Extrasolar Planets: Likely Locations For Life?

As of September 2002, about 100 planets had been reported orbiting stars other than our own Sun. These planets are called “extrasolar planets” (meaning planets outside our solar system), or simply “exoplanets.” The popular media have used these discoveries to fuel speculation that life may exist, or may even be abundant, outside the Earth. But a consideration of the characteristics of the exoplanets so far reported makes this seem improbable. Exoplanets can be grouped into the following categories:

1. “Hot Jupiters” or “Roasters”

These are planets that move around their stars in small circular orbits. They are called “Jupiters” because their mass is estimated to be similar to that of the planet Jupiter in our solar system. Unlike our Jupiter, however, they remain very close to their stars — often closer even than Mercury is to our Sun. “While current technology can detect only giant planets with the approximate mass of Jupiter, about half these planets have been found to have orbits that, within our own Solar System, would lie between the Sun and its closest planet, blistering-hot Mercury” (Butler, p. 67). Many of these “hot Jupiters” complete a full orbit within only a few days. The surface temperatures of such planets are much too high to permit life. For example, the first exoplanet reported orbiting a sun-like star (the star 51 Pegasi) has an estimated surface temperature of 1800° C (Spradley, p. 35). Other “hot Jupiters” include planets orbiting the stars Tau Bootis, Rho Coronae Borealis, and Upsilon Andromedae. “The big shock is that these planetary systems are nothing like the one we inhabit” (Hughes, p. 651).

2. “Eccentric Jupiters”

Planets in this group have orbits that are farther out from their stars. These orbits are highly eccentric, however — they are not circular, but are shaped like elongated ellipses. A large planet with such a “wild” orbit would tend to gravitationally disturb or even collide with any smaller inner planets, precluding the survival of Earth-like planets around the same star. “Unfortunately, Jupiter-mass planets toss Earth-mass planets around like Tinkertoys in a train wreck” (Marcy & Butler, p. 45). “The objects orbiting HD 114762, 70 Virginis, and 16 Cygni B are so massive and have such eccentric orbits that other planets in the stars’ ‘life zones’ would either be kicked out of their systems entirely, or would crash into the big planets. If one is prospecting for life, one would be advised to look elsewhere” (Naeye, p. 46). The planet recently reported to be orbiting the star Epsilon Eridani, only 10.5 light years from Earth, has an orbit of this shape (Cowen 2000, p. 85).

3. Other Exoplanets

The other exoplanets so far reported typically have a Jupiter-like mass. It is probable that they are cold, lifeless gas planets similar to Jupiter, with stormy, violent winds and intense gravity. “Although many fall within the tantalizing liquid-water habitable zone—the region around a star where liquid water could theoretically exist—none of these are thought to be able to support life” (Svitil, p. 50). Some planets in this group, such as those orbiting 55 Cancri and Lalande 21185, share their star with “hot Jupiters” — implying that these systems as a whole would not permit life to survive (Spradley, p. 37).

“All of the extrasolar planets thus far discovered orbiting main-sequence stars are more massive than Saturn, and most either orbit very close to their stars or travel on much more eccentric paths than do any of the major planets in our Solar System” (Lissauer, p. 660).

“. . . the theoreticians, who have laboured long and hard to explain how our Solar System formed . . . are greatly concerned that the new systems have very little in common with the one we live in and clearly had a radically different process of generation” (Hughes, p. 652).

“This new planet [orbiting Iota Horologii] adds to the suspicion that our solar system with its neat, circular, coplanar orbits, may be the exception rather than the rule” (noted exoplanet discoverer Geoffrey W. Marcy, quoted in Cowen 1999, p. 106).

“The extrasolar planets could be a warning that we have been too smug,” says [University of Hawaii astronomer Tobias] Owen. “It could be that we really don’t understand something fundamental about planet formation.” (Sincell, p. 51)

“The extrasolar planets could be a warning that we have been too smug,” says [University of Hawaii astronomer Tobias] Owen. “It could be that we really don’t understand something fundamental about planet formation.” (Sincell, p. 51)

“The nearly circular orbits of the planets in our solar system led astronomers to expect that planets around other stars would reside in circular orbits too. After all, planets probably form in circular protoplanetary disks, such as the disks seen in the Orion Nebula. The gas and dust in these disks follow circular orbits, and friction within these disks would circularize the orbits of newly forming planets in much the same way that friction circularizes the flow of water going down a bathtub drain.
“But most of the extrasolar planets found so far reside in highly eccentric orbits, not circular. Indeed the 18 extrasolar planets with the largest orbits all reside in eccentric orbits. Most of those orbits are more than twice as elongated as the orbits of Earth, Jupiter, or Saturn. . . .
“The predominance of elliptical orbits implies that planetary systems with circular orbits may be the exception rather than the norm. Apparently, our nine planets were just far enough apart and low enough in mass to avoid this chaos. The nine planets do perturb one another, but not enough to cause close passages. The planetary house of cards that we call our solar system may be one of the rare systems that remains just barely stable” (Marcy & Butler, pp. 44f.).

A listing of extrasolar planets is available at http://www.obspm.fr/encycl/catalog.html

References

Butler, R. Paul. 2000 (Feb). Prospecting for Planets. Natural History, Vol. 109 No. 1, pp. 67-69.

Cowen, R. 1999 (Aug 14). Extrasolar planet with an Earthlike orbit. Science News, Vol. 156 No. 7, p. 106.

Cowen, R. 2000 (Aug 5). Evidence grows for nearby planetary system. Science News, Vol. 158 No. 6, pp. 84f.

Hughes, David W. 1998 (Feb 12). Wobbly pursuit of extrasolar planets. Nature, Vol. 391, pp. 651f.

Lissauer, Jack J. 1999 (Apr 22). Three planets for Upsilon Andromedae. Nature, Vol. 398, pp. 659f.

Marcy, Geoff, and Paul Butler. 2000 (Mar). Hunting Planets Beyond. Astronomy, Vol. 28 No. 3, pp. 42-47.

Naeye, Robert. 1997 (Apr). The Strange New Planetary Zoo. Astronomy, Vol. 25 No. 4, pp. 42-49.

Sincell, Mark. 2000 (Mar). Switched at Birth. Astronomy, Vol. 28 No. 3, pp. 48-51.

Spradley, Joseph L. 1999 (Mar). Extrasolar Planets and Religious Responses. Perspectives on Science and Christian Faith, Vol. 51 No. 1, pp. 34-38.

Svitil, Kathy A. 2000 (Mar). Field Guide to New Planets. Discover, Vol. 21 No. 3, pp. 48-55.

Ward, Peter D., and Donald Brownlee. 2000. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus.