http://news.nationalgeographic.com/news/2008/02/080219-planets-life_2.html
Aalok Mehta in Boston, Massachusetts
National Geographic News
February 18, 2008
More than half of the sunlike stars in the galaxy could have terrestrial planets with the potential to harbor life, a new study suggests.
The research, announced yesterday at a meeting of the American Association for the Advancement of Science in Boston, Massachusetts, is just one of a set of recent findings that suggest the roster of potential life-harboring worlds is huge—even in our own solar system.
"Our observations suggest that between 20 percent and 60 percent of sunlike stars form rocky planets like our solar system's," said Michael Meyer, an astronomer at the University of Arizona, at a press briefing Sunday.
Meyer and his team used NASA's Spitzer Space Telescope to study heat from the dust around sunlike stars of various ages, much like looking at "the smoke you see rising from chimneys in Boston on a cold day."
Such hot dust implies that larger rocky bodies are forming and colliding in the "messy" business of planet formation, Meyer explained.
Planet-forming dust was found at one to five times the distance from the sun to Earth, Meyer said (see an interactive map of the solar system.)
The dust was also seen in young stars but was absent from most stars older than 300 million years—a perfect fit with current models of planetary formation, he added.
(Read related story: "Newborn Planet Found Orbiting Young Star" [January 3, 2008].)
The study will appear in an upcoming edition of the Astrophysical Journal Letters.
Earth-Size Refugees?
At the briefing, scientists also advanced the possibility that our solar system contains hundreds or even thousands more dwarf planets like Pluto, hidden from view in the distant region known as the Kuiper belt.
There is a growing body of evidence that the poorly understood region contains several Earth- or Mars-size planets and many tinier bodies, said NASA planetary scientist Alan Stern, adding that this could very well be a "new Copernican revolution" in our understanding of planets.
"What we thought is, our outer solar system is actually our middle solar system," Stern said.
It would be a vindication for Pluto, which was recently "demoted" from full planet status by astronomers after a lively and controversial debate.
Pluto might be the best known representative of a third major class of planets, the dwarfs, "which could be far more common than either the terrestrial or gas giant planets," Stern said.
The initial solar system was quite cluttered with small bodies, he explained, but these were swept out as the four gas giant planets—Saturn, Jupiter, Uranus, and Neptune—finished forming.
Evidence for that can be seen in Uranus, which is lying almost on its side compared to the other planets, Stern said.
It must have been struck by a massive object several times the mass of Earth—an extraordinary coincidence if there were only a few such bodies around.
Exploring the Kuiper belt will be a slow process, Stern pointed out, as the objects in it are extremely difficult to find because of their distance from Earth.
These worlds would mostly be rocky bodies with icy surfaces, though larger ones might be able to harbor gassy envelopes.
But there is also the possibility that some could have "warm, wet interiors," Stern said.
Some scientists think it is "likely Pluto has an ocean in its interior, as does [Jupiter's moon] Europa and many of the other satellites of other planets," Stern said.
In the future, we might focus the search for life on such worlds, which could be far more common than planets like Earth with liquid water on their surface, he added.
Slow Search for Life
But directly detecting the kinds of planets that could harbor life remains a huge challenge, said Debra Fischer of San Francisco State University.
Such planets fall into an "anti-sweet spot," she said—far too small to detect using any of the common planet detection methods, which have so far found about 250 or more extrasolar planets.
But scientists have reasons to remain optimistic, she said.
If a planet with the right mass is found at the right distance from a star—in the so-called Goldilocks zone, where it is neither too hot nor too cold—most of the work is done.
"The raw materials for life are common," she said. "Water is probably the most common molecule in the universe."
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