New estimates suggest that roughly 50 percent of sun-like stars could have planets the size of Earth orbiting in a place where liquid water might exist on their surface.
The results also indicate that almost all sun-like stars have a planetary system of some sort.
“If you could randomly travel to a star, it will have planets,” said astronomer Francois Fressin from the Harvard-Smithsonian Center for Astrophysics, during a press conference today here at the American Astronomical Society 2013 meeting.
The finding uses data from NASA’s Kepler space telescope, which is currently scanning 150,000 stars in the constellation Cygnus for evidence of planets. Astronomers analyzed this data using a program called Transiting ExoEarth Robust Reduction Algorithm (TERRA) to try and estimate the percentage of planets that Kepler is missing.
Kepler looks for the very tiny dimming of a star’s light that can be detected when an exoplanet passes in front of it, causing a mini eclipse. The telescope is good at detecting the dimming from a larger Jupiter-sized planet, but a tiny Earth-like planet will cause such a slight change that Kepler might miss it. TERRA found that Kepler had missed about 37 Earth-like planets in its data analysis, which means it’s missing about 25 percent of these worlds.
The findings suggested that smaller extrasolar planets form more frequently than larger ones, a result consistent with previous research. But TERRA found that the trend doesn’t hold true past a certain point. Planets twice the diameter of Earth are about as common as those that are Earth-sized or smaller, a result that astronomers hadn’t previously seen.
The results are heartening for anyone who likes imagining a universe full of life. Previous analysis suggested billions of Earth-like planets could exist in our galaxy, and recently another team announced the discovery of 15 potentially habitable exoplanets.
The astronomers do note that many of the worlds found aren't necessarily habitable, that exoplanets two or three times the size of Earth are likely to be ice giants like Neptune, and that one-sixth of stars have Earth-sized planets orbiting much too closely to their host star. Still.
Universe Today's Nancy Atkinson, meanwhile, writes in "Exciting Potential for Habitable ExoMoons" about the potential of exomoons, the moons of extrasolar planets. Most of the exoplanets first detected by astronomers were massive planets, gas giants like Jupiter or Saturn. Even though many of these planets did orbit in the habitable zones of their stars, they themselves could not support Earth-type life. But what of planet-sized moons of gas giants? In the Upsilon Andromedae system that I blogged about a couple of weeks ago, for instance, Upsilon Andromedae b does orbit within Upsilon Andromedae A's habitable zone and might be of a size to support relatively massive moons.
The hunt progresses.
Imagine moons like Europa or Enceladus that are orbiting distant gas giant exoplanets located in the habitable zone of their star. What would be the potential for life on those moons? Hopefully one day we’ll find out. That could be the scenario at an exoplanet that has been found by the Planet Hunter citizen science project. This is the second confirmed planet found by Planet Hunters, and PH2 b is a Jupiter-size planet in the habitable zone of a Sun-like star.
“There’s an obsession with finding Earth-like planets but what we are discovering, with planets such as PH2 b, is far stranger,” said Chris Lintott of Oxford University and Zooniverse. “Jupiter has several large water-rich moons – imagine dragging that system into the comfortably warm region where the Earth is. If such a planet had Earth size moons, we”d see not Europa and Callisto but worlds with rivers,lakes and all sorts of habitats – a surprising scenario that might just be common.”
Astronomers with Planet Hunters estimate the surface temperature PH2 b is 46 degrees Celsius. That’s a “just right” temperature for there to be liquid water, but it is extremely unlikely that life exists on PH2 b because it is a gas planet, and might be similar to Jupiter, so there is no solid surface or liquid environment for life to thrive. But if this planet is anything like the gas giant planets in our solar sytem, there could be a plethora of moons orbiting them.
“We can speculate that PH2 b might have a rocky moon that would be suitable for life, said lead author of the paper that has been published in arXiv, Dr Ji Wang, from Yale University. I can’t wait for the day when astronomers report detecting signs of life on other worlds instead of just locating potentially habitable environments. That could happen any day now.”
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The team said that with 19 similar planets already discovered in habitable zones, where the temperature is neither too hot nor too cold for liquid water, the new finds suggest that there may be a “traffic jam” of all kinds of strange worlds in regions that could potentially support life.
Although most of these planets are large, like Neptune or Jupiter, these discoveries increase the sample size of long-period planet candidates by more than 30% and almost double the number of known gas giant planet candidates in the habitable zone, Wang said. “In the future, we may find moons around these planet candidates (just like Pandora around Polyphemus in the movie Avatar) that allows life to survive and evolve under a habitable temperature.”