6.5 Other Planetary Systems

Our goals for learning:• How do we detect planets around other

stars?• How do extrasolar planets compare with

those in our own solar system?• Do we need to modify our theory of solar

system formation?

How do we detect planets around other stars?

Planet Detection

• Direct: Pictures or spectra of the planets themselves

• Indirect: Measurements of stellar properties revealing the effects of orbiting planets

Gravitational Tugs• The Sun and

Jupiter orbit around their common center of mass.

• The Sun therefore wobbles around that center of mass with the same period as Jupiter.

Gravitational Tugs• Sun’s motion

around solar system’s center of mass depends on tugs from all the planets.

• Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

Astrometric Technique• We can detect

planets by measuring the change in a star’s position in the sky.

• However, these tiny motions are very difficult to measure (~0.001 arcsecond).

Doppler Technique• Measuring a star’s

Doppler shift can tell us its motion toward and away from us.

• Current techniques can measure motions as small as 1 m/s (walking speed!).

The peak strength of the Doppler shift tells us something about the planet’s mass.

The period of the Doppler shift tells us the radius of the planet’s orbit (Kepler’s 3rd law).

We can also detect planets if they eclipse their star

Fraction of starlight blocked tells us planet’s size

Transits and Eclipses

• A transit is when a planet crosses in front of a star.• The resulting eclipse reduces the star’s apparent

brightness and tells us the planet’s radius.• By measuring time between transits, a measurement of

planet mass can be obtained.• Secondary eclipse (of planet by star) also measurable.

1995: first normal exoplanet discovered

• Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period

• Short period means small orbital distance

• But mass similar to Jupiter’s!

How do extrasolar planets compare with those in our solar system?

Measurable Exoplanet Properties

• Orbital period, distance, and shape

• Planet mass (or lower limit)

• Planet size (for transiting planets)

• Planet density (mass divided by volume)

• Density tells us about a planet’s composition (rocky, ice giant, gas giant, etc.)

Orbits of Extrasolar Planets• Most of the

detected planets have greater mass than Jupiter.

• Planets with smaller masses are harder to detect with the Doppler technique.

Orbits of Extrasolar Planets• Most of the

detected planets have orbits smaller than Jupiter’s.

• Planets at greater distances are harder to detect with the Doppler technique.

Surprising Characteristics

• Some extrasolar planets have highly elliptical orbits.

• Some massive planets orbit very close to their stars: “Hot Jupiters.”

Over 500 known extrasolar planets as of December 2010.

Most are more massive than Jupiter and closer to their star than Earth is to the Sun.

Revisions to the nebular theory of solar system formation are necessary!

Hot Jupiters

Revisiting the Nebular Theory

• Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).

• The discovery of “hot Jupiters” has forced a reexamination of nebular theory.

• “Planetary migration” or gravitational encounters needed to explain “hot Jupiters.”

Planetary Migration• A young planet’s

motion can create waves in a planet-forming disk.

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

Gravitational Encounters

• Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.

• Multiple close encounters with smaller planetesimals can also cause inward migration.

Planets: Common or Rare?

• One in ten stars examined so far have turned out to have planets.

• Others may still have smaller (Earth-sized) planets that cannot be detected using current techniques, or planets in larger orbits that require observations over decades to detect (Saturn takes nearly 30 years just to orbit the Sun once)

Is Earth Unusual?• Data aren’t sensitive

enough yet to tell if planets like Earth are common or rare

• Available methods can only detect big planets (2 or more Earth masses; usually much more than 2)

• CoRoT or Kepler satellites might detect Earth-mass planets soon

First Rocky Exoplanet Detected in 2009

• Most known exoplanets are large and have low densities – most similar to jovian planets in our solar system

• The orbiting CoRoT telescope discovered a planet with radius only 70% larger than Earth’s

• Ground-based observations show the planet’s mass is less than 5 times Earth’s

• Together, the observations reveal that the planet’s density is similar to Earth’s - the first confirmation of a “rocky” exoplanet

Artist’s conception of the view of the rocky planet’s parent star (Corot-7) from above the surface of the planet (Corot-7b).Image from ESO / L. Calcada.

• Density = Mass / Volume

• The planet’s mass was determined using the radial velocity method:

The planet gravitationally ‘tugs’ on the star, shifting the wavelength of light the star emits back and forth. The amount of shift here gives a mass ~5 times Earth’s.

• Volume = 4/3 R3

• The planet’s radius R was determined using the transit method:

The amount of light measured from a star decreases when a planet passes in front. The amount of decrease indicates the planet’s size.

How Can We Find a Planet’s Density?

Periodic decreases in light from the star are caused by a planet with radius 1.7 times Earth’s passing in front.

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Close to Earth-sized, but not Earth-like

• Although planet Corot-7b’s density is close to Earth’s, differences abound: it orbits its star in ~20 hours (faster than any known exoplanet) - so close that its rocky surface may be molten

• With the existence of Earth-like planets now demonstrated, astronomers have reason to hope that the Kepler mission will discover more such planets

Detection of more rocky exoplanets (‘Super-Earths’) like those in this artist’s depiction should come rapidly, thanks to dedicated space telescopes and improving ground-based detection capabilities. Image from D. Aguilar, Harvard Smithsonian CfA.

A Thousand New Planets

• Prior to 2011, scientists knew of about 500 planets around other stars, detected over 15 years

• NASA’s Kepler spacecraft has been monitoring more than 150,000 stars since 2009 for repeated, brief dimmings from a planet passing in front of a star

• Results from the first four months of Kepler observations were recently announced, adding more than 1200 probable new planets to the list

Kepler’s field of view, with the locations of likely planets colored according to their size. Kepler has discovered myriad planets in a small portion of the sky in only four months.

~10°

• The frequency of the dimmings gives the orbital duration, and therefore the planet's distance from the star.  The amount of dimming gives the planet's size.

• Kepler has detected about 70 Earth-sized planet candidates - many more than known previously

• Kepler has detected about 50 candidates in the ‘Habitable Zone’ of their star (the orbital distance where temperatures should be about right for liquid water and possibly life), compared to about 5 known previously

How Many Earths?

Size vs. orbital period of 1200 new planet candidates (yellow), with previously known transiting planets (purple) and previously announced Kepler planet candidates (blue) for reference

Kepler Candidates in the Habitable Zone Size of Kepler planet candidates determined to be in the Habitable Zone of their star. Sizes range from smaller than Earth to larger than Jupiter, with most comparable to Neptune.

Jupiter

Earth

Neptune

The Big Picture

• Follow-up observations using other techniques are necessary to confirm the Kepler planets and (in most cases) determine their mass

• Only short period (close-in) planets are detectable using four months of data. Kepler’s entire 3-year mission should find planets farther out, including many more in their stars’ Habitable Zones

Kepler’s field of view covers only a small portion of the sky. The discoveries it has made in only four months suggest that planets commonly form around other stars. Image courtesy Carter Roberts.

What have we learned?• How do we detect planets around other stars?

• So far, we are only able to detect extrasolar planets indirectly by observing the planet’s effects on the star it orbits. Most discoveries to date have been made with the Doppler technique, in which Doppler shifts reveal the gravitational tug of a planet (or more than one planet) on a star.

What have we learned?

• What have other planetary systems taught us about our own?

• Planetary systems exhibit a surprising range of layouts, suggesting that jovian planets sometimes migrate inward from where they are born. This lesson has taught us that despite the successes of the nebular theory, it remains incomplete.