New discoveries raise hopes for exoplanet searches

Posted: January 8, 2002

The discoveries of a brown dwarf and a circumstellar disk make it increasing likely that telescopes will soon be able to directly observe Jupiter-sized planets orbiting other stars, astronomers said Monday.

In a pair of unrelated discoveries, astronomers said they has directly observed a circumstellar disk of dust surrounding a newborn star as well as a brown dwarf orbiting another star. The two discoveries were announced Monday at a meeting of the American Astronomical Society in Washington, DC.

One team of astronomers reported that they had discovered a protoplanetary disk, oriented edge-on as seen from Earth, around a young star that is part of quadruple star system in the small cluster MBM 12, about 900 light-years from the Earth. The disk is only the tenth edge-on protoplanetary disk yet discovered and the first in a quadruple star system.

Protoplanetary disk
Top left: A normal ground-based infrared image, from the 2MASS survey, shows a wide binary system. Top right: Gemini adaptive optics partially corrects for blurring by the Earth's atmosphere, resolving one star in the wide binary into a close pair of stars. Bottom left: A longer exposure with Gemini adaptive optics reveals a fainter extended object near the upper close pair. Bottom right: A close-up of this faint object shows it to be an edge-on protoplanetary disk. Credit: UC Berkeley/CfA/Gemini Observatory/NOAO/NSF
The disk completely surrounds the star, believed to be only two million years old. "What we're looking at is an example of a dusty disk that will probably evolve into a young planetary system over the next several million years," said Ray Jayawardhana, a University of California Berkeley astronomer who presented the findings at the meeting. He added that they had already seen evidence that dust grains in the disk were starting to clump together, an early step towards the formation of planets.

Another group of astronomers announced Monday that they had directly observed a brown dwarf around a nearby Sun-like star. The brown dwarf, an object more massive that a planet but not large enough to sustain fusion like a star, is between 55 and 78 times the mass of Jupiter, and may orbit as close as 14 astronomical units (2.1 billion kilometers) to its star, 15 Sge. The object, first detected in June 2001 as a dim object near the star, was observed for several months to make sure it was gravitationally bound to the star and not a background star. Spectra of the object confirmed that its temperature, 1500-1800 kelvins, or less than a third the temperature of the Sun's photosphere, was consistent with it being a brown dwarf.

The brown dwarf is the closest directly detected to a star, and could influence models of planet formation. "This companion is probably too massive to have formed the way we believe planets do," said Michael Liu of the University of Hawaii, who led the research effort. "This finding suggests that a diversity of processes act to populate the outer regions of other solar systems."

Brown dwarf
Orbiting far from the star is a companion about 65 times the mass of Jupiter as shown in this illustration. It is a brown dwarf, more massive than a planet, but too small to produce energy by nuclear fusion as a star does. It is glowing with its own heat. Credit: Gemini Observatory
What ties the two seemingly disparate research projects together is both the technique used to make the discovery and its implications for the search for extrasolar planets, or exoplanets. Both groups relied on images obtained with adaptive optics systems on some of the largest groundbased telescopes in the world: the 8-meter Gemini North telescope and the 10-meter Keck 2 telescope, both located atop Mauna Kea, Hawaii. Adaptive optics systems use mirrors that change their shape many times a second to compensate for the blurring effects of the Earth's atmosphere. Such systems can create images as sharp as the best images from the Hubble Space Telescope.

An additional advantage of using groundbased telescopes with adaptive optics systems is that these telescopes are much larger than the Hubble Space Telescope, and can thus capture more light, making it possible to see dimmer objects. "This is exactly the combination needed to look for new planets," said Alan Boss of the Carnegie Institution of Washington, an astronomer not involved with either group of astronomers.

These discoveries also prove that current telescopes should be able to directly observe some kinds of exoplanets, including those as small as Jupiter. "It sounds crazy, but it's not," said Jayawardhana. "It's totally possible."

While Jupiter-sized planets orbiting Sun-like stars are still too dim to be directly observed, Jayawardhana said that it should be easier to detect these planets shortly after they form around new stars. At that time they are about 10,000 times brighter at infrared wavelengths than when they are mature and cooler. While these planets are still 100,000 times dimmer than their stars, they should be visible to large telescopes with adaptive optics systems. "It's technically possible now to directly image a young Jupiter around a young star," he said.

Jayawardhana showed one image of what could be such a planet orbiting a star, but noted it was too soon to conclude that the object was a planet and not a brown dwarf or background star. He said that anywhere from six to 18 months of followup observations were required to confirm that the object was orbiting the star, and that its spectra matched what would be expected from a planet, with traces of water and methane.

"These discoveries are marvelous," Boss said of the protoplanetary disk and brown dwarf finds, "but they are just a tantalizing appetizer of things to come."