Download Extrasolar planets

1B11
Foundations of Astronomy
Extrasolar Planets
Liz Puchnarewicz
[email protected]
www.ucl.ac.uk/webct
www.mssl.ucl.ac.uk/
1B11 Extrasolar Planets
ie planets around other stars, are discovered using three
main detection methods:
1. Astrometry
2. Radial velocities
3. Transits
1 and 2 rely on detecting the orbital motion of a star about the
centre of mass of its planetary system.
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…
1B11 Astrometric method
centre of
gravity
MSTAR
planet
MP
r1
r2
r1
MP

r2 MSTAR
For the Sun and
Jupiter:
r1
3
 10
r2
R2 ~ 5.2 AU
=> r1 = 5.2 x 10-3 AU = 1.2 RSUN
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!
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
4
3
T 
a
GMSTAR
2
2
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,
m P:
 2G 
v

 T 
1
3
mP sin i
2
3
M STAR
If i isn’t known, we only have mPsini.
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.
1B11 Transits
If a distant star was transited by a Jupiter-like planet, a 1%
drop in flux from the star would be observed.
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 = 1.42RJ
r = 0.27 g/cm3
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.