This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star.
Cowboys & Aliens are Coming!
July 29, 2011 --
If aliens are going out of their way to kick up dust in the Wild West, as they do in the upcoming movie "Cowboys & Aliens," they must be coming from somewhere. Life could take root on a moon or a meteorite. But to nurture the kind of life that could destroy our saloons and harass our livestock, a planet might be the most suitable. So far, Kepler, a NASA orbiting telescope that searches for planets beyond our solar system, has detected over 1,200 exoplanets. Surely there must be a few candidates among this group that could meet some of the most basic requirements to host life? Explore some far-out worlds that could support aliens, be they cattle-rustling characters or a more peaceful people.
First, let's lay out some basic criteria. Kepler hasn't identified many rocky worlds and a solid surface is essential for life to take root. Size matters: The mass of the planet helps astrophysicists infer what it's made of. Some planets are Earth-sized. Others are several times the size of our planet. And then there are gas giants, which can range from "Neptune sized" to "super-Jupiters." Orbit: To support life, a planet must be in a stable orbit around its star -- no planets with wonky orbits that will eventually dump them into their star for a fiery death. Goldilocks Zone: This is a region not too hot or too cold that gives the planet enough distance from its parent star to have liquid water, key for life. Loner Stars: Single stars make better parents. In 2010, a pair of closely orbiting binary stars was spotted surrounded by what could be the debris of former planets. Unknowns: Some factors for life can't be confirmed one way or the other from the data available about extrasolar planets. These include: water, chemical compounds such as ammonia; a nitrogen-rich atmosphere; a magnetic field to repel solar and cosmic radiation; and more. BUT, some planets do have a head-start, beginning with Gliese 581D.
Located a mere 20 light-years away, practically our backyard in cosmic terms, Gliese 581d is situated on the "outer fringes" of the Goldilocks zone, orbiting a red dwarf star. The planet may be warm enough and wet enough to support life in much the same manner as Earth. It might also contain a thick carbon atmosphere. If we ever need a new Earth and have the means to get there, Gliese 581d may be our best bet for now.
When it was first detected and reported last year in Astrophysical Journal, Gliese 581g appeared to be the perfect candidate for a true "Earth-like" planet. Located in the same star system as Gliese 581d (and detected earlier), Gliese 581g seemed to be the right size and located within a habitable zone away from its parent star. Gliese 581g was said to have three times the mass of Earth, making it possible for the planet to hold an atmosphere. However, since its discovery, follow-up studies have alleged that Gliese 581g might have been a false alarm. In other words, the planet might not exist at all.
Dubbed a "waterworld" and located a mere 42 light-years from Earth, GJ 1214b orbits near a red dwarf star about one-fifth the size of our sun. What makes this planet unique is that it appears to be primarily composed of water, although GJ 1214b is 6.5 times the mass of Earth and 2.7 times wider, which classifies it as a "super-Earth." This planet also has a steamy atmosphere composed of thick, dense clouds of hydrogen, which, although it might not the case with this planet, could incubate life.
Situated 150 light-years from Earth, HD 209458b is a planet that holds traces of water vapor in its atmosphere, and also contains basic organic compounds that, on Earth, foster the development of life. But there are two factors working against HD 209458b as a suitable habitat. The planet is very hot due to its close proximity to its parents star, and it's a gas giant, so no solid surfaces.
If Kepler-10b were located further from its parent star, it might have had a chance of hosting life. Kepler-10b was the first "iron-clad proof of a rocky planet beyond our solar system" back in 2001. It was even dubbed the "missing link" of extrasolar planetary research. When it comes to the search for life, though, Kepler 10-b is missing a lot of other ingredients -- just minor things like water or an atmosphere.
When venturing to a new star system to explore the possibility of extraterrestrial life, trying a star that has already shown itself to nurture planets -- even if they're not the kind you're looking for -- could be a promising strategy. Project Icarus, an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination, has identified two stars located within 15 light-years that might fit the bill: "epsilon Eridani, a single K star 10.5 light-years away, and the red dwarf GJ 674, 14.8 light-years away." Indirect evidence has also shown that epsilon Eridani may already hold smaller worlds scientists simply haven't detected yet. Also, red dwarf star systems generally may be a safe haven for life.
Are We Alone?
Taking into account the number of exoplanets that have been detected, as well as the vastly greater number that are estimated to be out there, some astrophysicists are convinced that extraterrestrial life is inevitable. After all, the Milky Way may be loaded with as many as 50 billion alien worlds. Some even think we'll find alien life by 2020. Others, however, say it may not exist at all. Recently, astrophysicists David Spiegel of Princeton University and Edwin Turner from the University of Tokyo suggested we might be alone in the universe, based on their interpretation of the Drake equation, a formula meant to determine loosely the probability of the existence of life beyond Earth. According to their analysis, just because life on Earth took shape early, endured and prospered doesn't mean the same process would naturally and inevitably occur elsewhere in the universe. Discovering life elsewhere, however, would be the only means of settling this debate. Unless the aliens find us first, of course.
- A planet has been found in the "habitable zone" -- a region where liquid water could exist.
- The newly confirmed planet, Kepler-22b, is the smallest yet found to orbit in the habitable zone of a star similar to our sun.
- Previous research hinted at the existence of near-Earth-size planets in habitable zones, but confirmation had been elusive.
A planet about twice the size of Earth has been confirmed to exist right in the middle of the "habitable zone" around its star, which is much like our own.
Previous research had hinted at the existence of such Earth-like planets, where liquid water could exist, but this is the first time such a life-friendly alien planet has been confirmed.
The planet, known as Kepler-22b, is among 29 confirmed and 2,326 candidate worlds found by a team of astronomers using NASA's Kepler Space Telescope.
Kepler-22b is the smallest planet yet to be found beyond our solar system in the region most conducive to life as we know it on Earth.
"If the greenhouse warming was similar on this planet and if it had a surface, its temperature would be something like 72 degrees Fahrenheit, a very pleasant temperature here on the Earth," William Borucki, lead Kepler researcher at NASA's Ames Research Center in Moffett Field, Calif., said at a press briefing Monday.
The planet is around 2.4 times wider than Earth -- making it a "super-Earth." It takes 289.9 days to fly around its parent star, which is very much like our sun.
"It's almost a solar twin," Batalha said.
More work is needed before scientists will be able to tell if Kepler-22b is rocky like Earth, gaseous like Neptune, or more probably, a mix. But its discovery is a milestone on the road to finding bona fide Earth-like planets.
"We don't know anything about the planets between Earth-size and Neptune-size because in our solar system we have no examples of such planets," said Natalie Batalha, Kepler deputy science team lead at San Jose State University.
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star.NASA/Ames/JPL-Caltech
The Kepler team's targets also will be scrutinized by the independent SETI Institute, which surveys stars in the Milky Way for non-naturally occurring radio signals, a project known as the Search for Extraterrestrial Intelligence, or SETI.
"There are so many special things about Earth," SETI Institute director Jill Tarter told Discovery News. "We have an example of one. In a physics experiment, when when there are multiple outcomes you want to run that experiment many times. We haven't been able to do those experiments yet. We don't know whether the Earth as it is and life as we know it here -- the way we got here was very unusual, that elsewhere things go in a different way. Or are we common?
"This is a work in progress," she said. "In this field, number two is the all-important number because as soon as we find a different, a separate, an independent example of life somewhere else, we're going to know that it's ubiquitous throughout the universe."
So far, Kepler-22b seems to be flying solo, with no sibling planets, but that conclusion may change. Smaller worlds like Earth require more time for the Kepler team to make observations.
The telescope is staring at about 150,000 stars in the constellations Cygnus and Lyra looking for the miniscule dimming of light that happens when a planet or planets pass across or "transit" -- the stars.
In addition to confirming Kepler-22b, the telescope team discovered more than 1,000 new planet candidates. The recent haul nearly doubles its previously known count. Ten of these candidates are near-Earth-size and orbit in the habitable zone of their host stars.
These finds require follow-up observations to verify they are actual exoplanets and not observational anomalies. Among the finds were two other small planets orbiting stars smaller and cooler than our sun, with orbits more closely resembling those of Venus and Mars.
Kepler requires at least three transits to verify a signal as a planet.