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

2. 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?

3. 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

4. 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)

5. 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

6. 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)

7. 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!

8. 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

9. Radial Velocity Planet Searches
How-to, with movie
Take a spectrum with a big telescope and very precise (and STABLE) spectrograph

10. 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)

11. 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!

12. 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

13. 51 Pegasi: Sky View

14. 51 Pegasi: Hot Jupiter?
51 Peg period indicates a VERY small orbital radius (how small?)

15. 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????

16. 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

17. ES-Planet Population
As of this morning, 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