Extra-Solar Planet Populations

Stephen Eikenberry

4 November 2010

AST 2037

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Radial Velocity Planet Searches• So … need a speedometer to measure star velocity versus

time• To a precision of a few meters per second!• Across distances of many light years!!!

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• How? Doppler shift of spectral lines

51 Pegasi• In 1995, Mayor & Queloz announce the discovery of

an orbital signature with amplitude = 50 m/s in a 4.23-day period around star 51 Pegasi

• Mass = 0.5 MJUP First extra-solar planet

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51 Pegasi: Hot Jupiter?• At that location, expected

temperature is VERY high (about 2000K or higher!)

• So … Jupiter-like planet, but closer than Mercury “Hot Jupiter”

• How do you make something like that????

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Planet Bonanza• Geoff Marcy & Paul Butler quickly confirmed 51

Pegasi• They had lots of archival data from searches for

Jupiter-type planets (periods >10 years, so they were still “in progress”)

• No on even thought to look for short-period MASSIVE planets (why would they be easier?)

• Found many “Hot Jupiters” – most extra-solar planets known today are Hot Jupters

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ES-Planet Population• As of this morning, 228 861 planets are now known to orbit

other stars (!!)• All of this has happened in about 15 years – someone

currently finds a new planet every couple of days or less• These planets are NOT generally like our Solar System

objects – WHY?

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Upsilon Andromedae• First multiple planet system

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Upsilon Andromedae• First multiple planet system

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HD 209458• Another Hot Jupiter

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HD 209458• What is a “transit”?

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HD 209458• A transit observed• Note: only ~1% dip

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HD 209458 - Results• Just at the entry moment into transit, for a brief

instant, only the upper atmosphere of the planet absorbs any starlight

• With a powerful enough spectrograph, we can look for absorption lines at this instant

• Result: COMPOSITION of the planet atmosphere• HD 20948b contains – WATER!!

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Gliese 581c• Low-mass planet, with mass 5 Mearth

• Orbit semi-major axis 0.07 AU• Low-mass star

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Gliese 581c• Just inside Habitable Zone• Gliese 581d just outside HZ

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Eccentricity

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Circumbinary Planets

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Transiting Planets in the HZ

Image Credits: NASA Borucki et al. 2012Orosz et al. 2012

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Planets in Habitable Zones• Many planets are

currently known in the Habitable Zone around their parent stars

• Most of these are gas giants no solid surface

• But … gas giants in our Solar System have lots of moons

• What happens to Europa if you move it/Jupiter to a distance of 1 AU?

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Planets in Habitable Zones• What happens to Europa

if you move it/Jupiter to a distance of 1 AU?

• Really?• Europa mass is closer to

our moon’s mass – why no water there?

• So … need giant planets in the HZ with giant moons …

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Our Solar System is Not Typical

• Sun is an unusually massive star• Our solar system has no planets with orbital periods

less than Mercury• Our inner solar system does not have a

super-Earth-size to Neptune-size planet• Most planetary systems don’t have a Jupiter analog

→ Planet formation theories should typically produce planetary systems unlike our own

Borucki+ 2011; Batalha+ 2012; Howard+ 2012; Fressin+ 2013; Burke+ in prep

Terrestrial Planet Finder• Ultra-high-contrast imager satellite• Capable of finding Earth-mass planets in HZ around nearby

stars

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Summary• We have found hundreds of planets around other stars• Overwhelming majority are massive gas giants, many close

to their parent star• This is because they are easiest to find with the Doppler

technique; but Kepler mission (transits) is changing that• Have found: multiple planet systems (20+); planet

atmospheres; some low-mass (probably solid) planets• Eccentricity seems more common than circular orbits;

problems for life• So far, only a few gas giants in the HZ; first possibly-solid

planets; maybe moons could host life (??)• Future searches will be sensitive to Earth-mass planets in

the HZ

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Exoplanet Zoo: Examples• We have found

hundreds of planets around other stars

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Exoplanet Zoo: Examples• Multiple-planet systems

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Exoplanet Zoo: Examples• OGLE-2005-BLG-390Lb – 5.5 Earth-mass, far from star

(T=-360 ˚F)• COROT-7b: 4.8 Earth-mass, density = Earth-density (!);

close to star (T ~1300K)• Gliese 581g: 3-4 Earth-mass; T ~- 25 ˚F to +160 ˚F; in the

Habitable Zone (!), but tidally locked

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