitable. The pressure cannot be too high to too low either, certain reactions only occur in a given pressure range. If the atmosphere contains too little carbon dioxide and water vapour then life will not form.

For animals to develop the atmosphere will need oxygen, thus a process like photosynthesis needs to develop. Oxygen is a highly reactive gas, therefore if we find it in abundance in an atmosphere it is a key sign of life. There needs to be a process actively producing it to match the rate it is used in reactions. 18

The planet’s position in its galaxySeveral astrobiologists believe that the position of a planet’s solar system in the galaxy will affect the development of life. They propose that galaxies, like solar systems, have a habitable zone. Life is fairly delicate, too much or too little heat, too much ionising radiation and too many charged particles can easily destroy it. Ward and Brownlee say that the centre of galaxies has all of these. The centres of galaxies have many more stars per unit volume. Some of these stars, such as magnetars, produce gamma rays and other charged particles into space. Ward and Brownlee believe it is unlikely any life could live near these stars. This defines the inner boundary of the galactic habitable zone.

In the outermost regions of the Milky Way, the heavy metal composition is lower. This is due to the rate of star formation being lower so the rate heavy elements are produced is lower. Ward and Brownlee believe that this means fewer terrestrial planets of an Earth-size are produced. This defines the outer boundary of the habitable zone.

However, I believe, this factor is, in fact, of little concern. The planets we are most interested in are in the immediate solar neighbourhood. Astronomers will be able to produce better quality data from these compared to stars in the outer reaches of the galaxy. We will also have a much higher chance of actually reaching these planets.

Which factors have the greatest effect?From the arguments, for and against, each factor discussed thus far, I have chosen a set of factors that I believe to have the largest effect on the development of life. This subsection will give a summary of why each factor has been chosen.

1. Type of star and the distance a planet lies from it.

This has been chosen as it has a direct effect on the temperature of the planet. Generally this distance is represented using a measurement of the semimajor axis: half of the longest length of the orbital ellipse. This, therefore, means a direct effect on whether liquid water is present 19

on the surface.

2. Atmospheres and Oceans

Ideally, we could find life that is alive today. Meaning we would have the ability to communicate if that was what Earth decided to do. Thus we want to find biosignatures in the atmosphere. If

Peter Ward, p. 98.18

‘Semimajor Axis’, Merriam Webster <http://www.merriam-webster.com/dictionary/semimajor%20axis?19utm_campaign=sd&utm_medium=serp&utm_source=jsonld> [accessed 13 Novemeber 2016].

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we were to use this factor when looking for exoplanets we could use it to find oxygen and thus good evidence for animal life. This can be found with absorption spectra of the atmosphere.

3. Planetary Mass

I believe the two most important factors that are directly affected by the mass are the atmospheric density and the type of planet. It seems likely that life needs a surface thus a terrestrial type to from. The atmospheric density is important as it can contribute to the temperature and pressure of the atmosphere.

Which exoplanet detection method works best for finding planets with these factors?Astronomers are currently using several methods to detect exoplanets. These either detect planets directly or by inferring them from other characteristics (indirect). Each method allows us to find a different set of properties. This can mean one method is better for finding planets with the set of properties I have determined to be the most significant.

In this section I will cover each method we have used to detect exoplanets and which factors can be inferred. Timing methods are excluded as they generally involve pulsars which are a type of dying star which won’t be hospitable to life.

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Direct imaging

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Figure 3: Direct Image of the region around the star Fomalhaut20

So far only 73 exoplanets have been detected using direct imaging. However, all these planets 21

were detected directly meaning signs of their existence we seen in actual images of the region around the star. Direct imaging is the only method able to do this. This is advantageous as it means it is less likely to be a false positive. With current technology, only planets with masses greater than that of Jupiter; that spend at least part of their orbit far from the star and are hot, usually from being young, can be found using direct imaging.

To directly image a planet the starlight usually needs to be eliminated from the image. Some astronomers use a coronagraph which is a mask over telescope lens. Alternatively, one method that is yet to be carried out in practice is with occulters. These are starshades that lie far from the telescope. It’s the shape of an occulter that, theoretically, makes it so effective. The starlight diffracts away from the telescope lens, ideally leaving behind only other light sources. 22

‘NASA’s Hubble Reveals Rogue Planetary Orbit For Fomalhaut B’, Nasa, 2013 <https://www.nasa.gov/mission_pages/hubble/20science/rogue-fomalhaut.html> [accessed 20 November 2016].

‘Catalog’, The Extrasolar Planet Encyclopaedia, 2016 <http://exoplanet.eu/catalog/> [accessed 17 November 2016].21

Ben R. Oppenheimer Wesley A. Traub, ‘Direct Imaging of Exoplanets’, in Exoplanets, ed. by Sara Seager (The University of 22Arizona Press, 2010), pp. 111–56.

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“Finding an Earth twin around a Sun-like star is like trying to see a firefly fluttering less than a foot from a huge searchlight—when the searchlight is 2600 miles away.”

—Sara Seager

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Figure 4: Direct imaging of a planet orbiting a brown dwarf 23

Once an image of the region around the star has been captured information about it can be determined. Ideally, multiple images should be taken to decrease uncertainties in the properties.

By looking at changes in several photos of the star, the projected motion of the planet can be found. This is the distance moved per year during the time we have been observing it. An estimate of the semimajor axis can also be found from the image, albeit with uncertainties as it must be assumed to be on the plane approximated from the image.

Using Kepler’s third law, the orbital period can then be found. The law states that the square of the orbital period is directly proportional the cube of the semimajor axis.

Where P is the orbital period is the gravitational constant is the mass of the star is the mass of the planet and is the semimajor axis. The mass of the planet is negligible when compared with

the mass of the star. Thus:

‘What Is a Brown Dwarf?’ <http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question62.html> [accessed 11 July 2016].23

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By modelling the planet with circular motion, the tangential speed can then be found. For this to be accurate it is assumed to be at periapsis. From the values found thus far, a lower limit on the eccentricity can be found. These measurements have large uncertainties due to the inclination of the orbital plane of the planet; the apsidal orientation and the uncertainties in the estimation of the stellar mass. 24

The luminosity of a planet can also be found from the image. This is used to approximate the mass. A model is used that relates luminosity change over time as a function of the mass and age. The mass is found by looking up the mass the planet should have for it have cooled down by an amount in its age. Mainly, the uncertainties come from the model. Most significant is the age plots which have very large range bars.

Planets can be detected using direct imaging by observing dust rings around the star. These dust rings can be imaged and then computer models can be used to find a best fit for a planet that causes the shape of these rings. 25

Paul Kalas, James R. Graham, Eugene Chiang, Michael P. Fitzgerald, Mark Clampin, Edwin S. Kite, Karl Stapelfeldt, Christian 24

Marois, John Krist, ‘Optical Images of an Exosolar Planet 25 Light Years from Earth’, Science, 322.5906 (2008), 25.

Paul Kalas, James R. Graham, Eugene Chiang, Michael P. Fitzgerald, Mark Clampin, Edwin S. Kite, Karl Stapelfeldt, Christian 25

Marois, John Krist.

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Figure 5: Luminosity versus age diagram. Hot stars and cold stars and shown by solid and dashed lines respectively.26

‘Characterization of the Gaseous Companion κ Andromedae B’, Astronomy & Astrophysics. Supplement Series, 562 (2014), 20.26

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Transit eventsThe transit method is by far the most used method today. The Kepler Mission has discovered thousands of exoplanets using it. It is also rather effective in finding data about the planet. When an object passes between an observer and the source, the light source is obstructed. This is what the transit method uses. A transit is “the passage of a relatively small body across the disk of a larger body, usually a star”. Thus, we monitor a set of stars in the sky measuring 27

the intensity of the light. If there is a dip in the light over a period of time we can predict the size of an object that caused it. We plot the relative flux (brightness) against time.

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Figure 6: Typical transit plot for a planet-star transit event. 28

The transit method is particularly useful for finding the radius (thus the size) of a planet due to the direct relation between the size and drop of photon flux . 29

One important factor to consider is that the system you’re observing will not be orientated perfectly for measurement. It’s rare to get a planet to orbit on the same plane as the observational plane. This is due to the inclination of a planet. However, unlike other methods, this can be found using the transit method.

The size of a planet can be found as the fractional change in photon flux is equal to the ratio of the two disc areas:

‘Transit | Astronomy’, Encyclopedia Britannica <https://www.britannica.com/topic/transit-astronomy> [accessed 12 July 2016].27

Niel Brandt, ‘Transit Plot’, 2002 <http://www2.astro.psu.edu/users/niel/astro1/slideshows/class44/019-extra_transit_plot.jpg>.28

Paul Anthony Wilson, ‘The Exoplanet Transit Method’, Paul Anthony Wilson <https://www.paulanthonywilson.com/exoplanets/29

exoplanet-detection-techniques/the-exoplanet-transit-method/> [accessed 8 July 2016].

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Where F is the stellar flux, R is the radius of a body. s denotes a property of the star and p denote a property of a planet.

Some of the time the orbital period can be found as it is short (tens of days ). For any star, 30

planets with a short orbital period will generally orbit close to their star, thus a small semimajor axis. Which can be found again through Kepler’s third law.

The effects of limb darkening need to be taken into account. A star is not a disc of constant brightness. It decreases as you move from the centre. This produces a shallower transit curve meaning resulting a smaller value for . Thus it produces a smaller value for the planet’s mass. 31

SpectroscopyFrom direct imaging and Transit Events, we are able to find in much more detail about the chemical composition. Spectra are taken of the star and the planet. These are then compared to find absences at certain frequencies. Each gas has its own “fingerprint”. By comparing to 32

experimental data we can find which gasses cause the absorptions. We tend to do this in both the infrared and visible light sections of the EM spectrum.

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Figure 7: The spectrum of Earth as a distant planet. 33

‘A 1.9 Earth Radius Rocky Planet and the Discovery of a Non-Transiting Planet in the Kepler-20 System’, The Astronomical 30

Journal, 152.6 (2016), 33.

Wilson.31

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 10.32

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 10.33

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Radial velocity and astrometric measurementsThe radial velocity and astrometry methods both utilise “stellar wobble”: how much a star moves due to the presence of a planet. Data can be inferred from these measurements.

Objects with a large mass will exert a force on other masses nearby (the force is proportional to the mass of the body). Both planets and stars have a large mass. Thus, the planet and star both induce movement on each other. For a system with one star and one planet, the two objects will orbit around a single point: their common centre of mass.34

The astrometry method uses cameras like with direct imaging. However, rather than trying to find a planet movement of the star across the star is monitored. This requires great angular precision.

From the period of this precession the semimajor axis can be determined. Then from the acceleration of the star the force from the planet can be found. This is directly proportional the mass of the planet.

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Figure 8: Movement of a star with time. 35

Tahir Yaqoob, Exoplanets and Alien Solar Systems (newearthlabs, 2011), chap. 3.34

Mark Elowitz, ‘How Are Extrasolar Planets Detected?’, Markelowitz <http://www.markelowitz.com/Exoplanets.html> [accessed 12 35

July 2016].

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So far only two (massive) planets have been claimed using the astrometric method . However, 36

the ESA GAIA spacecraft is expected to collect data for several thousands of new planets .37

A different technique to analyse the “stellar wobble” is the radial velocity method. This method makes use of a phenomenon called the Doppler shift. “a change in the frequency with which waves from a given source reach an observer when the source and the observer are in motion with respect to each other so that the frequency increases or decreases according to the speed at which the distance is decreasing or increasing” . In simpler terms if an object is moving 38

towards you, the starlight is shifted to the blue end of the light spectrum, it is blueshifted. Whereas, if the object is moving away from you, the light from the star is shifted to the red end of the spectrum, it’s redshifted.

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Figure 9: The movement of a planet-star system with its resultant radial velocity curve.

From the amount the light is shifted we can produce a radial velocity curve. From here we can extract the data we need to find out properties. By processing the data we are able to find out its minimum mass, where is the inclination.

With modern spectrometers, the movement of a star can be found down to a matter of metres. 39

Finding Earth-like planets with this method is currently beyond the technological limits . It’s also 40

the most sensitive to close-in planets as the force from the star decreases in an inverse square law with increasing distance.

The radial velocity method can be used to approximate the mass and orbital period. Large uncertainties again come from the orientation of the system, the planets orbital plane is not “edge on” to us. If the planet has an inclination, the uncertainty in the mass is larger. If the planets plane of movement is perpendicular to the place we are observing we wouldn’t be able

Michael Perryman, Joel Hartman, Gáspár Á. Bakos Lennart Lindegren, ‘ASTROMETRIC EXOPLANET DETECTION WITH 36

GAIA’, The Astrophysical Journal, 797.14 (2014), 22.

Tinetti, Encrenaz and Coustenis.37

‘Definition of DOPPLER EFFECT’, Merriam Webster <http://www.merriam-webster.com/dictionary/Doppler+effect> [accessed 28 38

August 2016].

Elowitz.39

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 19.40

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to detect any signs of a planet. The inclination cannot be found using this method leaving the result as a minimum mass.41

Gravitational microlensingThe gravitation microlensing method is rather different the other methods. Any mass will alter the shape of space. A way to think about this is if you roll a ball over a trampoline when you stand on it, the ball moves in a curve. The photons moving through space will follow this curvature. The photons direction of movement will change due to the curvature.

One principle behind general relativity is that any mass will warp spacetime. The trajectories of any photons moving through this region of space will change, resulting in a lensing effect.

A dense field of millions of background stars are monitored. Any sudden changes in brightness 42

are measured, suggesting the light from a distant star is being focused. The path of the light is worked backwards incorporating a unknown mass, potentially a planet, if the path initially doesn’t work out correct.

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Figure 10: Illustration of the microlensing exoplanet discovery technique 43

‘Radial Velocity’, The Planetary Society <http://www.planetary.org/explore/space-topics/exoplanets/radial-velocity.html> [accessed 41

13 July 2016].

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 42.42

Seager, ‘Is There Life Out There? The Search for Habitable Exoplanets’, p. 42.43

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Which method will work best for detecting hospitable planets?

Table 2: Summary of the measurements that can be taken using each method.

An ideal method would be one that is able to produce measurements in all three of my chosen factors and more. Furthermore, the measurements produced should have small uncertainties and high probabilities when statistically analysed. None of these methods discussed are perfect. Each provides a different set of data with a different accuracy and precision.

Gravitational Microlensing provides the fewest data points but its measurements have one of the lowest percentage uncertainties. Its fundamental flaw for detecting habitable planets is the lack spectroscopy. An idea of the temperature can be gathered from the mass and semimajor axis but without the right composition, we have very little idea of whether the planet could harbour life. Furthermore, gravitational microlensing is very situational thus it’s more difficult to justify a larger scale usage.

Radial velocity and astrometry are somewhat better as it allows the path of the orbit to be more accurately plotted to find a more accurate temperature approximation. However, it has the same flaw as gravitational microlensing. One downside to radial velocity is the mass is only a minimum meaning the mass could be many magnitudes greater, thus less habitable.

The two remaining methods, transits and direct imaging, can both be used for spectroscopy. This is property is crucial to finding life as we can find whether the planet contains biosignatures. According to my set of factors, direct imaging wins as it supports all three properties, unlike transits which is missing a mass. However, direct imaging’s issue is that is that it brings large uncertainties and is currently on the frontier of our technology meaning many systems cannot be imaged. In the future however as our models improve and the technology like occulters are launched, we should see much better data from this method.

The ideal scenario today, however, is a combination of methods. Transits to measure properties of the motion of the planet and find its size accompanied with the radial velocity method to find the minimum mass which with the inclination found from the transits, the actual mass can be found.

Appendix: GlossaryExoplanet: A planet in orbit of a star other than the Sun.

Direct Imaging

Transit Astrometry Radial Velocity

Gravitational Microlensing

Mass Yes No Yes Minimum Yes

Radius No Yes No No No

Semimajor axis

Yes Yes Yes Yes Yes

Eccentricity Yes Yes Yes Yes No

Atmospheric properties

Yes Yes No No No

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Exoplanet Candidate: A proposed exoplanet that is yet to be confirmed. Extremophile: An organise that thrives in extreme environments. Orbital period: The time taken for a planet to complete a full revolution of motion around its parent star. Jovian: Jupiter-likeLong Period Comet: A comet that has a long orbital period usually due to a very eccentric orbit. Obliquity: The tilt of a planet. Semimajor axis: Half the length of the largest diameter of an ellipse. Photon: Quantum of light, particles used to represent the movement of light. Photon/Steller Flux: Number of photons observed per unit time. Doppler Shift/Effect: Apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer. Apsidal Orientation: The positions of the apoapsis and periapsis with respect to the star.

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