Search for Extra-Solar Planets Stephen Eikenberry 25 March 2019 AST 2037

Planets Around Other Stars None known prior to 1992 (!) In 1992, Alex Wolczan discovered 2 (now 3) planets around a neutron star But those seemed weird Probably supernova leftover bits In 1995, Michel Mayor & Didier Queloz discover a planet around 51 Pegasi – how?

Kepler’s Laws Kepler described 3 “laws” of planetary motion (for our solar system) Kepler did not have a physical basis for the laws (i.e. Newton’s laws of motion) He just found patterns in the motions of planets and used them to develop 3 guidelines that provided a good matching description Newton later used his physical laws of motion to show WHY Kepler’s rules for planet motion worked

Kepler’s First Law Planet orbits are ELLIPSES (what’s that?) The sun/star is at one “focus” of the ellipse Both the planet and the star orbit the center of mass The distance from center to focus is the “eccentricity” Circles are ellipses with eccentricity=0 (both foci at center)

Kepler’s Second Law Planet motion sweeps out equal areas of the ellipse in equal time Meaning … planet moves faster when it is closer to the star and slower when it is farther away

Kepler’s Third Law a3/P2 = Mtot a = semi-major axis of the ellipse (AU) P = period of the orbit (years) Mtot = total mass of the system (solar masses)

Orbital Reflex Motion For a star/planet system, the planet does most of the moving Its low mass means it is farthest from the center of mass Same period, larger distance means higher velocity (what is it for Earth? For Jupiter?) But you can’t see it (too faint) Star moves VERY little High mass, means small distance from COM (what is it for Sun/Earth? Sun/Jupiter?) But we can SEE the star!

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!!! How? Doppler shift of spectral lines

Radial Velocity Planet Searches How-to, with movie http://static.howstuffworks.com/flash/planet-hunting-rad-method.swf https://www.eso.org/public/usa/videos/eso1035g/ Take a spectrum with a big telescope and very precise (and STABLE) spectrograph

Radial Velocity: Information Jupiter has biggest reflex velocity effect on the Sun but this velocity is still small period is long Information we get Period (how?) Orbit distance (how?) Eccentricity Planet mass (note sin i uncertainty)

Radial Velocity: Information Jupiter has biggest reflex velocity effect on the Sun but this velocity is still small period is long Information we get Period (how?) Orbit distance (how?) Eccentricity Planet mass (note uncertainty!) Really planet MINIMUM mass!

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

51 Pegasi: Sky View

51 Pegasi: Hot Jupiter? 51 Peg period indicates a VERY small orbital radius (how small?)

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????

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

ES-Planet Population As of this morning, 228 861 3926 planets are now known to orbit other stars (!!) All of this has happened in about 20 years – someone currently finds a new planet every few days (or less!) These planets are NOT generally like our Solar System objects – WHY? Most planets were originally found using this “radial velocity” technique. But now – most planets are found via “transits”. Next time: Extra-solar planet transits