Reasons to Believe

Habitable Planets Rarer than Originally Thought

Tradesmen get a lot of business from people who start projects that appear simple. ("How hard can it be to install a new shower?") What initially looks like a straightforward task ends up requiring much more skill, time, and financial resources than expected. After many hours of frustration, and often many dollars spent, the exasperated homeowner calls the expert. In those circumstances, the skill and resourcefulness of the craftsman is readily appreciated.

Scientists encounter a similar situation as they study the process of building a life-supporting planet. Some people, usually the nonexperts, believe the conditions for producing life-supporting planets are neither rare nor terribly complex. However, as planetary scientists-the experts-make planetary system formation models more realistic, the evidence for fine-tuning in our solar system continues to grow.

Twenty years ago, the nine solar system planets (the International Astronomical Union recently disqualified Pluto, reducing the total to eight1) were the only known planets in the universe. Lacking the ability to make direct observations, most astronomers assumed that any extrasolar planetary systems would bear a strong resemblance to ours and would be rather abundant. But in recent years, direct observations began pouring in. As of August 2006, astronomers had discovered just over 200 extrasolar planets (including 148 stars with single planets and 20 stars with multiple planets). Virtually all are gas giants similar to Jupiter. Detection of these Jupiter-class planets has enabled scientists to assess the possibility that an Earth-like planet, capable of supporting long-standing liquid water, could exist around these stars.

Contrary to expectations, none of these planetary systems resemble the one where we live. All of the extrasolar "Jupiters" orbit their stars more closely or have a more eccentric orbit than Jupiter. Consequently, the gravitational instabilities resulting from the gas-giant orbits in most of the planetary systems would eject any Earth-like planet into interstellar space. Still, astronomers found that in a few planetary systems, Earth-like planets would be stable for a long time.

Since a hypothetical habitable planet might be stable in a small fraction of known systems, astronomers added more realism by investigating whether an Earth-like planet could even form in such systems. Using Monte Carlo simulations,2 a University of Colorado astrophysicist studied how the existence of gas giants with specific orbits influences the subsequent formation of any habitable terrestrial planets.3 The simulations revealed that any gas giant orbiting slightly closer to its star than Jupiter's location prevented the formation of watery, Earth-like planets. Additionally, if the gas-giant orbit was not circular, its orbit had to be even farther from the star to permit formation of a watery, terrestrial planet. However, this position makes the terrestrial planet less habitable, as increasing the gas-giant distance from the star diminishes its capacity to shield the terrestrial planet from comet and asteroid impacts.

Even this picture is not complete, though. Astronomers know from the calculated orbits of the gas-giant planets that they must have migrated from the location where they formed to their current orbit. A group of astrophysicists simulated how this migration would affect the formation of habitable terrestrial planets in four systems where a terrestrial planet appears stable.4 The simulations showed that the migration completely disrupted habitable-planet formation in three systems. In the fourth system, planets up to six-tenths the mass of Earth could form if 1) the gas-giant migration occurred very early, 2) the known gas-giant orbits are well determined, and 3) no other gas giants exist in the system. (A terrestrial planet just over half an Earth mass likely cannot support the long-standing plate tectonics that are critical for advanced life, but no planets larger than 0.63 Earth masses formed in the simulations.) However, the chance of forming any potentially habitable terrestrial planet diminishes dramatically if any of the three conditions are not met, particularly if the migration does not occur very rapidly.

Such exquisite fine-tuning provides a sobering reminder of the difficulty of the task. The naturalist is clearly seeing that "it's not as easy as it looks."

The bottom line is that as scientists' understanding of planetary formation increases and their simulations become more realistic, the naturalistic expectation of finding habitable terrestrial planets around other stars continues to fade. In contrast, the latest scientific findings continue to reveal the fingerprints of a supernatural Creator who intervened in cosmic history to ensure a life-supporting environment such as Earth.



  2. For more information on Monte Carlo simulations, see Dave Rogstad's article, "Beating the Odds in Monte Carlo," on page 8 of this issue.

  3. Sean N. Raymond, "The Search for Other Earths: Limits on the Giant Planet Orbits that Allow Habitable Terrestrial Planets to Form," Astrophysical Journal 643 (2006): L131-34.

  4. Sean N. Raymond, Rory Barnes, and Nathan A. Kaib, "Predicting Planets in Known Extrasolar Planetary Systems. III. Forming Terrestrial Planets," Astrophysical Journal 644 (2006): 1223-31.

Subjects: Extrasolar Planets, Solar System Design

Dr. Jeff Zweerink
