1B11 Foundations of Astronomy Extrasolar Planets

1. 1B11 Foundations of AstronomyExtrasolar Planets Liz Puchnarewicz emp@mssl.ucl.ac.uk www.ucl.ac.uk/webct www.mssl.ucl.ac.uk/

2. 1B11 Extrasolar Planets • ie planets around other stars, are discovered using three main detection methods: • Astrometry • Radial velocities • Transits • 1 and 2 rely on detecting the orbital motion of a star about the centre of mass of its planetary system.

3. 1B11 Darwin Darwin, an ESA mission due for launch in 2014, is a flotilla of six telescopes each one 1.5m across. They will observe in the IR to pick out planets from their central stars, using interferometry to make very high resolution images. NASA are planning a similar mission called the Terrestrial Planet Finder. They may well merge the missions…

4. 1B11 Astrometric method centre of gravity planet MP MSTAR r1 r2 For the Sun and Jupiter: R2 ~ 5.2 AU => r1 = 5.2 x 10-3 AU = 1.2 RSUN

5. 1B11 Could we see Jupiter using astrometry? So our Sun orbits around the Sun-Jupiter centre of gravity with an orbital radius of only 1.2 solar radii. 1.2 solar radii subtends an angle of 5.2 x 10-3 arcsec at 1pc – or 5.2 x 10-4 arcsec at 10 pc. This is not currently measurable!

6. 1B11 Radial velocity Instead of directly imaging the wobble of a star using astrometry, it is also possible to detect its motion from spectra, by looking for systematic wavelength shifts in emission/absorption line positions. With current telescopes, we can only measure velocities of at least 3 m/s. The Earth’s effect on the Sun is 0.1 m/s. By measuring T and finding MSTAR, we can calculate the semi-major axis of the orbit

7. 1B11 Planet masses from radial velocities If the mass of the star can be inferred (eg from an H-R diagram) and the inclination of the orbit to the plane of the sky, i, is known, then we can calculate a mass for the planet, mP: If i isn’t known, we only have mPsini.

8. 1B11 Radial velocities For Jupiter: v = 13 m/s and period, T = 12 years. For the Earth : v = 0.09 m/s and T = 1 year With a detection limit of 3 m/s, this makes Earth-like planets very hard to find. The first discovery of any extrasolar planet was in 1995 for the star 51 Peg. Now more than 120 Jupiter-size planets have been found around other stars using this method. They have orbits with short periods and high eccentricities and masses reaching as high as 10 Jupiters.

9. 1B11 Transits If a distant star was transited by a Jupiter-like planet, a 1% drop in flux from the star would be observed.

10. 1B11 Transits A planet had already been discovered around the star HD209458 by the radial velocity method. In 1999, a transit was observed at exactly the time predicted. Radial velocities of HD209458 Transit of HD209458 Distance = 150 light-years Period = 3.5 days => orbital distance of 0.05 AU Like the planet around 51Peg, the planet was found to be large and orbiting tightly around the star – these are also known as “hot Jupiters”. Mass = 0.62MJ Radius = 1.42RJr = 0.27 g/cm3

11. 1B11 Summary • The radial velocity method can only detect massive planets (at least one-fifth of the mass of Jupiter) with relatively short periods. • Most planets are detected very close to their stars (less than ~0.1AU) • 3-4% of solar-type stars have these planets • The small number of more distant planets found usually have eccentric orbits (e >~0.2) • No true Solar System analogies have been found so far.