A young extrasolar planet in its cosmic nursery (Forwarded)

Max-Planck-Institut fur Astronomie
Heidelberg, Germany

Contact:
Dr. Johny Setiawan
Max-Planck-Institut fur Astronomie, Heidelberg
Tel. + 49 6221 528-326
Fax: + 49 6221 528-246

Dr. Ralf Launhardt
Max-Planck-Institut fur Astronomie, Heidelberg
Tel.: + 49 6221 528-207
Fax: + 49 6221 528-246

Dr. Jakob Staude (Pressestelle)
Max-Planck-Institut fur Astronomie, Heidelberg
Tel.: +49 6221 528-229
Fax: + 49 6221 528-379

2008-01-02

A young extrasolar planet in its cosmic nursery

Astronomers from Heidelberg discover planet in a dusty disk around a newborn
star

Scientists at the Max Planck Institute for Astronomy in Heidelberg have
discovered the youngest known extrasolar planet. Its host star is still
surrounded by the disk of gas and dust from which it was only recently born.
This discovery allows scientists to draw important conclusions about the
timing of planet formation.

How do planetary systems form? How common are they? What is their
architecture? How many habitable earth-like planets exist in the Milky Way?
In the past decade, astronomers have clearly come closer to finding answers
to these exciting questions. With the discovery of the first planet orbiting
another Sun-like star in 1995, the field of extrasolar planet research was
born.

Today, almost 12 years later, more than 250 exoplanets have been discovered.
A group of scientists at the Max Planck Institute for Astronomy in
Heidelberg is also looking for these objects. A planet next to a bright star
appears like a glow-worm next to a lighthouse. It is (not yet) possible to
directly make images of most extrasolar planets. Therefore, astronomers
often use an indirect detection method.

As a planet orbits its host star, it pulls the star in periodically
alternating directions with its gravity. The star then sometimes moves a
little towards us, and at other times away from us. When it moves towards
us, the light waves are "compressed" which is equivalent to the light
becoming bluer. When the star moves away, the waves are "stretched" and the
light is "red-shifted". The periodic change of color, or shift of spectral
lines, known as the "Doppler effect", can thus reveal an unseen planet and
allows astronomers to derive a lower limit to its mass. So far this so
called "radial velocity" method remains the most successful technique in
detecting exoplanets. However, no planet has ever been found around a
new-born Sun-like star. The detection of young planets would provide the
most important key to understanding questions like: how and where do planets
form, and what timescales are involved in this process?

With this in mind, a team of astronomers from the Max Planck Institute for
Astronomy (MPIA) in Heidelberg (Germany) has monitored radial velocity
variations of about 200 young stars to search for extrasolar planets. One of
these was the nearby star TW Hydrae, which is only 8-10 million years old
(about 1/500th the age of our Sun). Like other stars at this young age, it
is still surrounded by a circumstellar disk of gas and dust, believed to be
the birthplace of planets.

The team has now discovered a planetary companion that orbits the young star
TW Hydrae within an inner hole in its disk (Figure 1). "When we monitored
the radial velocity of TW Hydrae, we detected a periodic variation that
could not arise from stellar activity and pointed towards the presence of a
planet," (Figure 2) said Johny Setiawan (MPIA), the leader of the
observational program. The detection was made with the FEROS spectrograph at
the 2.2m telescope belonging to the Max Planck Society and the European
Southern Observatory (ESO) at La Silla in Chile.

The newly-discovered planet, called TW Hydrae b, is a heavyweight; it is
about ten times as massive as Jupiter, the biggest planet in our Solar
System. The planet orbits its host star in only 3.56 days at a distance of
about 6 million kilometres (Figure 3). This corresponds to 4 percent of the
distance from the Sun to the Earth.

Stellar activity represents a critical problem for the detection of
extrasolar planets -- in particular when the star is young and its surface
is still very unstable. For example, when starspots (like those on our Sun)
are large, they can mimic radial velocity variations caused by an orbiting
planet. "To exclude any misinterpretation of our data, we have investigated
all activity indicators of TW Hydrae in detail. But their characteristics
are significantly different from those of the main radial velocity
variation. They are less regular and have shorter periods," said Ralf
Launhardt (MPIA), who coordinates several search programs for extrasolar
planets around young stars.

Planets form from dust and gas in a circumstellar disk shortly after the
birth of a star. Not all aspects of this process are yet understood.
However, the discovery of TW Hydrae b provides new constraints on planet
formation theories. Based on statistical studies, astronomers have estimated
the average lifetime of a circumstellar disk to be 10 - 30 million years.
This would then be the maximum time available to form planets in a disk. The
detection of TW Hydrae b now provides the first direct measurement of a true
upper limit of the formation time of a giant planet: it cannot be older than
its host star, i.e., 8 - 10 million years. "This is one of the most exciting
discoveries in the study of extrasolar planets," said Thomas Henning, the
director of the Planet and Star Formation Department at MPIA. "For the first
time, we have directly proven that planets indeed form in circumstellar
disks. The discovery of TW Hydrae b opens the way to linking the evolution
of circumstellar disks with the processes of planet formation and
migration." It is the ideal system to test numerical models of planet
formation.

Scientists at the Max Planck Institute for Astronomy are currently
developing next-generation instruments to detect extrasolar planets with
other techniques, such as direct imaging, measuring the tiny reflex motion
of a star in the plane of the sky (astrometry), or the dimming effect when a
planet moves in front of the star (transit photometry). In the near future,
these instruments will open the door to finding other extrasolar planets
that cannot be detected by the radial velocity method. We will get a better
understanding of planet formation when we understand the diversity of the
planetary systems. We will then be able to place our own Solar System in a
universal context. Finally, perhaps in the future we will be able to answer
the question: "Are we alone in the Universe?"

Original publication:
J. Setiawan, Th. Henning, R. Launhardt, A. Muller, P. Weise, M. Kurster
A young massive Planet in a star-disk system
Nature, 2. January 2008

IMAGE CAPTIONS:
[Figure 1:
http://www.mpia.de/Public/Aktuelles/...080103_2gr.jpg
(77KB)]
The newly discovered giant planet orbits around its young and active host
star inside the inner hole of a dusty circumstellar disk (artist view).
Pictu Max Planck Institute for Astronomy

[Figure 2:
http://www.mpia.de/Public/Aktuelles/...103_1en_gr.jpg
(101KB)]
The diagram shows the variatios of the radial velocity of TW Hydrae as
observed in early 2007. The data can best be described as an oscillation
with a period of 3.6 days, caused by a giant planet orbiting around the
star.
Pictu Max Planck Institute for Astronomy