Observation Supports Herndon Model of Planetary Formation

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.