A Flurry of Exoplanet Discoveries

Brown dwarf disk
This artist's concept depicts a disk of gas and dust around a young brown dwarf. Low-mass disks like this should be capable of forming planetary systems with perhaps one gas giant and several Earth-size bodies.
Courtesy NASA / JPL / Caltech.
Discoveries of extrasolar planets are coming so fast and furious that even the planet hunters themselves can barely keep up. The current tally stands at around 150 known planets, though the exact number depends on what objects one chooses to call "planets." This week, astronomers announced several important new findings at a conference in Aspen, Colorado.

The discoveries include an object with such a low mass — just one-fifth the mass of Pluto (roughly 0.0004 Earth mass) — that it can hardly be considered a "planet." The object orbits the pulsar B1257+12, which was already known to harbor three terrestrial-mass planets. Alex Wolszczan (Penn State University) and Maciej Konacki (Caltech) discovered this asteroid-mass body, which is by far the lowest-mass object detected outside the solar system, by tracking minuscule changes in the arrival times of the pulsar's radio blips over 15 years of observation. The object orbits the pulsar at approximately 2.7 astronomical units, which in our solar system would put it in the asteroid belt. The precise timing of the pulsar's blips allows Wolszczan and Konacki to rule out any further planetary-mass objects in this system.

The European planet-hunting team led by Michel Mayor (Geneva Observatory, Switzerland) announced eight new planets at the conference. Mayor also reported that his team's radial-velocity observations are achieving 1 meter-per-second precision, which should enable it to detect planets with less than 10 Earth masses within a year. The team led by Geoffrey W. Marcy (University of California, Berkeley) and R. Paul Butler (Carnegie Institution of Washington) has also achieved this precision.

Two independent teams, led by Kevin L. Luhman (Harvard-Smithsonian Center for Astrophysics) and Subhanjoy Mohanty (also at the Center for Astrophysics), have reported numerous circumstellar disks of gas and dust around young brown dwarfs. Using the Spitzer Space Telescope, Luhman's group found one of these disks around a brown dwarf that has only about 15 Jupiter masses — making it the lowest-mass brown dwarf known to be surrounded by a disk. These findings indicate that planets can form around very low-mass objects.

Carbon planet vs. silicate planet
S&T: Gregg Dinderman. Source: Marc J. Kuchner / Sara Seager.

Marc J. Kuchner (Princeton University) and Sara Seager (Carnegie Institution of Washington) presented theoretical models showing that some exoplanets may consist primarily of carbon compounds. This is in marked contrast to Venus, Earth, and Mars, which are mostly made of silicates (silicon-oxygen compounds). The overall composition of carbon-based planets would resemble carbonaceous chondrite meteorites. High pressures deep underground could produce a jeweler's paradise: a layer of diamond many kilometers thick. Future space instruments such as a NASA Terrestrial Planet Finder mission might identify such planets by picking up an excess of carbon-rich molecules in their atmospheres.

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