Transcript Exoplaneter & Venuspassagen

HOW TO DETECT
EXOPLANETS
EAAE General Assembly
ESO HQ, Garching bei München, Germany
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
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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.
Observations indicate at least about 7% of all stars
have at least one planet.
Roughly 0.05 - 13 Jupiter masses (15 - 4000 Earth
masses). Lower limit due to sensitivity of instruments,
upper limit set by the definition of a planet. (Brown
dwarfs: about 13 - 75 Jupiter masses, above this are
the stars)
Properties of known Exoplanets
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Average distance from star: 0,02 - 5 AU
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)
Detection of exoplanets
Indirect methods
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Measurements of variations in stellar radial velocities
Detection of transits (Planet dims the light from a star
when it passes in front of it)
Direct methods
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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 109 times brighter than Jupiter and 1010 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
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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.
Transits
HD209458
(HD = Henry Draper (a star catalogue)
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The planet HD209458b was detected through
variations in the radial velocities of the star.
The star: F8 (Sun: G2)
Distance: about 154 light-years (in Pegasus)
7.65 mag
HD209458b
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Period 3.5 days
Average distance from star 0.045 AU
About 0.7 Jupiter masses
About 1.4 Jupiter radii
evaporating extended hydrogen atmosphere
around the planet
Tabel showing coming transits:
www.astro.caltech.edu/~dc/upcoming_transits.asc
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
 CCD
 V-filter
 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)
 Tycho Brahe observatory in Oxie outside Malmö
 36 cm teleskope
 CCD
OGLE-TR-56B
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Was detected by the transit method
Existence confirmed by radial velocity
measurements of the star
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
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1% (0.01 mag) for a planet of the size of Jupiter
0.01% (0.0001 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.
 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
Search for transits
 Transitsearch
www.transitsearch.org
Amateur network for observing stars with known
planets in order to find transits
skolor.nacka.se/samskolan/anders/