[full article at Source: http://www.sciencemag.org/cgi/content/abstract/330/6004/653; must be AAAS member for access]

by Andrew W. Howard, Geoffrey W. Marcy, John Asher Johnson, Debra A. Fischer, Jason T. Wright, Howard Isaacson, Jeff A. Valenti, Jay Anderson, Doug N. C. Lin, Shigeru Ida
Science 29 October 2010: Vol. 330. no. 6004, pp 653 - 655, DOI: 10.1126/science.1194854

Excerpt: The questions of how planets form and how common Earth-like planets are can be addressed by measuring the distribution of exoplanet masses and orbital periods.
...based on precise Doppler measurements of 166 Sun-like stars. We measured increasing planet occurrence with decreasing planet mass (M). Extrapolation of a power-law mass distribution fitted to our measurements, df/dlogM = 0.39 M–0.48, predicts that 23% of stars harbor a close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoretical models of planet formation predict a deficit of planets in the domain from 5 to 30 Earth masses and with orbital periods less than 50 days. This region of parameter space is in fact well populated, implying that such models need substantial revision....

... In the three largest mass domains, our survey is complete because these planets impart easily detectable Doppler signatures (K > 9 m s–1). In the two lowest-mass domains, there are markedly higher planet occurrence rates, despite reduced sensitivity.

...There is a substantial increase in planet occurrence with decreasing planet mass. ...The distribution of planets in the mass/orbital-period plane (Fig. 1) reveals important clues about planet formation and migration. Planets with M sini = 10 to 100 MEarth and P < 20 days are almost entirely absent. There is also an over-density of planets starting at P < 10 days and M sini = 4 to 10 MEarth and extending to higher masses and longer periods. These patterns suggest different formation and migration mechanisms for close-in low-mass planets as compared to massive gas-giant planets.

...the bulk of their predicted low-mass planets reside near the ice line, well outside of the P < 50 days domains analyzed here. In fact, these models predict a "planet desert" precisely in the domain of mass and period where we detect an over-density of planets. The desert emerges naturally in the simulations (3, 4) from fast migration and accelerating planet growth. Most planets are born near or beyond the ice line, and those that grow to a critical mass of several Earth masses either rapidly spiral inward to the host star or undergo runaway gas accretion and become massive gas giants. Our measurements show that population synthesis models of planet formation are currently inadequate to explain the distribution of low-mass planets.
The Kepler mission (30) is currently surveying 156,000 faint stars for transiting planets as small as Earth. Our power-law model predicts that Kepler will detect a bounty of close-in small planets: an occurrence rate of 22% for P < 50 days and M sini = 1 to 8 MEarth, corresponding to 1 to 2 Earth radii, assuming terrestrial, Earth-like density (5.5 kg m–3). When the mission is complete, we estimate (see SOM) that Kepler will have detected the transits of 120 to 260 of these plausibly terrestrial worlds orbiting the ~104 G and K dwarfs brighter than 13th magnitude (31, 32).

---

See also "Solar Systems Like Ours May Be Common" (ScienceMatters@Berkeley, Volume 7, Issue 57).

Return to News Archive