NASA

An artist's conception shows NASA's Kepler space telescope observing a planetary transit.

NASA is getting plenty of advice — and sympathy — as it assesses whether its Kepler planet-hunting telescope can be revived after the failure of its reaction-control system. The reactions from scientists and engineers range from repair tips to an Audenesque elegy. Here's a sampling:

How to fix Kepler
The reason why the $600 million Kepler spacecraft can no longer search for planetary transits is that two of its four gyroscopic reaction wheels can no longer spin. Mission managers say Kepler needs at least three of those wheels in working order to hold its position still enough to stare at alien stars.

The most recent part to fail is known as reaction wheel 4. The mission's deputy project manager, Charlie Sobeck, told reporters that the Kepler team could try putting some reverse torque on that wheel in hopes of freeing it up.

Two other possibilities were raised by Scott Hubbard, who headed NASA's Ames Research Center during the development of the Kepler mission and is now a consulting professor of aeronautics and astronautics at Stanford University.

One option would be to try turning on reaction wheel 2, which failed last July. "It was putting metal on metal, and the friction was interfering with its operation, so you could see if the lubricant that is in there, having sat quietly, has redistributed itself, and maybe it will work," Hubbard said in a Stanford Q&A.

"The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability," he said. The mission's principal investigator, Ames' Bill Borucki, said on Wednesday the thrusters couldn't hold the spacecraft stable enough for planet-hunting. Nevertheless, it might be one of the options under consideration.

For the time being, Kepler has been put into a holding pattern that should minimize its thruster fuel consumption and give the Kepler team several months to weigh all the options, the costs and the potential scientific benefits.

Going beyond Kepler
Even if the Kepler spacecraft can't be revived, Borucki says that only half of the data collected so far have been fully analyzed. He estimates it'll take another two years or so to complete the analysis.

Meanwhile, NASA has just given the go-ahead its next planet-hunting satellite: the Transiting Exoplanet Survey Satellite, or TESS. That $200 million project would put a telescope array in space in 2017 to perform an all-sky survey, looking for exoplanets in orbit around the nearest and brightest stars. That strategy is markedly different from the one used by Kepler, which stared at a relatively small patch of sky straddling the constellations Cygnus and Vega.

This October, the European Space Agency plans to launch a space probe called Gaia to conduct a census of more than a billion stars in the Milky Way. Gaia could detect thousands of distant planetary systems, and measure their orbits and masses using a technique known as astrometry.

ESA is working on another planet hunter called the Characterizing Exoplanets Satellite, or CHEOPS, which is due for launch in 2017. CHEOPS would conduct high-resolution transit observations of stars that have already been found to host planets. 

The $8.8 billion James Webb Space Telescope, which NASA bills as the successor to the Hubble Space Telescope, could conceivably analyze the atmospheres of alien planets. It's currently due for launch in 2018.

Paying tribute to Kepler
NASA's associate administrator for science, John Grunsfeld, said it's too early to consider Kepler "down and out." But many astronomers fear that Kepler's planet-hunting days are finished.

"I think 'The mission is not over' means 'the mission is over,'" Caltech's Mike Brown said in a Twitter update on Wednesday. "Might be other things it can do. But, kids, I think the mission is over."

Alan Boss, an astrophysicist at the Carnegie Institution for Science who's part of the Kepler team, was similarly downbeat. In an email sent to AAAS MemberCentral, he called this week's setback a "disaster":

"I am afraid that the loss of this second reaction wheel effectively means the partial loss of Kepler's main science goal: determining the frequency of Earth-sized planets orbiting their stars at distances such that liquid water could occur on the planets' surfaces. Kepler has taken an outstandingly impressive four years of data, but we still need another three or so years of outstandingly impressive data to be certain of the frequency of Earth-size planets. Right now we have enough data to make an intelligent extrapolation about what that number is, but that is not the same as actually determining that number. Kepler was planned to do that for us. There is no other mission in sight that can reproduce for us what Kepler was in the process of doing. The upcoming (2017) NASA TESS Mission will help to push the exoplanet field forward, but it is not designed to find Earthlike planets around sunlike stars, like Kepler was."

"This is one of the saddest days in my life. A crippled Kepler may be able to do other things, but it cannot do the one thing it was designed to do."

Another Kepler team member, Geoff Marcy of the University of California at Berkeley, told KQED that he felt dizzy and teary-eyed over the spacecraft's situation. "It’s a loss for our species," he said. "That sounds dramatic, but we pride ourselves as a species of exploration, seeking answers beyond the horizon, answers about our place in the universe. And Kepler was answering those questions."

Marcy went so far as to tweak W.H. Auden's poem "Funeral Blues" to pay tribute to Kepler. Here's the astronomer's elegy to a spacecraft:

Stop all the clocks, cut off the Internet,
Prevent the dog from barking with a juicy bone,
Silence the pianos and with muffled drum
Bring out the coffin, let the mourners come.

Let jet airplanes circle at night overhead
Sky-writing over Cygnus: Kepler is dead.
Put crepe bows round the white necks of doves,
Let the traffic officers wear black cotton gloves.

Kepler was my North, my South, my East and West,
My working week, no weekend rest,
My noon, my midnight, my talks, my song;
I thought Kepler would last forever: I was wrong.

The stars are still wanted now; let's honor every one,
Pack up the moon and dismantle the sun,
Pour away the ocean and sweep up the woods;
For nothing will ever be this good.

With thanks to W.H.Auden.

For a video rendition of "Funeral Blues," check out this clip from "Four Weddings and a Funeral."

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA

An artist's conception shows NASA's Kepler space telescope observing a planet making a transit across an alien star. (Star and planet not to scale.)

NASA's planet-hunting Kepler space telescope suffered a second failure in its reaction-wheel control system, forcing a suspension of its search for alien planets while the space agency determines whether the four-year mission is truly finished.

"It's certainly not good news," Charles Sobeck, deputy project manager for the $600 million mission at NASA's Ames Research Center, told reporters Wednesday.

But Sobeck and other mission managers emphasized that there was still a chance that the probe could be revived. "I wouldn't call Kepler down and out just yet," said John Grunsfeld, associate administrator for science at NASA Headquarters.

The problem has to do with the reaction wheels that are part of Kepler's fine-pointing system. The space telescope identifies worlds in far-off solar systems by watching for the telltale dips in starlight when the planet's disk passes over its parent sun. But in order to make those observations, Kepler has to hold itself in a precise position with the aid of four gyroscopic reaction wheels. One of the wheels failed last July, but Kepler could still do the job with the other three.

On Sunday, however, the spacecraft put itself into safe mode when it couldn't stay in its proper orbit around the sun, 40 million miles (64 million kilometers) from Earth. When the mission team did its regular check-up with Kepler on Tuesday, they found that a second reaction wheel wasn't working. In a mission update, NASA said the problem was probably caused by "a structural failure of the wheel bearing."

That forced an end to Kepler's planet quest. "We need three wheels in service to give us the pointing precision to make this work," the mission's principal investigator, William Borucki of NASA Ames, told NBC News.

Sobeck said the spacecraft itself could remain stable as long as it had fuel for its thrusters, but the thrusters aren't capable of providing the precise pointing that Borucki and his colleagues need. Over the next several months, members of the Kepler team will assess their technical options, and gauge what kind of science could be accomplished using those options, said Paul Hertz, astrophysics director at NASA Headquarters. 

There's still a chance that the reaction-wheel system could be restored — for example, by trying to spin the wheel backward and forward, just as you might spin the wheels of a car that's stuck in a snowdrift. Sobeck said it might even be possible to revive the wheel that was shut down last July. "When we turn it on, it just might start spinning, we don't know," he said.

But mission managers also acknowledged that Kepler's planet-hunting days may be over. Hertz pointed out that the spacecraft outlasted its 3.5-year primary mission, and was currently into an extended mission costing $20 million a year.

Even if the spacecraft's control system can't be revived, it will still take another couple of years to analyze the trillions of bits of data already collected, Borucki said. The mission has already identified 132 confirmed planets and 2,740 additional candidates yet to be confirmed. Some of those worlds are thought to lie within the habitable zones of their planetary systems.

"The prime reason for the existence of this mission is to determine whether Earths are common or rare in our galaxy," Borucki said. So far, the evidence suggests that there are billions of Earth-size planets in the Milky Way. Scientists have not yet identified an Earth-size planet in an Earthlike orbit around a sunlike star, but Borucki voiced confidence that the crucial evidence was tucked away somewhere in the readings that have already been beamed down from Kepler.

"I am really delighted, frankly, with what we've accomplished," he said.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA's Kepler planet-hunting probe has identified two potentially habitable planets only a little bigger than Earth, circling a star that's 1,200 light-years away. The planets could conceivably be covered by a global ocean, and they may well lead the growing list of alien worlds that can host life as we know it.

"These two planets are our best candidates for planets that might be habitable," said Bill Borucki, a space scientist at NASA's Ames Research Center who is the principal investigator for the $600 million Kepler mission.

The two habitable-zone planets, Kepler-62e and Kepler-62f, are part of a five-planet system that lies in the constellation Lyra, within a patch of sky that's been monitored by the Kepler space telescope over the past four years. The Kepler-62 parent star is about two-thirds the size of our own sun and about a fifth as bright.  Three of the star's confirmed planets circle the star in orbits so close that they'd be too hot for life. But the e and f planets are considered to lie in a zone where liquid water could exist, a ring of space that's defined as the habitable zone.

Water worlds?
Two members of the Kepler science team say their modeling suggests the two planets could be "water worlds" — with no land in sight.

"These planets are unlike anything in our solar system. They have endless oceans," Lisa Kaltenegger, an astronomer at the Max Planck Institute for Astronomy and the Harvard-Smithsonian Center for Astrophysics, said in a news release. "There may be life there, but could it be technology-based like ours?"

The report on the Kepler-62 system was published online on Thursday by the journal Science, and was the focus of a NASA news conference timed to coincide with publication. The water-world analysis, authored by Kaltenegger and Harvard's Dimitar Sasselov and Sarah Rugheimer, is contained in a separate paper that has been accepted for publication in The Astrophysical Journal.

The characterization of the two planets' habitability is based on an analysis of their size, plus what's known about the parent star. The Kepler data can show how wide a planet is, and how quickly it makes its orbit, by analyzing the telltale dips in light as the planet passes over its parent star. But Kepler can't make direct observations of a planet's mass. So, in Kepler-62's case, scientists had to make educated guesses about the planets' mass, composition and whether they had atmospheres.

Kepler-62e is 1.6 times as wide as Earth and orbits its star every 122.4 Earth days. Kepler-62f is 1.4 times Earth's width, with an orbital period of 267.3 Earth days. "It's highly likely they're rocky planets," Borucki told NBC News. "They might be water worlds, but they are so different, we just don't know."

David A. Aguilar / CfA

This artist's conception shows Kepler-62f as an ice-covered world, and Kepler-62e as an Earthlike planet with dense clouds. Other planets follow closer-in orbits and are not considered habitable.

NASA Ames / JPL-Caltech

The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring two-thirds the size of the sun and one-fifth as bright.

What would life be like?
Astrobiologists say the fact that the planets are bigger than Earth wouldn't be an obstacle for life. In fact, some experts argue that a super-Earth is more likely to have life than an Earth-sized planet. "If you and I walked on it, our weight would double," Borucki said. "But my weight has doubled since I was a teenager ... so we could do it."

If the planets had atmospheres like Earth's, Kepler-62e's surface temperature would be 86 degrees Fahrenheit (30 degrees Celsius), while Kepler-62f's temperature would be 19 degrees below zero F (-28 degrees C), Borucki said. "You'd see the sun being substantially larger than our sun, because it's so much closer," he said. "But it'd be darker, like walking around on a cloudy day."

In their research paper, Kaltenegger and Sasselov assume that Kepler-62e has a slightly cloudier atmosphere than Earth's, and that Kepler-62f has a thick carbon-dioxide atmosphere with a strong greenhouse effect. Without a thick atmosphere, Kepler-62f could get chillier than Mars. It might even look more like a Europa-style iceball than a Kevin Costner-style water world.

"Kepler-62e probably has a very cloudy sky, and is warm and humid all the way to the polar regions," Sasselov said. "Kepler-62f would be cooler, but still potentially life-friendly. The good news is, the two would exhibit distinctly different colors and make our search for signatures of life easier on such planets in the near future."

Habitable worlds ahead
Kepler-62e and Kepler-62f aren't the first habitable-zone planets to be identified by the Kepler team, and they won't be the last. A year and a half ago, Kepler-22b came to light as the mission's first potentially habitable planet. It's 2.4 times wider than Earth, which puts it halfway between our planet and Neptune on the size scale. Kepler-47c, unveiled last year, is also a habitable-zone planet — but it's 4.6 times wider than Earth, which makes it Neptune-sized.

This January, the science team discussed the habitability of another candidate planet, then known as KOI 172.02. The existence of that world has now been confirmed under the name Kepler-69c, with a size that's 1.7 times Earth's width. "Today we can announce that this is a bona fide planet," Thomas Barclay, an astronomer at Ames Research Center, said during Thursday's news conference. 

Three months ago, Kepler-69c was hailed as potentially the most Earthlike world detected beyond our solar system, but now researchers say Kepler-62e and Kepler-62f could be stronger contenders.

There will be more contenders ahead: Borucki said about four dozen of the more than 2,700 candidate planets being tracked by Kepler lie within their stars' habitable zones, and it takes about a year to confirm each candidate's existence through detailed analysis. "We really wish we were faster," he told NBC News. "I really wish we could knock off one a week."

Boruckin and his colleagues are poring through the oceans of observations coming in from the Kepler telescope, and although the spacecraft has had its problems, he's hoping that the flood of data will continue for years to come.

"When you're born a scientist, they leave out the gene for saying, 'We have enough data,'" Borucki joked.

More about the planet hunt:

• Alien Earths in our backyard?
• 17 billion hot Earths in our galaxy
• NBC News on planets | Cosmic Log on Kepler

Borucki, Kaltenegger, Sasselov and Barclay are among 64 authors of the Science paper, "Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone." Barclay and Borucki are among 31 authors of "A Super-Earth-Sized Planet Orbiting in or near the Habitable Zone around Sun-like Star," published in The Astrophysical Journal. Kaltenegger, Sasselov and Rugheimer are the authors of "Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the Case of Kepler-62e and -62f."

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA / Ames / JPL-Caltech

An artist's conception shows the planet Kepler-37b. This planet is slightly larger than our own moon and orbits its host star every 13 days. Its surface temperature is probably around 427 degrees Celsius (800 degrees Fahrenheit).

The discovery of a strange new world about the size of Earth's moon has shattered the record for the smallest known alien planet, scientists say.

The newfound alien planet Kepler-37b is the first exoplanet discovered to be smaller than Mercury. It whips around its parent star every 13 days and has a roasting surface temperature of about 800 degrees Fahrenheit (427 degrees Celsius), researchers said. It not a promising contender for life, they added.

Astronomers found Kepler-37b and two other, larger planets (called Kepler-37c and Kepler-37d) orbiting a star about 215 light-years from Earth using NASA's prolific Kepler space telescope. Finding such a small exoplanet with the Kepler spacecraft was a stretch, but some attributes of Kepler-37b's parent star made the discovery possible.

The star has few sunspots and is bright relative to its planet, making it easier for the Kepler spacecraft to spot the telltale dimming that takes place when a planet passes in front of its star, which scientists call a transit. That method revealed not just the presence of Kepler-37b, but two siblings traveling in orbits farther from the parent star. [Gallery: The Smallest Alien Planets]

"There are not many signals masking the transit," study leader Thomas Barclay of NASA's Ames Research Center in Moffett Field, Calif., told Space.com. "What makes this exceptional was [that] this dip of brightness was just 22 parts per million."

Too hot to host life
Kepler-37b and its siblings, 37c and 37d, are probably uninhabitable, scientists said. All three planets lie close to their parent star, well inside the Earth-sun distance (called astronomical units, or AU). One astronomical unit is about 93 million miles (150 million kilometers).

The moon-sized Kepler-37b is so close to its parent star, at just 0.10 AU, that it probably has no atmosphere or liquid water on its surface. The next planet out, Kepler-37c, is slightly smaller than Earth and may have an atmosphere, but it orbits the star at 0.14 AU — a location that's not in the star's habitable zone, where liquid water could exist on the surface.

NASA / Ames / JPL-Caltech

As shown in this comparative graphic, two of the three planets orbiting Kepler-37 are smaller than Earth, while the third is twice Earth's size. Kepler-37b is about 80 percent the size of Mercury,

The biggest planet in the newfound alien solar system is Kepler-37d. It is about twice the size of Earth and orbits the parent star at a distance of 0.2 AU.

"This could hold an atmosphere, but it's unlikely to be a rocky planet — more likely to be gassy — simply because of its size. It could hold some kind of liquid at the surface," Barclay said.

The next step, Barclay added, will be to look for Mercury-sized exoplanets at greater distances from the host star Kepler-37. More planets could be orbiting the star and await discovery.

"We're looking at it very carefully," Barclay said. "There's nothing yet, but something may appear in the data."

Starlight tells the tale
Barclay and his team took great care to confirm the existence of planets around Kepler-37. The researchers knew that a dip in the star's brightness identified by the Kepler spacecraft could have come from several types of sources, including another star that might be passing in front of the Kepler-37 target. So they ran a computer simulation to see if the newfound planet candidates could be false positives.

Using a tool called Blender from the Harvard-Smithsonian Center for Astrophysics, the researchers simulated several false positive scenarios in order to eliminate them. The results made researchers more than 99 percent confident that the planet candidates are actual planets, Barclay said.

More about planets from Cosmic Log

The science team managed to obtain a close approximation of the size of the Kepler-37 star, in addition to spotting its planetary retinue. The star's quiescent nature allowed the researchers to measure it with asteroseismology, a technique that uses acoustic oscillations on the star's surface. The method is similar to how researchers probe the Earth's interior with seismic devices during earthquakes.

The uncertainty for a star's size is typically 20 to 30 percent, Barclay said. In this case, using asteroseismology, the researchers narrowed the uncertainty to 3 percent.

Measurements showed that Kepler-37 is about 75 percent the size of Earth's sun and 80 percent as massive. This places the star within the same stellar class as our sun.

The $600 million Kepler mission launched in March 2009, and has found more than 2,740 candidate planets so far. The spacecraft searches for small dips in stars' light caused by orbiting worlds that pass in front of them periodically, dimming their brightness.

Follow Elizabeth Howell @howellspace, or Space.com @Spacedotcom. We're also on Facebook and Google+.

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© 2013 Space.com. All rights reserved. More from Space.com.

ESO

An artist's impression shows a planetary system around a red dwarf star.

An analysis of data from NASA's Kepler planet-hunting mission suggests that about 6 percent of all red dwarf stars should have habitable, Earth-sized planets — and because red dwarfs are the most common stars in our galaxy, the nearest Earthlike planet could be as close as 13 light-years.

"We thought we would have to search vast distances to find an Earthlike planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," Harvard astronomer Courtney Dressing, the lead author of the data-crunching study, said in a news release.

That doesn't mean we can just hop out the back door and head for a red dwarf: Although 13 light-years is relatively close in astronomical terms, it would still take more than 50,000 years to cover that distance using current propulsion technology. But the finding could lead astronomers to cast a wider net in the search for the conditions conducive to extraterrestrial life.

Dressing presented her team's findings on Tuesday during a news briefing at the Harvard-Smithsonian Center for Astrophysics, or CfA. The research paper is expected to be published in The Astrophysical Journal.

Red dwarfs are thought to account for about 75 percent of the stars in the Milky Way: They're smaller, cooler and fainter than our sun — so faint, in fact, that no red dwarf is visible to the naked eye. But the fact that they're cooler means that closer-in planets are more likely to be habitable.

That's good news for planet-hunters: The Kepler space telescope, which was launched in 2009, detects alien worlds by looking for the subtle dimming of starlight as a planet passes in front of its alien sun. Closer-in planets would cover more of the star's disk, making them easier to detect.

Dressing and her colleagues sifted through the 158,000 stars targeted by the Kepler probe to identify all the red dwarfs. She said the stars' sizes and temperatures were calculated using computer models "that are more appropriate for these small red stars." Previously, those stellar characteristics were derived using a less precise, one-size-fits-all type of computer model, she said.

The fresh analysis showed that almost all of the stars were smaller and cooler than previously thought. That means the worlds detected around those planets would be proportionately smaller as well, bringing more of them into the Earth-sized category.

The astronomers identified 95 Kepler candidate planets that are circling red dwarfs. When they ran those candidates through their fine-tuned computer model, they found that three of them were roughly Earth-sized, with the right temperature to sustain liquid water and life. And when they factored in their estimates for the proportion of planets that would have gone undetected, due to the limitations of the Kepler mission's observing method, they concluded that 6 percent of all red dwarfs should have an Earth-sized, habitable planet.

"That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought," the CfA's David Charbonneau, a co-author of the study, said in Wednesday's news release.

Because our sun is surrounded by a swarm of red dwarfs, the statistics suggest that the most probable distance for such a habitable planet would be 13 light-years, if all the surrounding stars could be examined with a suitable telescope. Kepler isn't designed for such a survey — but a new type of space telescope, or a big enough network of ground-based telescopes, could take on the job. For example, NASA's James Webb Space Telescope, currently due for launch in 2018, could study the thermal characteristics and perhaps even the atmospheres of nearby red-dwarf planets, Charbonneau said.

The researchers said that a habitable planet circling a red dwarf would be markedly different from Earth: It would probably be locked into an orbit that kept one side of the planet perpetually facing its alien sun. Charbonneau said the heat could conceivably be transported around the globe via a thick atmosphere or ocean.

Also, red dwarfs are known to be quite variable in their emissions, with occasional strong flares of ultraviolet light. "If that were to happen on Earth, it would cause havoc," Charbonneau told journalists.

But Dressing said alien life could conceivably adapt to such stresses. "You don't need an Earth clone to have life," she said.

Red-dwarf planets might have at least one edge over Earth in the habitability department: Astrobiologists have estimated that our planet could be rendered inhospitable to life in the next couple of billion years, due to a long-term increase in solar radiation. Red dwarfs are different in that regard. "They are incredibly long-lived," Charbonneau told journalists. "They never show their age."

He said it's conceivable that some of the planets circling red dwarfs could remain habitable for 10 billion years or more.

Caltech astronomer John Johnson said the newly reported research marks one more step toward taking the search for alien Earths out of the realm of science fiction and putting it squarely in the realm of science fact. "Now I think the conversation is starting in earnest," he said. 

More about the planet search:

• Estimate suggests our galaxy has 17 billion hot Earths
• How to take a trip to Alpha Centauri ... and why
• Kepler space telescope recovers from glitch
• Cosmic Log archive on planets

The three habitable-zone planetary candidates identified in this study are Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI 2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI 854.01, 1.7 times the size of Earth in a 56-day orbit. All three are located about 300 to 600 light-years away and orbit stars with temperatures between 5,700 and 5,900 degrees Fahrenheit (3,150 to 3,260 degrees Celsius). For comparison, our sun’s surface is 10,000 degrees F, or 5,500 degrees C.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

First published 11 a.m. ET Feb. 6. Last updated 12:53 p.m. ET Feb. 6.

NASA / JPL-Caltech / T. Pyle

An artist's rendering shows a typical close-in Earth-size planet, Kepler-20e, which is about 0.87 times as wide as our planet but orbits its parent star more closely than Mercury orbits our sun.

A simulation based on data from NASA's planet-hunting Kepler mission has determined that about one out of every six stars has an Earth-sized planet, which would translate to at least 17 billion such worlds in our Milky Way galaxy. And that's not even counting the alien Earths we'd want to live on.

These 17 billion planets would be circling their parent stars more closely than Mercury orbits our own sun — which means that, in many cases, the planets would be too hot for liquid water to exist. A few such worlds already have been found, including a "lava planet" known as Alpha Centauri Bb that's just 4.3 light-years away from us.

Someday, the type of simulation that astronomers used to estimate the number of hot Earths can be used to estimate how many habitable Earths could provide a home for life as we know it in the Milky Way. But not just yet.

"For an estimate of Earth-sized planets in the habitable zone, it's simply too early to call," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, or CfA.

Fressin and his colleagues lay out their estimates for Earth-sized planets, as well as bigger worlds, in a paper that's been accepted for publication in the Astrophysical Journal. Their research is being discussed today at the American Astronomical Society's winter meeting in Long Beach, Calif.

The estimates are based on a list of 2,400 planet candidates that have been detected by the Kepler probe since its launch in 2009. Kepler looks for planets in a patch of sky overlapping the constellations Cygnus and Lyra, by checking for the faint dimming of a star as an alien world passes across its disk. One of the challenges is to make sure the dimming is really caused by a planet, rather than some other phenomenon such as an eclipsing binary star. Another challenge is that Kepler is sure to miss some planets, because those planets are not in a position to block the light of its parent star, as seen from Earth.

Now that the Kepler mission has been churning out detections for more than three years, there's enough of a database to arrive at some statistical conclusions about the total number of planets in the Milky Way — at least 100 billion. There's also enough data to determine what the breakdown of detections should be, and even how many of those detections will be wrong.

"We have a knowledge of false positives that's good enough that we can do a study from scratch," Fressin said.

The simulation suggests that the false-positive rate should vary depending on the size of the planet candidates, from a low of 6.7 percent for small Neptune-scale planets to a high of 17.7 percent for Jupiter-type giants. The false-positive rate for close-in planets between 0.8 and 1.25 times as wide as Earth is 12.3 percent. When all these factors were added to the calculations, the astronomers arrived at a breakdown for five types of planets currently detectable by Kepler:

• 17 percent for Earths with orbital periods up to 85 days.
• 26 percent for super-Earths (1.25 to 2 times as wide as Earth) with orbits up to 145 days.
• 26 percent for small Neptunes (2 to 4 times Earth's width) with orbits up to 245 days.
• 3 percent for large Neptunes (4 to 6 times Earth's width) with orbits up to 418 days.
• 5 percent for giants (6 to 22 times Earth's width) with orbits up to 418 days.

The results indicate that for every size of planet except for gas giants, the type of star doesn't matter. Earth-sized planets should be just as likely to form around red dwarfs as around sunlike stars. That runs counter to what was previously thought.

"Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods," the CfA's Guillermo Torres, a co-author of the study, said in a news release.

The researchers emphasized that these are just minimum estimates — and that as Kepler provides more planet candidates at smaller scales and wider orbits, the numbers could increase. Eventually, such simulations could spit out a long-sought number: the tally of Earth-sized planets in the Milky Way expected to have conditions capable of supporting life.

"This result is a significant step towards the determination of eta-earth, the occurrence of Earthlike planets in the habitable zone of their parent stars," they wrote in their research paper.

More about the planet search:

• Kepler mission adds 461 potential planets to list
• 'Exocomets' are common across the Milky Way
• 2013 might be the year for first 'alien Earth'
• Alien planets face danger from binary stars
• Cosmic Log archive on the planet search

In addition to Fressin and Torres, the authors of "The False Positive Rate of Kepler nd the Occurrence of Planets" include David Charbonneau, Stephen Bryson, Jessie Christiansen, Courtney Dressing, Jon Jenkins, Lucianne Walkowicz and Natalie Batalha.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.