Like the Age of Copernicus, the Kepler space telescope's survey of one small swath of the Milky Way is changing mankind's view of the Universe: dramatically increasing popular awareness of the likelihood that life exits beyond our Solar System. According to Stephen Kane—at NASA's Exoplanet Science Institute at CalTech —"not only are planets common in the galaxy, but there are more small planets than large ones. This is encouraging news for investigations into habitable planets."
Previous research hinted at the existence of near-Earth-size planets in habitable zones, but clear confirmation proved elusive. Two other small planets orbiting stars smaller and cooler than our sun recently were confirmed on the very edges of the habitable zone, with orbits more closely resembling those of Venus and Mars.
Kepler also has discovered more than 1,000 new planet candidates, nearly doubling its previously known count. Ten of these candidates are near-Earth-size and orbit in the habitable zone of their host star. Candidates require follow-up observations to verify they are actual planets.
"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.
Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or "transit," the stars. Kepler requires at least three transits to verify a signal as a planet.
"Fortune smiled upon us with the detection of this planet," said William Borucki, Kepler principal investigator at NASA Ames Research Center at Moffett Field, Calif., who led the team that discovered Kepler-22b. "The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season."
The Kepler science team uses ground-based telescopes and NASA's Spitzer Space Telescope to review observations on planet candidates the spacecraft finds. The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.
Kepler-22b is located 600 light-years away. While the planet is larger than Earth, its orbit of 290 days around a sun-like star resembles that of our world. The planet's host star belongs to the same class as our sun, called G-type, although it is slightly smaller and cooler. Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed.
Congressional funding for Kepler — which has identified 1,235 candidate alien planets to date and recently discovered the first exoplanet with two suns in its sky — is due to run out in November 2012, but an extension may be approved is the new mission proposal is approved. The NASA Kepler team will know by next spring whether it's approved.
"I think the discoveries we're making are showing what could be done if we continue to extend it," said Charlie Sobeck, Kepler deputy project manager at NASA's Ames Research Center in Moffett Field, Calif. "So we're hopeful, but there's no guarantee."
When Kepler launched in 2009, the telescope's science mission was set to run through November 2012 — a lifetime of 3.5 years. But the instrument could operate for six years, or perhaps longer, if it receives more funding, team members have said.
It would cost about $20 million per year to keep the Kepler mission running at its current level of activity beyond November 2012, Sobeck added. The $600 million Kepler observatory launched in March 2009. Its mission is to find roughly Earth-size planets in or near the habitable zones of their parent stars — a just-right range of distances that could support liquid water and, perhaps, life as we know it on the alien worlds.Kepler's overall goal is to help scientists determine just how common such planets may be throughout our galaxy.
In its search for extraterrestrial life, NASA's Kepler space telescope has found two new planets orbiting a distant sun-like star, and the researchers who made the find say these two are the first ever the size of Earth or smaller --a critical factor in the search for life elsewhere in the universe. Until now, scientists said their instruments were not sensitive enough to detect them.
Two newly-found planets, called Kepler-20e and Kepler-20f --announced on December 20-- orbiting a star about 950 light-years away, are too distant to be seen directly. Scientists measured the faint dimming of their host star as they passed in front of it to estimate their size.
Kepler-20e and f are probably too hot to be friendly to life -- one of them circles its sun in just six Earth days, and the other does it in 19. This newest Kepler find increases the odds that some day soon we will find a planet of just the right size and temperature to have at least a chance of being a habitable.
In less than 20 years, astronomers have gone from not knowing if other planets exist in the universe, to our current Kepler catalog on almost 2000 planets. Our Milky Way galaxy may be home to at least two billion Earthlike planets, a recent study based on initial data from from Kepler space telescope shows -- a number that is actually far lower than many scientists anticipated.
Based on what Kepler's found so far, the study authors think that up to 2.7 percent of all sunlike stars in the Milky Way host so-called Earth analogs. As of this February, Kepler has confirmed 15 new planets and found an additional 1,235 planet candidates, including the smallest planet yet spied outside our solar system. Kepler will collect transit data for a minimum of three and a half years, allowing for a more complete planetary census at a later date.
"There are about a hundred billion sunlike stars within the Milky Way," said study co-author Joe Catanzarite, a scientist with NASA's Jet Propulsion Laboratory (JPL). "Two percent of those might have Earth analogs, so you have two billion Earth analog planets in the galaxy," he added. "Then you start thinking about other galaxies. There are something like 50 billion, and if each one has two billion Earthlike planets, it's mind boggling."
Although the figure seems large, Catanzarite and co-author Michael Shao, also of JPL, say their results actually show that Earths are "relatively scarce," which means a substantial effort will be needed to identify suitable target stars for followup missions designed to study the chemical signatures of Earth-size worlds. The chemical signals may hint whether the planets have oxygen atmospheres, liquid water -- or even signs of life.
The Kepler telescope has been scanning a patch of sky near the constellation Cygnus which can be considered a representative sample of what exists throughout the Milky Way. The Kepler space telescope analyze the light from the 156,000 stars in its field of view searching for stars that dim periodically—signs that significant objects are orbiting these stars.
To extrapolate the number of possible Earths in the Milky Way, Catanzarite and Shao started by defining an Earth analog based on a transiting planet's size and the distance at which the planet orbits its star."A famous 1993 paper calculated the inner and outer distance of the so-called Goldilocks zone"—not too hot, not too cold — "where liquid water could exist on a planet's surface," Catanzarite observed.
"But more recently people have been saying those boundaries are conservative. Maybe you could go closer or farther. For instance, because of greenhouse gases a planet could be farther away and still be warm, or because of clouds, which the previous models didn't take into account, you could be closer but keep the surface cool."
In general, though, an orbit similar to Earth's seems ideal: "Closer than Earth and you'll fry; water would turn to steam. If you're too far, water would freeze into ice."
"People generally agreed that the smallest habitable planet would be 0.8 Earth radii ... or roughly half of Earth's mass. The reason is that a lower mass planet wouldn't be able to hold on to an oxygen atmosphere," Catanzarite said. "Out to two Earth radii is the largest planet we'd call Earthlike. More massive planets start to accumulate thick hydrogen atmospheres, like Neptune or Uranus," with unbearable atmospheric pressures.
Other planets may exist that we can't see because of their orbital inclination, so the team used previous exoplanet data to make mathematical estimates for the probabilities of these unseen worlds.The results published online this month on arXiv.org showed that, according to the traditional boundaries of the Goldilocks zone, 1.4 percent of sunlike stars should have Earth analogs.
If you accept the wider, more modern version of the habitable zone, 2.7 percent of sunlike stars likely host Earths. The study authors predict that Kepler will eventually find 12 Earth analog planets in its field of view — and may have already found 4 such worlds among its initial candidates.
"This study completely underestimates the frequency of Earths," according to MIT planetary scientist Sara Seager, a member of the Kepler science team who noted that Kepler's just getting started, so its data is far from complete. "Say you're doing a census of the United States," she said. "If you go to California and knock on every door, you can then extrapolate out to the rest of the country. That's what Kepler's doing."
"If Kepler's really going to find the answer in a few years, I'm happy to just wait rather than speculate", Seager said. The other big concern is that, with the data Kepler can collect, it's impossible to say whether a given planet is truly Earthlike. Size alone, for example, doesn't say enough about habitability. "Earth and Venus are about the same mass and size," she said — and by some definitions both worlds fall in our sun's habitable zone.
"To me, for a planet to be called an Earth analog you have to have Earth's mass, size, orbit, and know whether there's liquid water on the surface. But you can't know that until we do atmosphere studies," she said. "Kepler can find only Earth-size planets — we'd never use the term 'Earth analog.' Some people are expecting the number of Earths to be higher," Catanzarite said, adding that Seager "may be right" and the new estimate is too low, "but it's still unclear."
"We're assuming that for all 156,000 stars, it's possible to detect all small planets of Earthlike radii and orbital distance. If [Kepler] can't, that would make our number an underestimate."
What about the possibility of life not only in the Milky Way, but also in the 50 billion galaxies beyond our galactic home-base? New findings from diverse fields are are being brought to bear regarding the central question of the 21st century: How common is life in the Universe? Where can it survive, Will it leave a fossil record, How complex is it.
For decades, scientists have been debating the conditions that are needed to replicate an Earth-like probablility of complexity beyond the microbial level. There's not much doubt in the minds of most astrobiologists that based on extremophile life we've discovered recently on Earth (see prior posts below), that life on the microbial level will be discovered sometime in the next twenty years on Mars or on one of Jupiter or Saturn's moons.
The three recent key findings for astrobiology are extremophiles, extrasolar planets, and a sense that water may be more ubiquitous even in our own solar neighborhood (in meteors like the Mars' Lafayette, Europa, and the ice frost on polar Mars). This picture has evolved quite suddenly with 1000-plus extrasolar planets found in just the last decade (and none known before around 1995).
Even in the oldest globular cluster star systems in our Milky Way galaxy -- choked with stars that were born more than 10 billion years ago -- there are enough "metals" to make earth-like worlds.
According to models of planet formation developed by Harvard's Dimitar Sasselov, a member of the Kepler team, such a planet should be about half again as large as the Earth and composed of rock and water, what the astronomers now call a “super Earth.”for evolved animal life to be present we need to find that sweet "Goldilocks" planet with an exceedingly complex host of conditions present that have given rise the "Rare Earth" hypothesis.
Kepler finds alien planets using what's called the transit method. The telescope detects the telltale dips in brightness caused when an alien planet crosses in front of, or transits, its star from Kepler's perspective. Kepler needs to witness three of these transits to firmly identify a planet candidate.
This technique has been extremely effective. In just its first four months of operation, Kepler discovered 1,235 exoplanet candidates.
So far, two dozen of them have been confirmed by follow-up observations — including Kepler-16b, a world with two suns that was announced recently.Kepler team members have estimated that 80 percent or so of the telescope's candidates will probably end up being the real deal. If that's the case, Kepler's finds to date would more than double the number of known alien planets, which currently stands at about 685.
"What we're seeing is this trend — the smaller the planet, the more of them there are," Sobeck told Space.com. "That's great news for the idea of finding Earth-like planets, or Earth-size planets. Once you have Earth-size planets, all it has to do is be in the right orbit, and it's habitable."
We've come to expect the unexpected," said Sara Seager, a Massachusetts Institute of Technology exoplanet specialist also on the Kepler science team. So, who knows what 2012 may bring.
The Daily Galaxy via NASA and washingtonpost.com
Image credit: Exo planet Glise 581 ESO
A CalTech analysis appears on arXiv.org. The Kepler science mission data, collected between May 2 and September 16, 2009, will be published in The Astrophysical Journal.