Max Planck Institute for Astronomy - Annual Report 2007

20
II. Highlights
II.1 Youngest Extrasolar Planet Discovered in a Circumstellar Disk
The question of how planets are created has always fascinated
astronomers. Even the very discovery of an extrasolar
planet orbiting around a sun-like star provided this
astronomy research branch an enormous observational
and theoretical impetus. At the MPIA the search for extrasolar
planets and the numerical simulation of planetary
formation is now an important research priority.
Within the context of a long-term research program,
a team at the institute has now found an eight to ten million
year old star, the youngest exoplanet so far, circling
around TW Hydrae. This discovery delivers for the first
time an upper limit to the time it takes for the formation
of Jupiter-sized planets. Because the central star still
possesses a circumstellar disk of gaseous dust, this is
also the first direct evidence that planets indeed form in
disks of this type. A further companion to a roughly 100
million year old star was found at the same time.
Until now, the most successful method of searching for
and discovering exoplanets has been the Doppler or radial
velocity method. It relies on the Doppler effect which
alternately shifts light to larger and smaller wavelengths
when the light sources and receivers move away or toward
each other. Should a planet orbit around a star, then
both bodies rotate around a common gravitational center
which generally is located within the star. The planetary
orbit thus causes a periodic stellar movement which
expresses itself in its spectrum as a periodically changing
Doppler-shift in the spectral lines, thus betraying the
existence of an otherwise not directly visible planet. The
measurement data directly deliver the planet’s orbital
period and a lower limit for its mass. The true mass may
be calculated when the orbital inclination against the
celestial plane is known.
By the end of 2007 approximately 250 extrasolar planets
had been discovered. They all orbit stars that are
at least 100 million years old (Fig. II.1.1). For various
reasons, the Doppler method could not be used where
the stars are younger. For example, most young stars
rotate very rapidly. This spreads the spectral lines and
reduces the precision with which the Doppler shift can
be measured. Add to this younger stars’ often strong and
periodically varying activity, such as pulsations, orbital
oscillations, and the occurrence of star spots. All these
phenomena make it more difficult to search for periodic
variability among the spectra.
Nevertheless, an MPIA research team began searching
for planets among young stars: in 2003, the program
was started at the Fibre-fed Extended Range Optical
Number of stars hosting planets
70
60
50
40
30
20
10
0
7
TW Hydrae
Fig. II.1.1: Age distribution of known extrasolar planets as of
the end of 2007.
Spectrograph (Fe r O s) on the MPG/esO 2.2-m telescope
at La Silla in Chile. The program covered roughly 200
stars with ages between 8 and 300 million years and
a distance of roughly 500 light years. Roughly 30 %
of these stars showed surprisingly diminished stellar
activity and relatively diminished orbital speeds. Thus
they were especially well suited for the radial velocity
method. The radial velocities could be measured with an
accuracy of 10 m/s so that it was easy to prove the existence
of particularly massive planets – so-called “white
Jupiters” – in very close proximity to their central star.
The TW Hydrae Planet
8 9
log [Age/yr]
10 11
Usually, to determine the Doppler shift of a star, very
many absorption lines within its spectrum are used simultaneously.
For the younger stars, on the other hand,
the MPIA team excluded those spectral lines which were
strongly affected by stellar activity. Among these were
the CA II H & K, Ha and Hb, as well as He I and Na
I lines. Periodic Doppler shifting was then searched for
by comparing a theoretical spectrum against the roughly
1300 remaining lines.
The team finally made a discovery at TW Hydrae,
a star that is 180 light years away. It is among the best
studied young stars in the sun’s environment. It has a
mass of roughly 0.7 sun masses, a luminosity of 0.2 solar
luminosities, and it is between 8 and 10 million years
old. Images taken by the Hu b b l e space telescope showed