1. Astrobiology Workshop June 29, 2006
Exoplanets
Astrobiology Workshop
June 29, 2006

2. Exoplanets: Around Solar-Type Stars
Discovery (since 1995) by
Doppler shifts in spectral lines of stars
Transits of stars by planets
Microlensing
Maybe imaging
Web Sites
exoplanet.org
exoplanet.eu
Solar System Planets
Terrestrial
Gas Giant
Ice Giant
Earth
Neptune
Jupiter
Saturn

3. Exoplanets: Around Solar-Type Stars
Characteristics
All (or almost all?) are gas or ice giants
Masses from 7ME up to > 13MJ (MJ = 320 ME)
Orbits are mostly unlike the Solar System
“Hot Neptunes” & “Hot Jupiters” (a < 0.4 AU) are common
Orbits are often very eccentric
Earths cannot be detected yet
Numbers (>180)
Probably at least 10-15% of nearby Sun-like Stars
18 Planetary Systems (stars with 2 or more planets)

4. Doppler Shift due to Stellar Wobble

5. Doppler Shift due to Stellar Wobble

6. Doppler Shift for a Star Orbited by a Planet

7. So How Hard Is It? Difficulty of Doppler Searches Jupiters
C.O.M. of Jupiter-Sun system (5.2 AU orbit radius) is near the Sun’s surface (M = 1,000 MJ)
Jupiter orbits the C.O.M. at 13 km/s
The Sun’s speed is smaller by the ratio of Jupiter’s mass to the mass of the Sun (10-3)
The Sun’s wobble due to Jupiter is only 13 m/s
The speed of light is 3x108 m/s
For the Doppler effect: / = v/c
So, we have to detect changes in wavelength  of spectral lines of less than one part in 107 to measure this!
Massive, close-in gas giants are much easier to detect

8. So How Hard Is It? Difficulty of Doppler Searches Earth
The Sun’s wobble due to the Earth is only about 10 cm/s !
Requirements for Any Planet
Very stable reference spectrum
Use of all the spectral lines in the spectrum
Problem: Velocity “noise” from motions in the star’s atmosphere is typically 1 to10 m/s !

9. Exoplanets from Doppler Shifts: General PictureV E M J

10. First Detection of an Exoplanet: 51 Pegasi

11. First Exo-Planetary System: Upsilon Andromedae
F8V
4.2 MJ
1.9MJ
0.7MJ

12. Eccentric Orbit Example: 16 Cygni b
1.7 MJ
G5V

13. S.S. Analog: 47 Ursa Majoris
2.5MJ
0.76MJ

14. 55 Cancri: A Four Planet System
Msini = 4.05 MJ
a = 5.9 AU (5,360 days)
Msini = 0.21 MJ
a = 0.24 AU (44.3 days)
Msini = 0.84 MJ
a = 0.12 AU (14.7 days)
Msini = MJ (14 ME)
a = AU (2.81 days)
Star Mass = 0.95 M G8V

15. Gliese 876 System: Gas Giants in 2:1 Resonance

16. Gliese 876 System: 6 to 8 Earth Mass Planet

17. Gliese 876 System: Three Known Planets
Msini = 1.89 MJ
a = 0.21 AU (61.0 days)
Msini = 0.56 MJ
a = 0.13 AU (30.1 days)
Msini = 5.9 ME
a = AU (1.94 days)
Star Mass = 0.32 M M4V

18. Gliese 876 System: The Movie

19. Systems Where Planets Transit the Star

20. Transiting Planet HD209458b Planet Mass = 0.69  0.05 MJ
Planet Radius = 1.43  0.04 RJ
Orbit a = AU
Orbit Period = 3.52 days
Star Mass = 1.05 M (F8V)

21. Transiting Planet HD209458b

22. Transiting Planet HD209458b: Absorption Line of Sodium

23. Transiting Planet HD149026b: A Massive Heavy Core
Planet Mass = 0.36 MJ
Planet Radius = 0.72  RJ
Orbit a = AU
Orbit Period = 2.88 days
Star Mass = 1.31 M G0IV

24. Image of a Planet?

25. Issues and Concerns: Planet Formation
Gas Giant Formation Theories
Solid Core Accretion followed by gas capture
Pro: Mechanism that can work
Con: Slow, expect formation at > few AU, may not be able to make super-Jupiters
Disk Instability due to self-gravity of the protoplanetary disk
Pro: Fast formation
Con: Real protoplanetary disks may not cool fast enough to fragment, may be hard to explain large solid cores
Hybrid: Core Accretion sped up by Disk Instability?
Evidence
Metallicity correlation may favor Core Accretion

26. Issues and Concerns: Planet Formation
Hot Neptunes & Jupiters?
Formation in Place
Probably not possible
Planet “Migration”
Planets can drift inward due to planet-disk interaction
Eccentricities?
How Are They Attained?
Multi-body interactions
Perturbations by nearby stars
Planet-disk interactions
Migration into orbital resonances
Overall
Incredible Diversity of Planetary Systems!

27. Issues and Concerns: Life
Why Are Hot Jupiters Bad?
Origin
Probably exist due to inward “migration” during planet formation
Effects
Sweep terrestrial planet material into the star as they migrate
Gas Giants near or inside the habitable zone make stable orbits for terrestrial planets difficult or impossible
Why Are Eccentric Gas Giants Bad?
Tend to disrupt terrestrial planet formation
Tend to destabilize terrestrial planet orbits and/or force the orbits to be eccentric, producing extreme seasons

28. Issues and Concerns: Life
Hope?
There ARE Solar System Analogs!
Gas giants at > few AU in nearly circular orbits
Over the next decade, more are likely to be found
Incredible Diversity of Environments!
And…

29. Maybe Close-In Gas Giants Have Earth-Like Moons