1B11 Foundations of Astronomy Extrasolar Planets

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1B11 Foundations of AstronomyExtrasolar Planets

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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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.

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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
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1B11 Astrometric method
centre of gravity
planet
MP
MSTAR
r1
r2
For the Sun and Jupiter
R2 5.2 AU gt r1 5.2 x 10-3 AU 1.2 RSUN
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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!
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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 Earths 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
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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 isnt known, we only have mPsini.
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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.
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1B11 Transits
If a distant star was transited by a Jupiter-like
planet, a 1 drop in flux from the star would be
observed.
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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 gt 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 1.42RJ r 0.27 g/cm3
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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 gt0.2)
• No true Solar System analogies have been found
  so far.