Researchers first detected a star-orbiting planet outside the solar system in 1995.1 It was found orbiting the star 51 Pegasi. Today, astronomers know of 347 extrasolar planets. While this is not a large number, it's enough to give astronomers confidence that they possess a complete sample of gas giants in a particular category. A team of American and Japanese astrophysical theoreticians is studying the sample planets to constrain physical models for planet formation. And these constraints, in turn, are revealing new evidences for the design of the solar system for the benefit of advanced life on Earth.
Almost all of the discovered extrasolar planets are more massive than Saturn (Saturn's mass is 95 times the mass of Earth). Surveys of extrasolar planets are now complete for planet masses exceeding 100 times the mass of Earth and for distances from their stars ranging between 9 and 186 million miles (15 and 300 million kilometers).
The American-Japanese team demonstrated that only a certain narrow set of planetary formation models could explain the statistics of the complete sample of extrasolar planets.2 All of these models predict that extrasolar planetary systems will manifest a "desert." That is, they will lack planets that are in the mass range of 30–50 times the mass of Earth and that are closer to their stars than Earth is to the Sun (less than 93 million miles). In addition, the successful models expect that these systems will possess an abundance of rocky planets approximating the mass of Earth that orbit their stars at distances less than our planet orbits the Sun.
Astronomers already possess the telescopes and instruments to test the team's first predictions. So far, limited observations reveal a complete lack of planets between 30 and 50 Earth masses with orbital distances of less than 93 million miles. Observing programs currently underway will soon offer complete verification or falsification of this calculation. Complete verification would dramatically increase astronomers' confidence in the team's second prediction.
Advanced life on Earth requires that the solar system manifest an absence of rocky planets as large as, or larger than, our planet anywhere near the vicinity of Earth's orbit. The presence of such planets would disturb the long-term stability of Earth's orbit, one of the fundamental requirements for the longevity and diversity of life on Earth and the eventual existence of advanced life. Thus, the researchers' expectation of finding an abundance of such planets appears to add to the already existing evidence that our solar system is uniquely fine-tuned for life.
This study demonstrates a pathway for efficient research on the design of the solar system and Earth. More detailed statistics of the easy-to-detect extrasolar planets combined with more comprehensive simulations of planet formation could provide even more definitive constraints on the kinds of Earth-sized planets natural processes could produce.
- M. Mayor and D. Queloz, "A Jupiter-Mass Companion to a Solar-Type Star," Nature 378 (November 23, 1995): 355.
- Kevin C. Schlaufman, D. N. C. Lin, and S. Ida, "The Signature of the Ice Line and Modest Type I Migration in the Observed Exoplanet Mass-Semimajor Axis Distribution," Astrophysical Journal 691 (February 1, 2009): 1322–27.