Extra-Solar Planet Populations George Lebo 10 April 2012 AST

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!!! 2 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 day period around star 51 Pegasi Mass = 0.5 M JUP  First extra-solar planet 3

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???? 4

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 one 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 5

ES-Planet Population As of November 19, 2012, 851 planets are known to orbit other stars in 670 Planetary Systems(!!) All of this has happened in about 20 years – someone currently finds a new planet every day These planets are NOT generally like our Solar System objects – WHY? 6

7 Planets found so far Note velocity limits No Earths so far – why?

Upsilon Andromedae First multiple planet system 13

Upsilon Andromedae A Four-Planet System as of 11/19/12 All Jupiter-Size First multiple planet system 14

HD Another Hot Jupiter, a = AU 15

HD What is a “transit”? 16

HD A transit observed Note: only ~1% dip 17

HD 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!! 18

Sun and Gliese 581 (Red Dwarf)

Gliese Planetary System

Gliese 581c Low-mass planet, with mass  5 M earth Orbit semi-major axis 0.07 AU Low-mass star 21

Planets in Habitable Zones Several planets are currently known in the Habitable Zone around their parent stars All 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? 22

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 … 23

Planets in Habitable Zones We don’t see any Earth- mass planets in the HZ Does this mean they do not exist? 24

Planets in Habitable Zones Future experiments will use other techniques than Doppler shifts to search for planets These will have the sensitivity to look for Earth-mass planets in the HZ Stay tuned! 25

Terrestrial Planet Finder Cancelled, 2011 Ultra-high-contrast imager satellite Capable of finding Earth-mass planets in HZ around nearby stars 26

Kepler 22-b, First Earth-Sized Planet in the Habitable Zone of a Sun-Like Star, May 2012

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 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; no solid planets; maybe moons could host life (??) Future searches will be sensitive to Earth-mass planets in the HZ 28