Planet Probabilities

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Science  22 Jun 2001:
Vol. 292, Issue 5525, pp. 2219
DOI: 10.1126/science.292.5525.2219a

Models of planet formation favor accretion into spheres in protoplanetary disks around young stars. The smaller, rocky planets (Earth-like) would form closer to their central star where the more refractory metals (elements heavier than helium) can accrete, and the larger, icy planets (Jupiter-like) would form farther from their central star where volatile-rich ices can accumulate. The Jupiter-like planets could then migrate inward, causing any Earth-like planets to be pushed into the star, increasing the star's concentration of metals (metallicity). About 40 “hot Jupiters” (Jupiter-like planets that have migrated to smaller orbits) have been detected, and at least 32 of these planets orbit stars that are richer in metals than typical solar-mass stars.

Lineweaver analyzes the correlation between stellar metallicity and hot Jupiters in order to estimate the distribution of Earth-like planets in the universe. High metallicity translates into a high probability of the presence of a hot Jupiter, whereas low metallicity would yield a low probability of finding any kind of planet. A moderate metallicity would result in the highest probability of an Earth-like planet, and Lineweaver estimates that 74% of the possible Earth-like planets in the universe are older than Earth by at least 2 billion years. Thus, in addition to the challenge of locating these Earth-like planets, we will need to consider how other life may have evolved, given a head start. — LR

Icarus151, 307 (2001).

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