Radial Velocity Detection of Planets:II. Results

• To date 1783 exoplanets have been discovered

• ca 558 planets discovered with the RV method. The others are from transit searches

• 98 are in Multiple Systems (RV)

→ exoplanet.eu

Telescope Instrument Wavelength Reference

1-m MJUO Hercules Th-Ar

1.2-m Euler Telescope CORALIE Th-Ar

1.8-m BOAO BOES Iodine Cell

1.88-m Okayama Obs, HIDES Iodine Cell

1.88-m OHP SOPHIE Th-Ar

2-m TLS Coude Echelle Iodine Cell

2.2m ESO/MPI La Silla FEROS Th-Ar

2.7m McDonald Obs. Tull Spectrograph Iodine Cell

3-m Lick Observatory Hamilton Echelle Iodine Cell

3.8-m TNG SARG Iodine Cell

3.9-m AAT UCLES Iodine Cell

3.6-m ESO La Silla HARPS Th-Ar

8.2-m Subaru Telescope HDS Iodine Cell

8.2-m VLT UVES Iodine Cell

9-m Hobby-Eberly HRS Iodine Cell

10-m Keck HiRes Iodine Cell

Campbell & Walker: The Pioneers of RV Planet Searches

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.

1988:

„Probable third body variation of 25 m s–1, 2.7 year period, superposed on a large velocity gradient“

Campbell, Walker, & Yang 1988

Eri was a „probable variable“

Filled circles are data taken at McDonald Observatory using the telluric lines at 6300 Ang as a wavelength reference

The first extrasolar planet around a normal star: HD 114762 with Msini = 11 MJ P = 84 d discovered by Latham et al. (1989)

51 Pegasi b: The Discovery that Shook up the Field

Discovered by Michel Mayor & Didier Queloz, 1995

Period = 4,3 Days

Semi-major axis = 0,05 AU (10 Stellar Radii!)

Mass ~ 0,45 MJupiter

51 Peg

Rate of Radial Velocity Planet Discoveries

The Brown Dwarf Desert

Mass Distribution

Global Properties of Exoplanets:

Planet: M < 13 MJup → no nuclear burning

Brown Dwarf: 13 MJup < M < ~80 MJup → deuterium burning

Star: M > ~80 MJup → Hydrogen burning

Brown Dwarf Desert: Although there are ~100-200 Brown dwarfs as isolated objects, and several in long period orbits, there is a paucity of brown dwarfs (M= 13–50 MJup) in short (P < few years) as companion to stars

An Oasis in the Brown Dwarf Desert: HD 137510 = HR 5740

The distinction between brown dwarfs and planets is vague. Until now the boundary was taken as ~ 13 MJup where deuterium burning is possible. But this is arbitrary as deuterium burning has little influence on the evolution of the brown dwarf compared to the planet

Brown Dwarfs versus Planets

Bump due to deuterium burning

A better boundary is to use the different distributions between stars and planets:

By this definition the boundary between planets and non-planets is 20 MJup

Mass Distribution at Low Masses

A note on the naming convention:

Name of the star: 16 Cyg

If it is a binary star add capital letter B, C, D

If it is a planet add small letter: b, c, d

55 CnC b : first planet to 55 CnC

55 CnC c: second planet to 55 CnC

16 Cyg B: fainter component to 16 Cyg binary system

16 Cyg Bb: Planet to 16 Cyg B

The IAU has yet to agree on a rule for the naming of extrasolar planets

Semi-Major Axis Distribution

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.

Eccentricity versus Orbital Distance

Note that there are few highly eccentric orbits close into the star. This is due to tidal forces which circularizes the orbits quickly.

Eccentricity distribution

Fall off at high eccentricity may be partially due to an observing bias…

e=0.4 e=0.6 e=0.8

=0

=90

=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!

2 ´´

Eri

Comparison of some eccentric orbit planets to our solar system

At opposition with Earth would be 1/5 diameter of full moon, 12x brighter than Venus

16 Cyg Bb was one of the first highly eccentric planets discovered

Mass versus Orbital Distance

There is a relative lack of massive close-in planets

Classes of planets: 51 Peg Planets: Jupiter mass planets in short period orbits

Another short period giant planet

• ~40% of known extrasolar planets are 51 Peg planets with orbital periods of less than 20 d. This is a selection effect due to:

1. These are easier to find.

2. RV work has concentrated on transiting planets

• 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)

Classes of planets: 51 Peg Planets

Butler et al. 2004

McArthur et al. 2004

Santos et al. 2004

Msini = 14-20 MEarth

Classes of planets: Hot Neptunes

Note that the scale on the y-axes is a factor of 100 smaller than the previous orbit showing a hot Jupiter

If there are „hot Jupiters“ and „hot Neptunes“ it makes sense that there are „hot Superearths“

Mass = 7.4 ME P = 0.85 d

CoRoT-7b

Hot Superearths were discovered by space-based transit searches

Mass = 1.31± 0.25 MEarth (Amplitude = 1.34 m/s)Period = 8.5 hours

Earth-mass Planet: Kepler 78b

Pepe et al. 2013, Howard et al. 2013

Classes of Planets: The Massive Eccentrics

• Masses between 7–20 MJupiter

• Eccentricities, e > 0.3

• Prototype: HD 114762 discovered in 1989!

m sini = 11 MJup

Red: Planets with masses < 4 MJup

Blue: Planets with masses > 4 MJup

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

• Most stars are found in binary systems• Does binary star formation prevent planet

formation?

• Do planets in binaries have different characteristics?

• What role does the environment play?• Are there circumbinary planets?

Why should we care about binary stars?

Classes: Planets in Binary Systems

Star a (AU)16 Cyg B 80055 CnC 540

HD 46375 300Boo 155 And 1540

HD 222582 4740HD 195019 3300

Some Planets in known Binary Systems:

There are very few planets in close binaries. The exception is Cep.

For more examples see Mugrauer & Neuhäuser 2009, Astronomy & Astrophysics, vol 494, 373 and references therein

If you look hard enough, many exoplanet host stars in fact have stelar companions

A new stellar companion to the planet hosting star HD 125612

Mugrauer & Neuhäuser 2009

Approximately 17% of the exoplanet hosting stars have stellar companions (Mugrauer & Neuhäuser 2009). Most of these are in wide systems.

The first extra-solar Planet may have been found by Walker et al.

in 1988 in abinary system:

Ca II is a measure of stellar activity (spots)

Cep Ab: A planet that challenges formation theories

2.13 AUa

0.2e

26.2 m/sK

1.76 MJupiterMsini

2.47 YearsPeriod

Planet

18.5 AUa

0.42 ± 0.04e

1.98 ± 0,08 km/sK

~ 0.4 ± 0.1 MSunMsini

56.8 ± 5 YearsPeriod

Binary Cephei

Cephei

Primary star (A)

Secondary Star (B)Planet (b)

The planet around 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 Cep this disk is truncated just at the ice line. No ice line, no solid core, no giant planet to migrate inward. 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

Effective Temperature: A=5760 K, B=5760 K

Surface gravity (log g): 4.28, 4.35

Log [Fe/H]: A= 0.06 ± 0.05, B=0.02 ± 0.04

16 Cyg B has 6 times less Lithium

These stars are identical and are „solar twins“. 16 Cyg B has a giant planet with 1.7 MJup in a 800 d period

Kozai Mechanism: One Explanation for the high eccentricty of 16 Cyg B

Two stars are in long period orbits around each other.

A planet is in a shorter period orbit around one star.

If the orbit of the planet is inclined, the outer planet can „pump up“ the eccentricity of the planet. Planets can go from circular to eccentric orbits.

This was first investigated by Kozai who showed that satellites in orbit around the Earth can have their orbital eccentricity changed by the gravitational influence of the Moon

Kozai Mechanism: changes the inclination and eccentricity

Planetary Systems: ~ 100 Multiple Systems

The first:

Some Extrasolar Planetary 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 0.6 0.1 0.27 61 2.0 0.2 0.10

47 UMa 1095 2.4 2.1 0.06 2594 0.8 3.7 0.00

HD 37124 153 0.9 0.5 0.20 550 1.0 2.5 0.4055 CnC 2.8 0.04 0.04 0.17 14.6 0.8 0.1 0.0 44.3 0.2 0.2 0.34 260 0.14 0.78 0.2 5300 4.3 6.0 0.16Ups And 4.6 0.7 0.06 0.01 241.2 2.1 0.8 0.28 1266 4.6 2.5 0.27HD 108874 395.4 1.36 1.05 0.07

1605.8 1.02 2.68 0.25HD 128311 448.6 2.18 1.1 0.25 919 3.21 1.76 0.17HD 217107 7.1 1.37 0.07 0.13 3150 2.1 4.3 0.55

Star P (d) MJsini a (AU) eHD 74156 51.6 1.5 0.3 0.65 2300 7.5 3.5 0.40

HD 169830 229 2.9 0.8 0.31 2102 4.0 3.6 0.33

HD 160691 9.5 0.04 0.09 0 637 1.7 1.5 0.31

2986 3.1 0.09 0.80

HD 12661 263 2.3 0.8 0.35

1444 1.6 2.6 0.20

HD 168443 58 7.6 0.3 0.53 1770 17.0 2.9 0.20HD 38529 14.31 0.8 0.1 0.28 2207 12.8 3.7 0.33HD 190360 17.1 0.06 0.13 0.01 2891 1.5 3.92 0.36HD 202206 255.9 17.4 0.83 0.44 1383.4 2.4 2.55 0.27HD 11964 37.8 0.11 0.23 0.15

1940 0.7 3.17 0.3

The 5-planet System around 55 CnC:

5.77 MJ

Red lines: solar system plane orbits

•0.11 MJ ••

0.17MJ

0.03MJ

0.82MJ

The Planetary System around GJ 581

7.2 ME

5.5 ME

16 ME

Inner planet 1.9 ME

Can we find 4 planets in the RV data for GL 581?

1 = 0.317 cycles/d

2 = 0.186

3 = 0.077

4 = 0.015

Note: for Fourier analysis we deal with frequencies (1/P) and not periods

The Period04 solution:

P1 = 5.37 d, K = 12.7 m/s

P2 = 12.92 d, K = 3.2 m/s

P3 = 66.7 d, K = 2.7 m/s

P4 = 3.15, K = 1.05 m/s

P1 = 5.37 d, K = 12.5 m/s

P2 = 12.93 d, K = 2.63 m/s

P3 = 66.8 d, K = 2.7 m/s

P4 = 3.15, K = 1.85 m/s

=1.53 m/s=1.2 m/s

Yes!

Published solution:

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 0.6 0.1 0.27 61 2.0 0.2 0.10

55 CnC 14.6 0.8 0.1 0.0 44.3 0.2 0.2 0.34

HD 108874 395.4 1.36 1.05 0.07 1605.8 1.02 2.68 0.25

HD 128311 448.6 2.18 1.1 0.25 919 3.21 1.76 0.17

2:1 → Inner planet makes two orbits for every one of the outer planet

→

→

2:1

2:1

→ 3:1

→ 4:1

→ 2:1

•

Eccentricities

Period (days)Red points: SystemsBlue points: single planets

EccentricitiesMass versus Orbital Distance

Red points: SystemsBlue points: single planets

Idea: If you divide the disk mass among several planets, they each have a smaller mass?

Exoplanets around low mass stars (Mstar < 0.4 Msun)Programs:

• ESO UVES program (Kürster et al.): 40 stars• HET Program (Endl & Cochran) : 100 stars• Keck Program (Marcy et al.): 200 stars• HARPS Program (Mayor et al.):~200 stars

Results:

• ~15 planets around low mass (M = 0.15-0.4 Msun)

• Giant planets (2) around GJ 876. Giant planets around low mass M dwarfs seem rare• Hot neptunes around several → low mass start tend to have low mass planets

Transiting surveys are finding more planets around M dwarfs

GL 876 System

1.9 MJ

0.6 MJ

Inner planet 0.02 MJ

Exoplanets around massive stars

Difficult with the Doppler method because more massive stars have higher effective temperatures and thus few spectral lines. Plus they have high rotation rates. A way around this is to look for planets around giant stars. This will be covered in „Planets around evolved stars“

Result: Only a few planets around early-type, more massive stars, and these are mostly around F-type stars (~ 1.4 solar masses)

Galland et al. 2005

HD 33564

M* = 1.25 solar masses

m sini = 9.1 MJupiter

P = 388 days

e = 0.34

F6 V star

HD 8673

A Planet around an F star from the Tautenburg Program

Mplanet = 14.6 MJup Period = 4.47 Years ecc = 0.72

Frequency (c/d)

Sca

rgle

Pow

erP = 328 days

Msini = 8.5 Mjupiter

e = 0.24

An F4 main sequence star from the Tautenburg programM* = 1.4 M

• Long period planet

• Very young star

• Has a dusty ring

• Nearby (3.2 pcs)

• Astrometry (1-2 mas)

• Imaging (m =20-22 mag)

• Other planets?

Eri: A „complete“ System

Clumps in Ring can be modeled with a planet here

(Liou & Zook 2000)

Radial Velocity Measurements of Eri

Large scatter is because this is an active star. It has been argued that this is not a planet at all, but rather the signal due to activity.

Hatzes et al. 2000

Scargle Periodogram of Eri Radial velocity measurements

False alarm probability ~ 10–8

Scargle Periodogram of Ca II measurements

Figure 10 from The HARPS-TERRA Project. I. Description of the Algorithms, Performance, and New Measurements on a Few Remarkable Stars Observed by HARPSGuillem Anglada-Escudé and R. Paul Butler 2012 ApJS 200 15 doi:10.1088/0067-0049/200/2/15

Period: 2501 ± days

Eccentricity: 0.61 ± 04

: 49 ± 4degrees

K: 19.0 ± 1.7 m/s

msini : 0.86 MJupiter

Period: 2651 ± 36 days

Eccentricity: 0.40 ± 0.1

: 141 ± 10 degrees

K: 11.8 ± 1.1 m/s

msini : 0.64 MJupiter

Hatzes et al. 2000 Anglada-Escude & Butler 2011

Anglada-Escude & Butler argue that the variations are due to an activity cycle.

False Planets

or

How can you be sure that you have actually discovered a planet?

HD 166435

In 1996 Michel Mayor announced at a conference in Victoria, Canada, the discovery of a new „51 Peg“ planet in a 3.97 d. One problem…

HD 166435 shows the same period in in photometry, color, and activity indicators.

This is not a planet!

What can mimic a planet in Radial Velocity Variations?

1. Spots or stellar surface structure

2. Stellar Oscillations

3. Convection pattern on the surface of the star

4. Noise

Fake Planets

Starspots can produce Radial Velocity Variations

Spectral Line distortions in an active star that is rotating rapidly

Rad

ial V

elo

city

(m

/s)

10

-10

0 0.2 0.4 0.6 0.8

Rotation Phase

Activity Effects: Convection

Hot rising cell

Cool sinking lane

•The integrated line profile is distorted.

•The ratio of dark lane to hot cell areas changes with the solar cycle

RV changes can be as large as 10 m/s with an 11 year period

This is a Jupiter!One has to worry even about the nature long period RV variations

Tools for confirming planets: Photometry

Starspots are much cooler than the photosphere

Light Variations

Color Variations

Relatively easy to measure

Ca II H & K core emission is a measure of magnetic activity:

Active star

Inactive star

Tools for confirming planets: Ca II H&K

HD 166435

Ca II emission measurements

Bisectors can measure the line shapes and tell you about the nature of the RV variations:

What can change bisectors:• Spots• Pulsations • Convection pattern on star

Span

Curvature

Tools for confirming planets: Bisectors

Correlation of bisector span with radial velocity for HD 166435

Spots produce an „anti-correlation“ of Bisector Span versus RV variations:

How do you know you have a planet?

1. Is the period of the radial velocity reasonable? Is it the expected rotation period? Can it arise from pulsations?

• E.g. 51 Peg had an expected rotation period of ~30 days. Stellar pulsations at 4 d for a solar type star was never found

2. Do you have Ca II data? Look for correlations with RV period.

3. Get photometry of your object

4. Measure line bisectors

5. And to be double sure, measure the RV in the infrared!

Figueira et al. 2010, Astronomy and Astrophysics, 511, 55

Points: IR measurements, Solid line is the orbital solution using optical radial velocity measurements, but with one-third the optical amplitude → No planet!

A constant star

The Non-Planet around TW Hya

Period = 3.24 d

K = 0.5 m/s

Msini = 1.13 MEarth

FAP = 0.02%

Is Alpha Cen Bb really there?

False alarm probability (FAP) ~ 0.02 %

Dumusque et al. 2012

Claimed detection:

False alarm probability = 0.4

False alarm probability = 0.00010

Data

Fake Planet

Maybe not!

Radial Velocity Planets30 90 1000Period in years →

Red line: Current detection limits

Green line detection limit for a precision of 1 m/s

Summary Radial Velocity Method

Pros:

• Most successful detection method• Gives you a dynamical mass• Distance independent

• Important for transit searches

Summary

Radial Velocity Method

Cons:• Only effective for cool stars.

• Most effective for short (< 10 – 20 yrs) periods

• Only high mass planets, but getting closer to Earth mass planets

• Only get projected mass (msin i)

• Other phenomena (pulsations, spots, etc.) can mask as an RV signal. Must be careful in the interpretation

The Radial Velocity Method is successful, but highly biased – we only know about planets around solar-

type stars!

Summary of Exoplanet Properties from RV Studies

• ~10 % of normal solar-type stars have giant planets

• < 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)

• Massive planets tend to be in eccentric orbits and have large orbital radii