The Doppler Method, or the Radial
Velocity Detection of Planets:
II. Results
Telescope
1-m MJUO
1.2-m Euler Telescope
1.8-m BOAO
1.88-m Okayama Obs,
1.88-m OHP
2-m TLS
2.2m ESO/MPI La Silla
2.7m McDonald Obs.
3-m Lick Observatory
3.8-m TNG
3.9-m AAT
3.6-m ESO La Silla
8.2-m Subaru Telescope
8.2-m VLT
9-m Hobby-Eberly
10-m Keck
Instrument
Hercules
CORALIE
BOES
HIDES
SOPHIE
Coude Echelle
FEROS
2dcoude
Hamilton Echelle
SARG
UCLES
HARPS
HDS
UVES
HRS
HiRes
Wavelength Reference
Th-Ar / Iodine cell
Th-Ar
Iodine Cell
Iodine Cell
Th-Ar
Iodine Cell
Th-Ar
Iodine cell
Iodine cell
Iodine Cell
Iodine cell
Th-Ar
Iodine Cell
Iodine cell
Iodine cell
Iodine cell
Campbell & Walker: The Pioneers of RV Planet Searches
1988:
1980-1992 searched for planets around 26
solar-type stars. Even though they found
evidence for planets, they were not 100%
convinced. If they had looked at 100 stars
they certainly would have found
convincing evidence for exoplanets.
Campbell, Walker, & Yang 1988
„Probable third body variation of 25 m s–1, 2.7 year
period, superposed on a large velocity gradient“
The first (?) extrasolar planet around a normal star: HD 114762
with M sin i = 11 MJ discovered by Latham et al. (1989)
Filled circles are data taken at McDonald Observatory using the telluric lines at 6300 Ang.
The mass was uncomfortably high (remember sin i effect) to
regard it unambiguously as an extrasolar planet
The Search For
Extrasolar Planets
At McDonald Observatory
Bill Cochran & Artie Hatzes
Harlan J. Smith
2.7 m Telescope
1988 - present
Phillip MacQueen, Paul Robertson,
Erik Brugamyer, Diane Paulson, Robert
Wittenmyer, Stuart Barnes
Michael Endl
Hobby-Eberly
9 m Telescope
2001 - present
51 Pegasi b: the 1st extrasolar planet:
P = 4.3 days!!!
a = 0.05 AU !!!
M sin i = 0.45 M Jupiter
Michel Mayor & Didier Queloz 1995
A HOT JUPITER
1997: The first 2.7 m
Survey Planet:
P = 2.2 yrs a = 1.67 AU
M ~ 1.7 M Jupiter
More Planets / Brown Dwarfs
(co-)discovered
with the 2.7 m Telescope:
Eps Eri b:
Gam Cep:
HD 137510 b:
HD 13189 b:
Beta Gem b:
HD 91699 b:
And then the discoveries started rolling in:
“New Planet Seen Outside Solar System”
New York Times
April 19, 1996
“10 More Planets Discovered”
Washington Post
August 6, 2000
“First new solar system discovered”
USA TODAY
April 16, 1999
Global Properties of Exoplanets:
Mass Distribution
The Brown Dwarf Desert
Planet: M < 13 MJup → no nuclear burning
Brown Dwarf: 13 MJup < M < ~80 MJup → only deuterium burning
Star: M > ~80 MJup → Hydrogen burning
Up-to-date Histograms with all ~ 500 exoplanets:
One argument: Because of unknown sin i these are just
low mass stars seen with i near 0
i decreasing
probability decreasing
Number
Semi-Major Axis Distribution
Semi-major Axis (AU)
The lack of long period planets is a selection effect since these take a long
time to detect
The short period planets are also a selection effect: they are the easiest to find
and now transiting surveys are geared to finding these.
Updated:
Eccentricity distribution
Fall off at high eccentricity may be partially due to an observing
bias…
e=0.4
e=0.6
e=0.8
w=0
w=90
w=180
…high eccentricity orbits are hard to detect!
For very eccentric
orbits the value of the
eccentricity is is often
defined by one data
point. If you miss the
peak you can get the
wrong mass!
At opposition with Earth would
be 1/5 diameter of full moon,
12x brighter than Venus
e Eri
2 ´´
Comparison of some eccentric orbit planets to our solar system
Mass versus
Orbital Distance
Eccentricities
There is a relative lack of massive close-in planets
Classes of planets: 51 Peg Planets: Jupiter mass
planets in short period orbits
Discovered by Mayor & Queloz 1995
Classes of planets: 51 Peg Planets
• ~35% of known extrasolar planets
are 51 Peg planets (selection effect)
• 0.5–1% of solar type stars have
giant planets in short period orbits
• 5–10% of solar type stars have a
giant planet (longer periods)
Somehow these giant planets ended
up very close to the star!
=> orbital migration
Classes of planets: Hot Neptunes
Santos et al. 2004
Butler et al. 2004
M sin i = 14-20 MEarth
If there are „hot Jupiters“ and „hot Neptunes“ it makes sense that
there are „hot Superearths“
CoRoT-7b
Mass = 7.4 ME
P = 0.85 d
Classes: The Massive Eccentrics
• Masses between 7–20 MJupiter
• Eccentricities, e > 0.3
• Prototype: HD 114762 discovered in 1989!
m sini = 11 MJup
Classes: The Massive Eccentrics
There are no massive planets in circular orbits
Planet-Planet Interactions
Initially you have two giant
planets in circular orbits
These interact gravitationally.
One is ejected and the
remaining planet is in an
eccentric orbit
Lin & Ida, 1997, Astrophysical Journal, 477, 781L
Red: Planets with masses < 4 MJup
Blue: Planets with masses > 4 MJup
Planets in Binary Systems
Why should we care about binary stars?
• Most stars are found in binary systems
• Does binary star formation prevent planet formation?
• Do planets in binaries have different characteristics?
• For what range of binary periods are planets found?
• What conditions make it conducive to form planets?
(Nurture versus Nature?)
• Are there circumbinary planets?
Some Planets in known Binary Systems:
Star
16 Cyg B
55 CnC
HD 46375
 Boo
 And
HD 222582
HD 195019
a (AU)
800
540
300
155
1540
4740
3300
There are very few planets in close binaries. One
exception is the g Cep system.
The first extra-solar Planet
may have been found by
Walker et al.
in 1992 in a
binary system:
Ca II is a measure of stellar activity (spots)
g Cephei
Planet
Period
Msini
2,47 Years
1,76 MJupiter
e
a
K
0,2
2,13 AU
26,2 m/s
Binary
Period
Msini
56.8 ± 5 Years
~ 0,4 ± 0,1 MSun
e
a
0,42 ± 0,04
18.5 AU
K
1,98 ± 0,08 km/s
g Cephei
Primary star (A)
Secondary Star (B)
Planet (b)
The planet around g Cep is difficult to form and on the
borderline of being impossible.
Standard planet formation theory: Giant planets form beyond
the snowline where the solid core can form. Once the core is
formed the protoplanet accretes gas. It then migrates
inwards.
In binary systems the companion truncates the disk. In the
case of g Cep this disk is truncated just at the ice line. No ice
line, no solid core, no giant planet to migrate inward. g Cep
can just be formed, a giant planet in a shorter period orbit
would be problems for planet formation theory.
The interesting Case of 16 Cyg B
These stars are identical and are „solar twins“. 16 Cyg B has
a giant planet with 1.7 MJup in a 800 d period, but star A
shows no evidence for any planet. Why?
Planetary Systems: ~50 Multiple Systems
Extrasolar Planetary Systems (18 shown)
Star
P (d) MJsini a (AU) e
HD 82943 221 0.9
0.7
0.54
444 1.6
1.2
0.41
GL 876
47 UMa
30
61
1095
2594
0.6
2.0
2.4
0.8
HD 37124 153
0.9
550
1.0
55 CnC
2.8
0.04
14.6 0.8
44.3 0.2
260
0.14
5300
4.3
Ups And
4.6
0.7
241.2 2.1
1266
4.6
HD 108874 395.4 1.36
1605.8 1.02
HD 128311 448.6 2.18
919 3.21
HD 217107 7.1 1.37
3150 2.1
0.1
0.2
2.1
3.7
0.27
0.10
0.06
0.00
0.5
2.5
0.04
0.1
0.2
0.78
6.0
0.06
0.8
2.5
1.05
2.68
1.1
1.76
0.07
4.3
0.20
0.40
0.17
0.0
0.34
0.2
0.16
0.01
0.28
0.27
0.07
0.25
0.25
0.17
0.13
0.55
Star
P (d) MJsini
HD 74156 51.6
1.5
2300
7.5
HD 169830 229
2.9
2102
4.0
HD 160691 9.5
0.04
637
1.7
2986
3.1
HD 12661
263
1444
HD 168443 58
1770
HD 38529 14.31
2207
HD 190360 17.1
2891
HD 202206 255.9
1383.4
HD 11964
37.8
1940
2.3
1.6
7.6
17.0
0.8
12.8
0.06
1.5
17.4
2.4
0.11
0.7
a (AU)
0.3
3.5
0.8
3.6
0.09
1.5
0.09
e
0.65
0.40
0.31
0.33
0
0.31
0.80
0.8
2.6
0.3
2.9
0.1
3.7
0.13
3.92
0.83
2.55
0.23
3.17
0.35
0.20
0.53
0.20
0.28
0.33
0.01
0.36
0.44
0.27
0.15
0.3
The 5-planet System around 55 CnC
0.17MJ
5.77 MJ
•0.11 M
J
Red lines: solar system plane orbits
0.82MJ
•
•0.03M
J
The Planetary System around GJ 581 (M dwarf!)
16 ME
7.2 ME
5.5 ME
Inner planet M sin i = 1.9 MEarth
Resonant Systems Systems
Star
P (d) MJsini a (AU) e
HD 82943 221 0.9
0.7
0.54
444 1.6
1.2
0.41
→
GL 876
30
61
55 Cnc
14.6
44.3
2:1
0.6
2.0
0.1
0.2
0.27
0.10
→ 2:1
0.8
0.2
0.1
0.2
0.0
0.34
→ 3:1
HD 108874 395.4 1.36
1605.8 1.02
1.05
2.68
0.07
0.25
→ 4:1
HD 128311 448.6 2.18
919 3.21
1.1
1.76
0.25
0.17
→ 2:1
2:1 → Inner planet makes two orbits for
every one of the outer planet
Eccentricities
•
Period (days)
Red points: Systems
Blue points: single planets
Mass versus Orbital Distance
Eccentricities
Red points: Systems
Blue points: single planets
On average, giant planets in planetary sytems tend to be lighter than single planets. Either 1)
Forming several planets in a protoplanetary disks „divides“ the mass so you have smaller
planets, or 2) if you form several massive planets they are more likely to interact and most get
ejected.
Summary
Radial Velocity Method
Pros:
• Most successful detection method
• Gives you a dynamical mass and orbital
parameters
• Distance independent
•
Will provide the bulk (~1000) discoveries in
the next 10+ years
•
Important for transit technique (mass determ.)
Summary
Radial Velocity Method
Cons:
•
Only effective for late-type stars
•
Most effective for short (< 10 – 20 yrs) periods
•
Only high mass planets (no Earths! maybe)
•
Projected mass (m sin i)
•
Other phenomena (pulsations, spots) can
mimic RV signal. Must be careful in the
interpretation (check all diagnostics)
Summary of Exoplanet Properties from RV Studies
• ~5% of normal solar-type stars have giant planets
• ~10% or more of stars with masses ~1.5 M‫ סּ‬have giant planets that tend to be
more massive (more on this later in the course)
• < 1% of the M dwarfs stars (low mass) have giant planets, but may have a large
population of neptune-mass planets
→ low mass stars have low mass planets, high mass stars have more planets of
higher mass → planet formation may be a steep function of stellar mass
• 0.5–1% of solar type stars have short period giant plants
• Exoplanets have a wide range of orbital eccentricities (most are not in circular
orbits). This indicates a much more dynamical past than for our Solar System!
• Massive planets tend to be in eccentric orbits and large orbital radii
• Many multiple systems, some in orbital resonances
• Close-in Jupiters must have migrated inwards!