Extrasolar Planet News: Superplanet or brown dwarf?

Ron Cowen

New observations of an oddball planetary system 150 light-years from Earth may force astronomers to rethink the textbook definition of a planet and the accepted idea about how such a body forms.

The observations suggest that either some planets are superheavy or that planets can form from disks of gas and dust that encircle not just a single star but two starlike objects.

Two years ago, when astronomers at the Geneva Observatory in Sauverny, Switzerland, reported their findings on the sunlike star HD 202206, nothing seemed out of the ordinary. The team announced that a body at least 17.4 times as heavy as Jupiter orbits the star. The unseen body resides at an average distance from the star of 0.82 times the Earth-sun distance.

The same team, led by Alexandre Correia of the University of Aveiro in Portugal, has now found evidence for a second unseen body orbiting HD 202206. This object is at least 2.4 times as heavy as Jupiter and resides at an average distance from the star of 2.55 times the Earth-sun distance, the researchers report (http://lanl.arxiv.org/abs/astro-ph/0411512).

According to International Astronomical Union standards, the heavier body is a failed star known as a brown dwarf. By the union's definition, brown dwarfs range from 13 to about 75 times the mass of Jupiter. That's heavy enough to burn deuterium at their cores but too light to burn any other nuclear fuel, as bona fide stars do.

In contrast, the lighter object would be classified as a planet, as long as it weighs less than the 13-Jupiter-mass cutoff, as the scientists strongly suspect. In that case, it would have formed from gas and dust coalescing within a disk of material that surrounded the star in its youth. This would make it the first planet known to orbit a pair of objects—a brown dwarf and a star.

That would be that—except for a curious feature. Every time the heavier, inner body goes around the star five times, the outer body goes around exactly once. This particular synchrony has never before been observed in a planetary system. Synchrony keeps objects in an especially powerful gravitational embrace.

Synchrony can't happen by chance, Correia says. The team suggests that the two bodies are birds of a feather, born in the same way and at the same time.

In that case, the heavier object orbiting HD 202206 wouldn't be a brown dwarf after all, but the heaviest planet known. If so, the disk from which the two planets arose would have to have been two to four times as heavy as expected, Correia told Science News.

Moreover, if the heavier body is a superplanet, "we have to rethink our definition of what is a brown dwarf and what is a planet," says Correia. Some objects massive enough to burn deuterium may be brown dwarfs, while others may be planets, he notes.

"Somewhat odd systems like this … challenge our overall thinking about the formation of giant planets and brown dwarfs," says Alan P. Boss of the Carnegie Institution of Washington (D.C.).

It's possible that the heavier object is a brown dwarf but that theorists haven't yet been clever enough to figure out why it's in sync with the outer planet, cautions Adam S. Burrows of the University of Arizona in Tucson.

********

Letters:

The resonance discovered in the star HD 202206 system is not unique, and it isn't true that "[t]his particular synchrony has never been seen before in a planetary system." Resonances abound in the solar system. The orbital periods of the Galilean satellites of Jupiter, many asteroids' orbits, Jupiter's orbit, and Pluto's orbit with Neptune are some examples. Resonances also exist in numerous extrasolar systems discovered over the past 10 years. Also, the extrasolar system around HD 37124 that consists of two planets with periods of 155 and 31 days shows a 5:1 mean motion resonance, so even the particular commensurability seen for HD 202206 is not new.

Derek Wallentinsen
San Pedro, CA

There are clearly other synchronies in our solar system. However, researchers say that the 5:1 synchrony is new and doesn't describe the planets of HD 37124, which have approximate periods of 150 days and 1,500 days. The word particular was intended to describe only the 5:1 synchrony, which has also been suggested as one of several ratios for two planets in another system, HD 160691.—R. Cowen

References:

Correia, A.C.M.., et al. In press. A pair of planets around HD 202206 or a circumbinary planet? Astronomy & Astrophysics. Preprint available at http://lanl.arxiv.org/abs/astro-ph/0411512.

Sources:

Alan P. Boss
Carnegie Institution of Washington
Department of Terrestrial Magnetism
5241 Broad Branch Road
Washington, DC 20015-1305

Adam S. Burrows
Steward Observatory
Department of Astronomy
University of Arizona
Tucson, AZ 85721

Alexandre M. Correia
Department of Physics
University of Aeiro
Santiago Campus
3810-192 Aveiro
Portugal

From Science News, Volume 166, No. 22, November 27, 2004, p. 341.