Search for Earth's Twin: New NASA Research Finds Close-in, Small Planets Common in Milky Way

"Detecting Earth in reflected light is like searching for a firefly six feet from a searchlight that is 2,400 miles distant," wrote a panel of astronomers, when describing the challenges facing the search for other planets in the universe. 

But new NASA reserach shows that nearly one in four stars similar to the sun may host planets as small as Earth, according to an extensive  new study funded by NASA and the University of California.

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 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 planetary systems 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 http://planetquest.jpl.nasa.gov.

 

Casey Kazan Via http://www.jpl.nasa.gov/news/news.cfm?release=2010-357&cid=release_2010-357

Comments

[Bob Greenwade]

And we grow ever closer and closer to actually finding a planet that potentially has Earth-like characteristics -- not just in size and orbit, but in surface environment.

I've been paying close attention for the first announcement of that.

[Steve Bergman]

Bob Greenwade said:

"""
I've been paying close attention for the first announcement of that.
"""

We're all waiting for the TPF, of course. It's funding is currently in limbo. But in the mean time, we still have something very exciting to look forward to which some readers might not know about. JWST is scheduled to launch in 2014 or 2015. And has the right instrumentation do something fairly amazing, in tandem with data from Kepler, and that of the ground-based gravitational wobble planet finding teams.

Once we know that a planet exists, and know its orbit, we can do something very clever. We can point JWST at it, and compare the spectrum of the system at a time we know the planet is off to the side, and reflecting the star's light, to the spectrum of the system when we know that the planet is behind the star. Subtract the latter from the former, and you end up with the spectrum of the light reflected by the planet. (Yes, it's slightly more complicated, since the planet both blocks and reflects the star's light. But still...)

Analyze that, and you find out something about the planet's atmosphere.

I was born in 1963. And I never expected to see the day that we could make such observations. In a few years, we will be able to say what, say, Gliese 581g's atmosphere is composed of.

-Steve

[Bob Greenwade]

I had to go look up a couple of things, so for those as ignorant as I:

TPF = Terrestrial Planet Finder
JWST = James Webb Space Telescope

Both have Wikipedia articles.

I was born two years ahead of you, Steve, and I actually expected that we'd be a tad closer than this. But then again few people at the time expected the space program to get as stalled out as it did after the moon program.

[Steve Bergman]

Bob,

Thanks for spelling those out. I *hate* it when other people use acronyms and just expect that I'll recognize them. And there I was doing it myself. Lazy me.

One thing that I've noticed, over the years, is that we've made tremendous progress in areas that I didn't expect, and little progress in areas where I expected we'd do better.

In my youth, I was big on manned space missions. In fact, I did not see a distinction between what we now call space exploration (manned missions) and what we now call space science (robotic missions). I guess I was too busy watching Star Trek to see the distinction.

But today, my assessment is that while both are underfunded, space science is where the limited funds should go.

If you were born 2 years ahead of me, that would make you 49. Are you beginning to feel the same time crunch I do when you hear, for example, that the Europa/Jupiter System Mission won't reach the Jupiter system until 2026, at the earliest?

-Steve

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[Bob Greenwade]

Steve, I hadn't really thought much about the distinction between robotic missions and manned missions, but I pretty much agree with how you describe it -- both need more funding, but the robotic missions can go further right now and should take priority.

I also think, in keeping with the original article here, that we need to put some emphasis on finding Earth-like planets, and on finding a way to migrate people there. Earth won't last forever; even if we manage to convert completely to renewable resources, and we manage to avoid planet-killing asteroids and supernovae, the sun will eventually enlarge and kill everything on the planet (except maybe waterbears and cockroaches). Just for the first two, we need a beta site.

Though I'm 49, I do have some hope for great longevity. Another recent article here suggested that the first person to live to be 1000 is already in his sixties, and for all that I'd love to do with my life I'd be as pleased as punch with half that. And such a life extension could keep me around long enough to see the development of the Alcubierre drive, and possibly even allow me to set foot on an extrasolar planet.

And as far as I'm concerned, anyone with a pivotal role to play in the establishment of Earth's first extrasolar colony will have contributed to the long-term salvation of the species as a whole.

[Steve Bergman]

"""
Earth won't last forever; even if we manage to convert completely to renewable resources
"""

True, indeed. But the most immediate and pressing problem we have is the relentless increase in our human population. And it is important to recognize that while space colonies would help to preserve our genetic heritage, emigration to space won't do much good for the Earth itself. World population, by humans, was 3 billion in 1960. It's 6.7 billion today. That works out to an average increase of 74 million people per year, over the period. And that rate has been increasing with time. Is it reasonable to think that we could send 74 million people per year to space colonies, just to maintain our current level of overpopulation? That's over 200,000 people per day.

Space colonies are a way to secure our genetic and, to a lesser extent, our cultural heritage(s). But I fear that the ultimate solution to the problem of Earth's finite resources will be Malthusian. And it is not hard to see that the process has already begun. One need only look to the 3rd world countries to see it.

If we did achieve immortality, or radically extended life-spans... I don't even want to think about that possible future. I'll happily die when my time comes. (OK. Maybe not so happily.) I think that there are fundamental genetic reasons that pushing past 120 years, for humans, is a virtual impossibility. But even extending the US and western Europe's 75 year average life-span to the rest of the world would have catastrophic consequences.

We face serious problems. And, alas, space colonies can't solve many of them. I wish I had an answer to the problem which was ethical. And I'm not willing to promote a "solution" which I do not consider to be ethical. So I expect that Malthus will end up winning by default.

Sorry for making such a "downer" of a post. But once I started typing, it kinda just wrote itself.

-Steve