The announcement of an Earth-like planet in the "habitable zone" of the star Gliese 581 has stirred immense interest in the media and blogs around the world. This appears to be the news many have been waiting for. The Daily Mail announced: It "forced bookies to slash odds on the existence of alien beings", and "This remarkable discovery appears to confirm the suspicions of most astronomers that the universe is swarming with Earth-like worlds."
The planet is thought to have a mass 5 times that of the Earth and a diameter 50% larger. The host star is a red dwarf with a small diameter and a surface temperature of about 3300 K. The planet is 14 times closer to the star than Earth is to the sun, and a full orbit is completed every 13 days. Taking into account the reduced radiation from the star and the closeness of the planet, the surface temperature of the planet is estimated in the range 0-40 C. This is sufficient to say that it lies in the "habitable zone" or (more evocatively) the "Goldilocks zone" where temperatures are just right for life.
However, the situation is not as comfortable as it might seem. A planet so close to the sun will become tidally locked with one side constantly in light and he other in darkness. Since it has an elliptical orbit, the case of Mercury might be a useful analogy. The discoverers of the planet say: "A detailed study will also need to consider the possible tidal locking of the planetary rotation to the orbital period". The implication is that liquid water (if water actually exists) is highly unlikely: most will be either in water vapour form or locked up as ice. The only real "Goldilocks zone" is at the terminator, which is sunset for unbridled excitement about the discovery.
Another problem concerns the light falling on the planet. Since the star is quite cool, the radiation peaks in the infrared. The radiation energy is relatively low compared with white light, insufficient to break chemical bonds necessary for plant-based photosynthesis. Whilst bacteria on earth appear to be able to exploit energy from hydrothermal vents , they inhabit a world that is far from our experience. So, even if the new planet's atmosphere was "just right" (which is also doubtful), Earth-like plant life would be impossible.
So, although some have spoken of an "oasis in space" and a "treasure", we ought to consider this new planet as very hostile to life and focus our excitement and energies on the "Goldilocks zone" represented by our own Earth.
The HARPS search for southern extra-solar planets
XI. Super-Earths (5&8M) in a 3-planet system
S. Udry, X. Bonfils, X. Delfosse, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and J.-L. Bertaux.
Astronomy & Astrophysics, accepted for publication
Abstract. This Letter reports on the detection of two super-Earth planets in the Gl 581 system, already known to harbour a hot Neptune. One of the planets has a mass of 5M_ and resides at the "warm" edge of the habitable zone of the star. It is thus the known exoplanet which most resembles our own Earth. The other planet has a 7.7M_ mass and orbits at 0.25AU from the star, close to the "cold" edge of the habitable zone. These two new light planets around an M3 dwarf further confirm the formerly tentative statistical trend for i) many more very low-mass planets being found around M dwarfs than around solar-type stars and ii) low-mass planets outnumbering Jovian planets around M dwarfs.
Schilling, G. Exoplanets: Habitable, But Not Much Like Home, Science 316, 27 April 2007: 528.
For the first time, astronomers have found an Earth-like planet that could be habitable. Like an oasis in space, the rocky world, possibly covered with oceans, orbits a puny red dwarf star just over 20 light-years away in the constellation Libra. "On the treasure map of the universe, one would be tempted to mark this planet with an X," says team member Xavier Delfosse of Grenoble University in France. [snip]
Found 20 light years away: the New Earth
Daily Mail, 25th April 2007
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