Alien Worlds: How Common Are Habitable Planets?
November 5, 2013

Habitable, Earth-Sized Planets Believed To Orbit One-Fifth Of All Sun-Like Stars

[ Watch the Video: Earth May Not Be So Unique After All ]

redOrbit Staff & Wire Reports - Your Universe Online

New statistical analysis of data obtained by NASA’s now-retired Kepler spacecraft has revealed that one out of every five sun-like stars have at least one Earth-sized planet in the habitable zone.

“What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing,” Erik Petigura, a University of California, Berkeley graduate student who led the research, said in a statement Monday.

Petigura, Andrew Howard of the Institute for Astronomy at the University of Hawaii, and UC Berkeley astronomy professor Geoffrey Marcy conducted the analysis of data collected during Kepler’s four-year search for potential habitable planets in our galaxy. Their research was funded in part by the National Science Foundation (NSF), and their findings appear online in the journal Proceedings of the National Academy of Sciences.

[ Watch the Video: Earth-Size Planets in Habitable Zone ]

“It's been nearly 20 years since the discovery of the first extrasolar planet around a normal star,” Howard said. “Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life. With this result we've come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy.”

“For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are,” he added. “"An abundance of planets orbiting nearby stars simplifies such follow-up missions.”

Of course, the study authors caution against assuming that planets are potentially habitable just because they are approximately the same size as Earth, travel in orbits similar to that of our planets, and orbit at a distance from a star that is neither too hot nor too cold to sustain life. There are other factors to consider, such as the thickness of the atmosphere and whether or not the world contains liquid water necessary to support living organisms.

For there to be liquid water, there needs to be a rocky surface to harbor it, the researchers said. To that end, last week a research team including Howard and Marcy reported that one Earth-sized planet discovered with Kepler was the same density as Earth, and likely had the same rock-and-iron composition.

While the planet was far too hot (an estimated temperature of approximately 2,000 Kelvin) to sustain life as we know it, Howard said that the discovery “gives us some confidence” that there are Earth-sized, rocky planets that would be better able to support life located in habitable zones.

Kepler, which was launched in 2009, photographed 150,000 stars every 30 minutes for four years. Using that data, NASA researchers found over 3,000 planet candidates, many of which are larger than Earth and unsuitable for life for various reasons.

In their research, Petigura, Howard and Marcy focused primarily on the 42,000 stars found to be similar to the sun, and discovered 603 candidate planets orbiting them. Of those, just 10 were said to be Earth sized, described by the researchers as one-to-two-times the diameter of our planet, and orbiting their star at a distance where they are heated to temperatures suitable to life (receiving one-fourth to four-times the light that Earth receives from the sun).

“What distinguishes the team's analysis from previous analyses of Kepler data is that they subjected Petigura's planet-finding algorithms to a battery of tests in order to measure how many habitable zone, Earth-size planets they missed. Petigura actually introduced fake planets into the Kepler data in order to determine which ones his software could detect and which it couldn't,” UC-Berkeley explained. “Accounting for missed planets, as well as the fact that only a small fraction of planets are oriented so that they cross in front of their host star as seen from Earth, allowed them to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their habitable zones.”

“The primary goal of the Kepler mission was to answer the question, When you look up in the night sky, what fraction of the stars that you see have Earth-size planets at lukewarm temperatures so that water would not be frozen into ice or vaporized into steam, but remain a liquid, because liquid water is now understood to be the prerequisite for life,” added Marcy. “Until now, no one knew exactly how common potentially habitable planets were around Sun-like stars in the galaxy.”

Image 2 (below): Astronomers use the term "habitable zone" to indicate an orbit not too far from the star such that water freezes, and not too close such that water vaporizes. Habitable zones are orbital areas where the heat from the star creates lukewarm temperatures at which liquid water can exist, and water is the presumed prerequisite for life. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley