Exoplanet formation surprise: Partially Filled Disk Gap

A team of researchers has discovered evidence that an extrasolar planet may be forming quite far from its star-- about twice the distance Pluto is from our Sun.

The planet lies inside a dusty, gaseous disk around a small red dwarf TW Hydrae, which is only about 55 percent of the mass of the Sun.

The discovery adds to the ever-increasing variety of planetary systems in the Milky Way.

The research is published in the Astrophysical Journal.

This dusty protoplanetary disk is the closest one to us, some 176 light-years away in the constellation Hydra.

The astronomers made Hubble Space Telescope observations over a wide range of wavelengths from visible to near infrared and modeled the color and structure of the disk in a way that has not been done before.

They found a deficit of disk material, or partial gap, at about 80 astronomical units (AU) (1 AU is the Earth/Sun distance).

Their models indicate that the depression is about 20 AUs wide, just slightly wider than necessary for a planet-opening gap and consistent with a planet of between 6 and 28 Earth masses.

The feature is seen at all wavelengths indicating it is structural and not a local compositional difference.

The team believes the evidence is strong for planet formation causing the gap.

This graphic shows a gap in a protoplanetary disk of dust and gas whirling around the nearby red dwarf star TW Hydrae, which resides 176 light-years away in the constellation Hydra, sometimes called the Sea Serpent. 

The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow. 

In the left image, astronomers used a masking device on the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer to block out the star's bright light so that the disk's structure could be seen. 

The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The illustration at right shows the gap relative to the star. 

The Hubble observations were taken on June 17, 2005. 

Credit: NASA, ESA,

"TW Hydrae is between 5 and 10 million years old, and should be in the final throes of planet formation before its disk dissipates," remarked coauthor Alycia Weinberger of the Carnegie Institution and principal investigator of the observations.

"It is surprising to find a planet only 5 to 10% of Jupiter's mass forming so far out since planets should form faster closer in. In all planet formation scenarios, it's difficult to make a low-mass planet far away from a low mass star."

The goal of these observations was to understand not only whether planets have formed, but also what conditions can result in planet formation and what chemical constituents are available for new planets.

Carnegie astrophysicist Alan Boss, who works on disk instability models, said "If the mass of this suspected planet is as low as it seems to be, this presents a real puzzle. Theory would say that it cannot exist!"