Extra-Solar Planets

Planetary Physics

Summer Term 2004

Lecture 10

Or: How to get on the Big Breakfast Show...

Extra-Solar Planets

Since October 1995, more than 120 planets have been detected around more than 100 stars like the Sun - why the sudden success?

None has been seen directly: all found by ‘wobble’ of parent star caused by orbiting planet - how?

None is like the Earth: all have masses more like Jupiter - why?

Extra-Solar Planets

Definitions: Star: > 0.08 solar masses (80 jupiter masses),

powered by H fusion; formed by gravitational collapse, occur singly or in multiples

Brown Dwarf: 13 jup < mass < 80 jup, some initial D fusion, then fade; formation and occurrence as normal stars

Planets: < 13 jup, no nuclear power source; formed by accretion from protoplanetary disc left behind from formation of parent star

In these HST pictures we see disks around stars, but also jets of material being blown out again, especially in the lowest picture (the ‘Rotten Egg’ nebula). Planets form from the disks.

Extra-Solar Planets

Methods of detection: Astrometry - long history, no detections Radial velocity surveys - very successful Brightness variations

– planet passes in front of star: drop in brightness

– gravitational lensing of star by planet: increase in brightness - only way to detect earth-mass planets

Interferometry - ground and space missions

Extra-Solar Planets

Astrometry: Very precise measurements of position of parent star,

relative to inertial frame, repeated over many years Centre of mass of system moves in straight line If planet exists, star’s motion will have slight

‘wobble’ about CoM motion - can find size and period of orbit, and mass of planet

Massive planets in wide, long-period orbits give largest wobble; nearby stars are best, but effects still too small to measure with present equipment

Extra-Solar Planets

Radial velocity surveys: Very precise measurements of radial

velocity of parent star, using doppler effect Single star will have constant velocity, but

if companions are present their orbital motions will be reflected in motion of star about CoM - look for periodic oscillations in radial velocity

Extra-Solar Planets

Limits of radial velocity measurements: Can find period, eccentricity and MP sin i, where i

is inclination of orbit to line of sight (just measure the radial velocity, = V1 sin i, where V1 is the orbital motion of the star – see Lecture 5)

Can not measure i : need transits for this Motion largest for large masses, short periods Limit set by oscillations in star’s size (2-3 m/s) Earth mass in 1-yr orbit gives only 0.1 m/s

Extra-Solar Planets

Gravitational lensing: Parent star (‘lens’) focuses light of

background star as it passes in front of it:

Observer Star Lens (and planet) time Planet causes blip in slow light variation

Extra-Solar Planets

History I: 1950 - 1970: various false astrometric

claims of Jupiter size companions, e.g. to Barnard’s star (van de Kamp)

1980s: pioneering radial velocity survey by Campbell & Walker - precision of 13 m/s, but no detections (small sample - only 21 stars)

Extra-Solar Planets

History II: Late 1980s, early 1990s: several large, high

precision RV surveys began - Marcy & Butler (Lick), Mayor & Queloz (OHP), Cochran & Hatzes (McDonald)

1995 October 6: first extrasolar planet round a ‘normal’ star announced by Swiss team (2 found earlier around a pulsar)

Marcy and Butler found this ‘wobble’ in the motion of the star 51 Pegasi, and were able to find a period of just over 4 days.

This shows the same observations, but now with all the different orbits superimposed to show the variation better.

Extra-Solar Planets

What did we expect? small mass planets (Earth-like) close to star:

radiation should evaporate most abundant element, hydrogen, leaving small rocky planets

massive gaseous planets (Jupiter-like: mainly hydrogen) far enough out to avoid evaporation - so expect long periods

circular orbits, as in solar system stars like the Sun

Extra-Solar Planets

What did we find? massive planets close to star! many eccentric orbits! no earth-like planets stars generally richer in heavy elements

(such as iron) than the Sun Of the more than 100 found so far, at least 10 of

the early one were around targets suggested by Kevin Apps (Sussex u.g.)

Extra-Solar Planets

Kevin’s involvement: November/December 1997 - e-mailed Marcy

requesting list of 300 targets for Keck 10-m, found 30 unsuitable and suggested 30 replacements

Became responsible for generating new targets (in 1999, nearly 400 of the 900 targets were his) and for searching the literature for their properties

April 1999: went to San Francisco to observe with a Lick Observatory telescope

August 1999: observed on Hawaii (Keck) May do PhD with Marcy from 2004

Extra-Solar Planets

Current results: 122 planets with masses 0.12 to 16.9 jup 41 ‘hot Jupiters’

(Jupiter mass, close to star: a < 0.4 AU) 57 ‘Jupiter analogues’

(Jupiter mass, far from star: P > 1 yr) 70 ‘eccentric’ planets

(high eccentricity orbits: e > 0.2) 13 systems with multiple planets (2 have 3)

The first 22 planets were very close to their parent stars

Three planets around Upsilon Andromedae: first ‘solar system’

The majority of the planets found so far have orbital radii less than 1 AU: 35% of them are less than 0.4 AU, although there is a growing number with larger radius, including one ‘beyond Jupiter’.

Semi-major axes of planetary orbits

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Semi-major axis (in AU)

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The majority of planets (~60%) have masses between 1 and 8 Jupiter masses, but there are now many known (~30%) with masses less than that of Jupiter (down to 0.12 jup: ~0.4 sat). But only lower limits are known without knowing i: needs transits.

Mass distribution for extra-solar planets

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M sin i (in Jupiter masses)

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7 November 1999: first-ever planetary transit observed for sun-like star HD 209458 by Henry, Marcy, Butler and Vogt

The radial velocity group of Marcy, Butler and Vogt found this evidence of a wobble in the motion of the star and alerted Greg Henry to observe its brightness.

This is the 0.8m Automatic Photoelectric Telescope of the Tennessee State University’s Fairborn Observatory in southern Arizona, which was used by Greg Henry to observe the planetary transit.

An attempt to verify the result on November 14 was clouded out, but other groups later verified it.

Average of four HST transit observations

Three more transits have now been detected, in the microlensing surveys, and one genuine microlensingevent:

One of the three transit events…. ….and the microlensing event

Extra-Solar Planets

The Future: Continuing radial velocity searches (lower mass

planets? more ‘solar systems’?) Transits – now four cases Gravitational lensing Ground-based astrometry Direct imaging? Difficult from the ground Space-based imaging (e.g. DARWIN) Space-based astrometry (e.g. GAIA)

An artist’s impression of the proposed 6-spacecraft Darwinmission, due to fly in about 2010. Each craft contains a large IR telescope, and they arespread over distances of 40 to 500 metres.

Extra-Solar Planets

Where next? Who knows!