The search for life outside our solar system just took an interesting turn.
Nearly every researcher who has ever looked for extrasolar planets capable of harboring life has looked for ones resembling Earth just enough to be habitable. They've looked for planets in the "Goldilocks zone" – not too hot and not too cold to harbor life as we know it. It's never really occurred to researchers that there might, in fact, be planets outside our solar system that are actually more hospitable to life than Earth.
Research discussed at the American Geophysical Union's annual convention recently concludes those assumptions may be wrong. It turns out there may be at least a few solar systems out there with orbiting planets that have even more of the right stuff than we do to hold life.
NASA's Kepler mission managed by the Ames Research Center has found literally dozens of potentially habitable planets outside our solar system – planets that sort of, kind of resemble Earth. But, in most cases, the assumption has been that most of the planets identified by Kepler aren't really all that similar to Earth.
New research at Ohio State discussed at AGU, however, indicates at least some of the planets orbiting those stars may very well be hotter – at least, in their interiors and cores, where it matters most -- and more dynamic than Earth.
In fact, the Ohio State research suggests, the interiors of any terrestrial planets in these systems are likely to be warmer than Earth – in some cases up to 25 percent warmer, which could mean they're more geologically active and therefore likely to hold enough liquid water to support some form of life (like microbes).
"If it turns out that these planets are warmer than we previously thought, then we can effectively increase the size of the habitable zone around these stars by pushing the habitable zone farther from the host star, and consider more of those planets hospitable to microbial life," said Cayman Unterborn, an Ohio State doctoral student who presented the results at AGU last week.
Unterborn and the geologists and astronomers at Ohio State decided to take a different approach in their search for alien life. They studied data on eight "solar twins" of Earth's sun (stars that are roughly the same size, age and composition as ours) in order to measure the amounts of radioactive elements in each. Using the datasets generated at a "planet searching" observatory in Chile, they searched the solar twins for radioactive elements like thorium and uranium.
On Earth, these elements are crucial to plate tectonics because they warm the interior of the Earth. In turn, plate tectonics help maintain water on the surface of our planet – so recognizing the factors and elements that reveal the possibility of plate tectonics on an alien planet can logically be used as an indicator that such a planet might hold water and be hospitable to life.
Surprisingly, the research team found more thorium than our own sun on seven of the eight "solar twins." That means that planets orbiting these stars probably contain more thorium than ours – leading to the possibility of plate tectonics, warmer interiors, surface water, etc. In short, planets circling these stars are, perhaps, more likely to be hospitable to the formation of life than our own Earth.
In fact, one of the stars they studied had 2.5 times more thorium than our sun, which means that terrestrial planets that formed around this particular star likely generate 25 percent more internal heat than the Earth does, extending the timeframe for plate tectonics necessary to allow life to form.
Unterborn, though, is cautious about what it all might mean.
"At this point, all we can say for sure is that there is some natural variation in the amount of radioactive elements inside stars like ours," he said. "With only nine samples…we can't say much about the full extent of that variation throughout the galaxy. But from what we know about planet formation, we do know that the planets around those stars probably exhibit the same variation, which has implications for the possibility of life."
His advisor, Wendy Panero, says the interplay between radioactive elements, plate tectonics and surface water is complex and not totally understood. It's "one of the great mysteries in the geosciences," Panero said. But research is beginning to show the same forces of heat in the Earth's interior that move its crust also regulate the amount of water in oceans as well.
For this reason, the presence of radioactive elements such as thorium in a planet's interior seem crucial to the way in which water is held on the surface of that planet – and, therefore, give astronomers an awfully interesting way in which to look for telltale signs of an extrasolar planet's ability to hold life.
What's more, such interior heat may also support life as well. For instance, right here on Earth, scientists have already shown that certain forms of microbes don't rely on our sun for energy; they live directly off the heat emanating from deep inside the Earth.
So, for these reasons – the fact that a planet's interior heat may be key to water on the surface and supporting some forms of microbial life – looking for radioactive elements such as thorium or uranium in extrasolar suns with planets around them offers researchers a new, exciting way to hunt for the possibility of life outside our solar system.