HOW TO DETECT EXOPLANETS EAAE General Assembly ESO HQ, Garching bei München, Germany March 4, 2005 March 4, 2005 Anders Västerberg European Association for Astronomy Education (EAAE) Saltsjöbadens Samskola, Sweden

Exoplanets (Extrasolar planets) Planets in orbits around other stars than our sun 144 known (Michel Mayor et al., The Geneva Extrasolar Planet Search Programmes) (obswww.unige.ch/~udry/planet/planet.html February 8, 2005) 144 known (Michel Mayor et al., The Geneva Extrasolar Planet Search Programmes) (obswww.unige.ch/~udry/planet/planet.html February 8, 2005) First found in 1995 by Michel Mayor & Didier Queloz of the Geneva Observatory. Orbiting 51 Pegasi. First found in 1995 by Michel Mayor & Didier Queloz of the Geneva Observatory. Orbiting 51 Pegasi. Observations indicate at least about 7% of all stars have at least one planet. Observations indicate at least about 7% of all stars have at least one planet. Roughly Jupiter masses ( Earth masses). Lower limit due to sensitivity of instruments, upper limit set by the definition of a planet. (Brown dwarfs: about Jupiter masses, above this are the stars) Roughly Jupiter masses ( Earth masses). Lower limit due to sensitivity of instruments, upper limit set by the definition of a planet. (Brown dwarfs: about Jupiter masses, above this are the stars)

Properties of known Exoplanets Average distance from star: 0, AU Average distance from star: 0, AU Most have fairly excentric orbits. (In the Solar system all planets except Mercury and Pluto have quasi-circular orbits) Most have fairly excentric orbits. (In the Solar system all planets except Mercury and Pluto have quasi-circular orbits) Period: 30 hours – 13 years (Jupiter: 5.20 AE, 318 earth masses, 12 years) Period: 30 hours – 13 years (Jupiter: 5.20 AE, 318 earth masses, 12 years)

Detection of exoplanets Indirect methods Measurements of variations in stellar radial velocities Measurements of variations in stellar radial velocities Detection of transits (Planet dims the light from a star when it passes in front of it) Detection of transits (Planet dims the light from a star when it passes in front of it) Direct methods Detecting planets on images (Possibly sucessful in one case, but needs confirmation) Hard because the stars are often more than one billion times as luminous as their planets and very close to them in the sky. (The Sun is 10 9 times brighter than Jupiter and times brighter than Earth at visible wavelengths). Instruments with the required sensitivity are under construction and planned to be operative around Detecting planets on images (Possibly sucessful in one case, but needs confirmation) Hard because the stars are often more than one billion times as luminous as their planets and very close to them in the sky. (The Sun is 10 9 times brighter than Jupiter and times brighter than Earth at visible wavelengths). Instruments with the required sensitivity are under construction and planned to be operative around 2007.

Measurements of stellar radial velocities Planets and stars move around their common center of gravity. Planets and stars move around their common center of gravity. The amplitude of the radial velocity for the Sun due to Jupiter is 13 m/s, for the Earth it is 1 dm/s. The amplitude of the radial velocity for the Sun due to Jupiter is 13 m/s, for the Earth it is 1 dm/s.

Transits HD (HD = Henry Draper (a star catalogue) The planet HD209458b was detected through variations in the radial velocities of the star. The planet HD209458b was detected through variations in the radial velocities of the star. The star: F8 (Sun: G2) The star: F8 (Sun: G2) Distance: about 154 light-years (in Pegasus) Distance: about 154 light-years (in Pegasus) 7.65 mag 7.65 mag

HD209458b Period 3.5 days Period 3.5 days Average distance from star AU Average distance from star AU About 0.7 Jupiter masses About 0.7 Jupiter masses About 1.4 Jupiter radii About 1.4 Jupiter radii evaporating extended hydrogen atmosphere around the planet evaporating extended hydrogen atmosphere around the planet Tabel showing coming transits:

HD209458b transit Copyright: ESA

Transits from extrasolar planets can be observed by amateurs… eg. Group in Jyväskylän Sirius Marko Moilanen et al. Nyrölä observatory 41 cm telescope 41 cm telescope CCD CCD V-filter V-filter Computer program for data reduction Computer program for data reduction

… and by students! Anders Nyholm (Bronze medal winner in the IAO 2002), Rymdgymnasiet in Kiruna, (an upper secondary school) observed transits of HD209458b Project work (compulsory in Swedish Schools – 1+ month of full time studies) Project work (compulsory in Swedish Schools – 1+ month of full time studies) Tycho Brahe observatory in Oxie outside Malmö Tycho Brahe observatory in Oxie outside Malmö 36 cm teleskope 36 cm teleskope CCD CCD

OGLE-TR-56B Was detected by the transit method Was detected by the transit method Existence confirmed by radial velocity measurements of the star Existence confirmed by radial velocity measurements of the star Average distance from star: 5 solar radii, period of revolution 30 hours. Average distance from star: 5 solar radii, period of revolution 30 hours.

The dimming of the light of a sun-like star at a transit 1% (0.01 mag) for a planet of the size of Jupiter 1% (0.01 mag) for a planet of the size of Jupiter 0.01% ( mag) for an earth-sized planet 0.01% ( mag) for an earth-sized planet The probability that transits will occur if a star has planets: 0.5% if the star has a planet at a distance of 1 AE. 0.5% if the star has a planet at a distance of 1 AE. If 10% of the stars have a planet at a distance of 1 AE, the light from about 2000 stars need to be monitored continuoiusly during one year if the stars are of the same size as the Sun – to find one single exoplanet using this method! If 10% of the stars have a planet at a distance of 1 AE, the light from about 2000 stars need to be monitored continuoiusly during one year if the stars are of the same size as the Sun – to find one single exoplanet using this method!

Links Exoplanets California & Carnegie Planet Search exoplanets.org California & Carnegie Planet Search exoplanets.org Search for transits Transitsearch Amateur network for observing stars with known planets in order to find transits Transitsearch Amateur network for observing stars with known planets in order to find transits

skolor.nacka.se/samskolan/anders/