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, stars 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 M P sin i, where i is inclination of orbit to line of sight (just measure the radial velocity, = V 1 sin i, where V 1 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 stars 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 Barnards 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 Kevins 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 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
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 Universitys 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.
Three more transits have now been detected, in the microlensing surveys, and one genuine microlensing event: 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 artists impression of the proposed 6-spacecraft Darwin mission, due to fly in about 2010. Each craft contains a large IR telescope, and they are spread over distances of 40 to 500 metres.