Rapid-born Planets Present 'Baby Picture' Of Our Early Solar System

The new findings in the Sept. 10 issue of Astrophysical JournalLetters not only reinforce the idea that giant planets like Jupiterform much faster than scientists have traditionally expected, but oneof the gas-enshrouded stars, called GM Aurigae, is analogous to our ownsolar system. At a mere 1 million years of age, the star gives a uniquewindow into how our own world may have come into being.

"GM Aurigae is essentially a much younger version of our Sun, and thegap in its disk is about the same size as the space occupied by our owngiant planets," says Dan Watson, professor of physics and astronomy atthe University of Rochester and leader of the Spitzer IRS Disksresearch team. "Looking at it is like looking at baby pictures of ourSun and outer solar system," he says.

"The results pose a challenge to existing theories ofgiant-planet formation, especially those in which planets build upgradually over millions of years," says Nuria Calvet, professor ofastronomy at the University of Michigan and lead author of the paper."Studies like this one will ultimately help us better understand howour outer planets, as well as others in the universe, form."

The new "baby planets" live within the clearings they have scoured outin the disks around the stars DM Tauri and GM Aurigae, 420 light yearsaway in the Taurus constellation. These disks have been suspected forseveral years to have central holes that might be due to planetformation. The new spectra, however, leave no doubt: The gaps are soempty and sharp-edged that planetary formation is by far the mostreasonable explanation for their appearance.

The new planets cannot yet be seen directly, but Spitzer'sInfrared Spectrograph (IRS) instrument clearly showed that an area ofdust surrounding certain stars was missing, strongly suggesting thepresence of a planet around each. The dust in a protoplanetary disk ishotter in the center near the star, and so radiates most of its lightat shorter wavelengths than the cooler outer reaches of the disk. TheIRS Disks team found that there was an abrupt deficit of lightradiating at all short infrared wavelengths, strongly suggesting thatthe central part of the disk was absent. These stars are very young bystellar standards, about a million years old, still surrounded by theirembryonic gas disks. The only viable explanation for the absence of gasthat could occur during the short lifetime of the star is that aplanet--most likely a gas giant like our Jupiter--is orbiting the starand gravitationally "sweeping out" the gas within that distance of thestar.

As with last year's young-planet findings, these observationsrepresent a challenge to all existing theories of giant-planetformation, especially those of the "core-accretion" models in whichsuch planets are built up by accretion of smaller bodies, which requiremuch more time to build a giant planet than the age of these systems.

The IRS Disks team discovered something else curious about GMAurigae. Instead of a simple central clearing of the dust disk, as inthe other cases studied, GM Aurigae has a clear gap in its disk thatseparates a dense, dusty outer disk from a tenuous inner one. Thiscould be either an intermediate stage as the new planet clears out thedust surrounding it and leading to a complete central clearing like theother "baby planet" disks, or it could be the result of multipleplanets forming within a short time and sweeping out the dust in a morecomplex fashion.

GM Aurigae has 1.05 times the mass of our Sun-a near twin--soit will develop into a star very similar to the Sun. If it wereoverlaid onto our own Solar System, the discovered gap would extendroughly from the orbit of Jupiter (460 million miles) to the orbit ofUranus (1.7 billion miles). This is the same range in which thegas-giant planets in our own system appear. Small non-gas-giantplanets, rocky worlds like Earth, would not sweep up as much material,and so would not be detectable from an absence of dust.

The Spitzer Space Telescope was launched into orbit on Aug. 25,2003. The IRS Disks research team is led by members that builtSpitzer's Infrared Spectrograph, and includes astronomers at theUniversity of Rochester, Cornell University, the University ofMichigan, the Autonomous National University of Mexico, the Universityof Virginia, Ithaca College, the University of Arizona, and UCLA.NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages theSpitzer Space Telescope mission for NASA's Science Mission Directorate,in Washington. Science operations are conducted at the Spitzer ScienceCenter at the California Institute of Technology, also in Pasadena.