Planets on which life might exist could be closer than we think, according to a new analysis of results from the Kepler Space Telescope.
"To put things in perspective," says Courtney Dressing "If we scale the Milky Way Galaxy down to the size of the United States we find that the nearest Earthlike Planet isn't across the country, it's not even across the state. In fact if we all were to take a field trip to New York City and we sit on one side of Central Park, we'd find that the nearest Earth like planet would be a stroll across the park away."
Earth-sized planets in the habitable zone are so common around M type (red dwarf) stars, and red dwarfs are so numerous around us, that the nearest planet with liquid water is probably right nearby, astronomically speaking. A best guess is just 13 light-years away, based on an analysis of the results of the Kepler findings.
Kepler mainly focused on sun-like stars, but these have proved more prone to obscuring variations than expected so attention has turned to fainter objects. Its methodology means Kepler will miss many planets around nearby stars if their orbits do not cause them to cross the face of the star from Earth. However, it does provide us with a statistical sample from which we can draw conclusions about the frequency of planets of particular types.
"We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," says Ms Dressing, a graduate student at the Harvard-Smithsonian Center for Astrophysics and lead author of a paper in The Astrophysical Journal.
Dressing investigated the M type stars Kepler studied among its 158,000 target stars. She found systematic errors in previous estimates that mean most of these were smaller and cooler than had been thought.
Such errors have major implications for planets found around the stars. Planets around Kepler's targets have their size estimated relative to the star. So if the star is smaller than we thought, the planet is too. Cooler stars have “habitable zones” closer in than warmer ones. Dressing admits definitions of that term are problematic, but has used the standard definition of temperatures where liquid water can exist.
Although not all of them have been confirmed, Kepler identified 95 possible planets around 64 red dwarfs from a sample of almost 4000. Allowing for the chances of a planet being picked up this is enough to suggest that 60% of these stars have planets smaller than Neptune. Allowing for multiple planet systems this translates to 0.86 such planets per star. Only three of those detected are the right size, and at the right distance, to be considered Earth-like, but again allowing for the Kepler's expected success rate this translates to 6% of red dwarfs having these sorts of companions. Naturally the error margins based on such a sample size are substantial, but using the curve of data from other sizes and locations Dressing places the lower bound at 0.015 habitable zone earth-sized planets per star.
Red dwarfs make up three quarters of the sun's neighbors, and if the 6% figure is correct then the expected distance to an excitement instilling planet would be 13 light years. Dressing estimates there are three Earth-size planets in the habitable zones of Red Dwarfs within 10 parsecs (32 light years) of the Earth.
Such a world would be very different from most science fiction exo-planets. Orbiting so close to its parent it would most likely rotate at the same rate that it orbits, like the moon, so that one side always faces its parent while the other always looks out towards the darkness of space. While parts of the planet would probably be too hot for life, and other parts too cold, the thicker the atmosphere the broader the intermediary temperate zone. "You don't need an Earth clone to have life," says Dressing.
M type stars have very long lifespans, so some are much older than the sun with plenty of time for life to develop. They do often have strong ultraviolet flares which could temporarily blast anything living on the sun-ward side of a close in planet. However, either a very healthy ozone layer or life restricted to twilight regions could be enough for this to not be a fatal problem.
Since longstanding methods of planet-finding favor heavier objects it is not surprising that those we have observed around nearby stars tend to be somewhat larger than the Earth, but this study's conclusions fit well with those reported earlier this month using different methodology.
Although a planet of mass similar to the Earth orbits Alpha Centauri B it has an estimated temperature of nearly 1000 C. Tau Ceti, the closest solo sun-like star has two planets with masses at least four times that of Earth, but they sit at the inner and outer edges of the zone where liquid water might exist.
Just two weeks ago the possibility of a planet at least twice the mass of the Earth was announced around GJ 180, 12 light years away, while a confirmed planet lies in the habitable zone of Gliese 682b, 16 light years away.
