Extra-Solar Planet Populations
George Lebo
10 April 2012
AST 2037
1
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
2
51 Pegasi
• In 1995, Mayor & Queloz announce the discovery of
an orbital signature with amplitude = 50 m/s in a 4.23day period around star 51 Pegasi
• Mass = 0.5 MJUP  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
• Planets found so far
• Note velocity limits
• No Earths so far –
why?
7
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 209458
• Another Hot Jupiter, a = 0.045 AU
15
HD 209458
• What is a “transit”?
16
HD 209458
• A transit observed
• Note: only ~1% dip
17
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!!
18
Sun and Gliese 581 (Red Dwarf)
Gliese Planetary System
Gliese 581c
• Low-mass planet, with mass  5 Mearth
• 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 Earthmass 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
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