Thread: Earth-sized planets may be common in our galaxy. (from ScienceDaily)

ScienceDaily (Oct. 28, 2010) — Nearly one in four stars similar to the sun may host planets as small as Earth, according to a new study funded by NASA and the University of California.

The study is the most extensive and sensitive planetary census of its kind. Astronomers used the W.M. Keck Observatory in Hawaii for five years to search 166 sun-like stars near our solar system for planets of various sizes, ranging from three to 1,000 times the mass of Earth. All of the planets in the study orbit close to their stars. The results show more small planets than large ones, indicating small planets are more prevalent in our Milky Way galaxy.

"We studied planets of many masses -- like counting boulders, rocks and pebbles in a canyon -- and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can't see the grains of sand, the Earth-size planets, but we can estimate their numbers," said Andrew Howard of the University of California, Berkeley, lead author of the new study. "Earth-size planets in our galaxy are like grains of sand sprinkled on a beach -- they are everywhere."

The study appears in the Oct. 29 issue of the journal Science.

The research provides a tantalizing clue that potentially habitable planets could also be common. These hypothesized Earth-size worlds would orbit farther away from their stars, where conditions could be favorable for life. NASA's Kepler spacecraft is also surveying sun-like stars for planets and is expected to find the first true Earth-like planets in the next few years.

Howard and his planet-hunting team, which includes principal investigator Geoff Marcy, also of the University of California, Berkeley, looked for planets within 80-light-years of Earth, using the radial velocity, or "wobble," technique.

They measured the numbers of planets falling into five groups, ranging from 1,000 times the mass of Earth, or about three times the mass of Jupiter, down to three times the mass of Earth. The search was confined to planets orbiting close to their stars -- within 0.25 astronomical units, or a quarter of the distance between our sun and Earth.

A distinct trend jumped out of the data: smaller planets outnumber larger ones. Only 1.6 percent of stars were found to host giant planets orbiting close in. That includes the three highest-mass planet groups in the study, or planets comparable to Saturn and Jupiter. About 6.5 percent of stars were found to have intermediate-mass planets, with 10 to 30 times the mass of Earth -- planets the size of Neptune and Uranus. And 11.8 percent had the so-called "super-Earths," weighing in at only three to 10 times the mass of Earth.

"During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter," Howard said. "It's natural for lots of these building blocks, the small planets, to be left over in this process."

The astronomers extrapolated from these survey data to estimate that 23 percent of sun-like stars in our galaxy host even smaller planets, the Earth-sized ones, orbiting in the hot zone close to a star. "This is the statistical fruit of years of planet-hunting work," said Marcy. "The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that's not counting Earth-size planets that orbit farther away from their stars in the habitable zone."

The findings challenge a key prediction of some theories of planet formation. Models predict a planet "desert" in the hot-zone region close to stars, or a drop in the numbers of planets with masses less than 30 times that of Earth. This desert was thought to arise because most planets form in the cool, outer region of solar systems, and only the giant planets were thought to migrate in significant numbers into the hot inner region. The new study finds a surplus of close-in, small planets where theories had predicted a scarcity.

"We are at the cusp of understanding the frequency of Earth-sized planets among celestial bodies in the solar neighborhood," said Mario R. Perez, Keck program scientist at NASA Headquarters in Washington. "This work is part of a key NASA science program and will stimulate new theories to explain the significance and impact of these findings."

NASA's Exoplanet Science Institute at the California Institute of Technology, Pasadena, Calif., manages time allocation on the Keck telescope for NASA. NASA's Jet Propulsion Laboratory, also in Pasadena, manages NASA's Exoplanet Exploration program office. More information about exoplanets and NASA's planet-finding program is at PlanetQuest: Exoplanet Exploration .

This is incredible. I admit, I obsess over exoplanets (I have the iPhone app!). And I can definately believe it. 23% - that's between 50 and 100 billion stars. If you take the Solar System as an example, there are (depending on whether or not you believe planetoids and moons of gas giants count) four, twelve, or twenty-ish roughly (1.5 to 0.1) Earth-mass bodies. Out of those, only one lies within the life-sustaining habitable zone (although several others are expected to harbor microbial life). That means that, if we presume disk formation in stars to be roughly the same throughout the galaxy, between 5% and 25% percent of said planets have a chance of supporting life (completely ignoring the possibility that they could all form outside of the habitable zone). Add up the math, and you get a pretty whopping number - 11.25 billion stars, plus or minus about 8 billion. Or 2% of the entire Galaxy. That is an amazing amount of planets!

I love the idea of other habitable planets. I think that humanity's ultimate goal is to colonize these planets, to save us in case of destruction and to extend our knowledge and reach (one reason why I watched "Avatar" four times). Every new discovery never ceases to amaze me.

Haha, yeah, I was a big Avatar nerd as well. I saw it four times in the theatres. While I'm aware that it is still sci-fi and a glorified remake of Fern Gully, I thought the world they portrayed was quite beautiful.

I'm a bit obsessed over Exoplanets as well. Although I don't have an iPhone, I try to keep up on the new discoveries as time goes on. Ever heard of CoRoT 7-b? It's a fascinating little world, go check it out on wikipedia or sciencedaily, definitely a planet for all the sci-fi nerds out there.

I would tend to agree that finding other habitable worlds is definitely one of mankind's ultimate goals. Our planetary home may be wonderful, but it won't last forever, and it is much more advantageous to be a 2 or 3 planet species if at all possible to have at least most of the bases covered. My great excitement in exoplanets isn't just the thought of alien life, but the thought of intelligences either similar or far beyond our own. We're all used to talking with humans, who have common ancestry and common experience, but imagine what it would be like to have a conversation with a being from a world not your own! Definitely an interesting thought.

That is very cool. I think it's really amazing how we're already finding planets with masses between 3 and 10 times the Earth's mass (although I'm not too sure I'd like to live on a planet with a surface temperature four to six times those on Mercury!). Finding a planet that is roughly Earth-sized is probably only a few years (or if you listen to PopSci, a few months) away!

Originally Posted by Jimmy Pelletier

I love the idea of other habitable planets. I think that humanity's ultimate goal is to colonize these planets, to save us in case of destruction and to extend our knowledge and reach.

Originally Posted by John Partika

I would tend to agree that finding other habitable worlds is definitely one of mankind's ultimate goals. Our planetary home may be wonderful, but it won't last forever, and it is much more advantageous to be a 2 or 3 planet species if at all possible to have at least most of the bases covered. My great excitement in exoplanets isn't just the thought of alien life, but the thought of intelligences either similar or far beyond our own. We're all used to talking with humans, who have common ancestry and common experience, but imagine what it would be like to have a conversation with a being from a world not your own! Definitely an interesting thought.

@Jimmy: Don't know how to tell you this but there is no "in case of destruction"...long before the end of the Universe, the Earth will have been destroyed! @Jimmy & @John: "Finding other habitable worlds is definitely one of mankind's ultimate goals" is far from true; it is a primary goal. Finding other uninhabited habitable worlds would be an ultimate goal otherwise the 'Star Trek Syndrome' kicks in and we'd ultimately be no better off than we are now. Just my opinion, your mileage may vary...

Originally Posted by David E. Eaton Sr.

@Jimmy: Don't know how to tell you this but there is no "in case of destruction"...long before the end of the Universe, the Earth will have been destroyed! @Jimmy & @John: "Finding other habitable worlds is definitely one of mankind's ultimate goals" is far from true; it is a primary goal. Finding other uninhabited habitable worlds would be an ultimate goal otherwise the 'Star Trek Syndrome' kicks in and we'd ultimately be no better off than we are now. Just my opinion, your mileage may vary...

@David: No, I think you really hit the nail on the head just right. It is our primary goal to find other worlds. But I said ultimate because we can't really start working on our main goal unless we get the rest of our smaller, less important goals straightened out first. We certaintly can't start spreading our reach to other worlds if we run out of natural resources or nuke the crap out of each other. But one of our main problems is that people get so preoccupied with fixing these small issues that they don't think about the farther future, so they think that space travel and colonization is "unreasonable" and "unrealistic." As for the "in case of destruction," I meant large-scale destruction within our lifetime (in the near future), where, if we were on one planet, we wouldn't be prepared to deal with it at all and which would cripple us terribly. If we were spread out over several planets, especially a large number of planets with a large number of people on each, the blow wouldn't be so bad.