NASA Kepler celebrates five years in space

The panel features an image of NASA Kepler's launch and artist concepts of milestone discoveries (l to r): Kepler-9b and c, Kepler-10b, Kepler-11, Kepler-16b, Kepler-22 and Kepler-64f. 

The final panel illustrates exoplanet discoveries: blue is previous; red is previous Kepler; gold is Kepler's on Feb. 26. 

Credit: NASA Ames/W. Stenzel

Five years ago today, on March 6, 2009, NASA Kepler Space Telescope rocketed into the night skies above Cape Canaveral Air Force Station in Florida to find planets around other stars, called exoplanets, in search of potentially habitable worlds.

Since then, NASA Kepler has unveiled a whole new side of our galaxy—one that is teeming with planets. Because of Kepler we now know that most stars have planets, Earth-sized planets are common, and planets quite unlike those in our solar system exist.

By analyzing Kepler data, scientists have identified more than 3,600 candidates believed to be planets, and verified that 961 of those candidates actually are planets, many as small as Earth.

Discoveries made using Kepler now account for more than half of all the known exoplanets.

William Borucki

"During the last five years, Kepler has produced results needed to take the next big step forward in humankind's search for life in our galaxy, providing information needed for future missions that will ultimately determine the atmospheric composition of Earth-sized exoplanets to discover if they could be habitable," said William Borucki, Kepler principal investigator at NASA's Ames Research Center in Moffett Field, Calif.

Kepler's finds include planets that orbit in the habitable zone, the range of distances from a star where the surface temperature of an orbiting planet may be suitable for life-giving liquid water.

This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. 

The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. 

Liquid water is essential for life on Earth.

Kepler-22's star is a bit smaller than our sun, so its habitable zone is slightly closer in. 

The diagram shows an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. 

The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star. It's about 2.4 times the size of Earth.

Image credit: NASA/Ames/JPL-Caltech

One example of a habitable zone planet found by the mission is known as Kepler-22b.

At 2.4 times the size of Earth, it is thought to be too big to be rocky and support life.

The artist's concept depicts Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.

Scientists believe other habitable zone planets found by the Kepler mission might be rocky, such as Kepler-62f, which is 40 percent larger in size than Earth.

A twin to Earth, a planet with the same temperature and size as Earth, has not yet been identified, but the analysis is far from over as scientists continue to search the Kepler data for the tiny signature of such a planet.

Other Kepler discoveries include hundreds of star systems hosting multiple planets, and have established a new class of planetary system where planets orbit more than one sun.

In August of last year, the mission ended its science observations after a faulty reaction wheel affected the telescope's ability to point precisely.

The mission may be able to operate in a different mode, and continue to do science.

This next-generation mission proposal, called K2, will be considered for funding by NASA in the 2014 Astrophysics Senior Review of Operating Missions.

NASA Kepler finds a very wobbly planet - Kepler-413b Binary System

This illustration shows the unusual orbit of planet Kepler-413b around a close pair of orange and red dwarf stars. 

The planet's 66-day orbit is tilted 2.5 degrees with respect to the plane of the binary star's orbit. 

The orbit of the planet wobbles around the central stars over 11 years, an effect called precession. 

This planet is also very unusual in that it can potentially precess wildly on its spin axis, much like a child's top. 

Credit: NASA, ESA, and A. Feild (STScI)

Imagine living on a planet with seasons so erratic you would hardly know whether to wear Bermuda shorts or a heavy overcoat.

That is the situation on a weird, wobbly world found by NASA's planet-hunting Kepler space telescope.

The planet, designated Kepler-413b, precesses, or wobbles, wildly on its spin axis, much like a child's top.

NASA's planet-hunting Kepler space telescope

The tilt of the planet's spin axis can vary by as much as 30 degrees over 11 years, leading to rapid and erratic changes in seasons.

In contrast, Earth's rotational precession is 23.5 degrees over 26,000 years.

Researchers are amazed that this far-off planet is precessing on a human timescale.

Precession : the axis of rotation of a precessing body itself rotates around another axis.

Kepler 413-b is located 2,300 light-years away in the constellation Cygnus. It circles a close pair of orange and red dwarf stars every 66 days.

Constellation Cygnus

The planet's orbit around the binary stars appears to wobble, too, because the plane of its orbit is tilted 2.5 degrees with respect to the plane of the star pair's orbit.

As seen from Earth, the wobbling orbit moves up and down continuously.

Kepler finds planets by noticing the dimming of a star or stars when a planet transits, or travels in front of them.

Normally, planets transit like clockwork. Astronomers using Kepler discovered the wobbling when they found an unusual pattern of transiting for Kepler-413b.

Veselin Kostov

"Looking at the Kepler data over the course of 1,500 days, we saw three transits in the first 180 days—one transit every 66 days—then we had 800 days with no transits at all. After that, we saw five more transits in a row," said Veselin Kostov, the principal investigator on the observation.

Kostov is affiliated with the Space Telescope Science Institute (STSCI) and Johns Hopkins University in Baltimore, Md.

The next transit visible from Earth's point of view is not predicted to occur until 2020.

This is because the orbit moves up and down, a result of the wobbling, in such a great degree that it sometimes does not transit the stars as viewed from Earth.

Astronomers are still trying to explain why this planet is out of alignment with its stars. There could be other planetary bodies in the system that tilted the orbit.

Or, it could be that a third star nearby that is a visual companion may actually be gravitationally bound to the system and exerting an influence.

Peter McCullough

"Presumably there are planets out there like this one that we're not seeing because we're in the unfavourable period," said Peter McCullough, a team member with the Space Telescope Science Institute (STSCI) and Johns Hopkins University.

"And that's one of the things that Veselin is researching: Is there a silent majority of things that we're not seeing?"

Even with its changing seasons, Kepler-413b is too warm for life as we know it.

Because it orbits so close to the stars, its temperatures are too high for liquid water to exist, making it inhabitable.

It also is a super Neptune—a giant gas planet with a mass about 65 times that of Earth—so there is no surface on which to stand.

Mini-Neptunes: First transiting planets in a star cluster discovered

In the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars. Credit: Michael Bachofner

All stars begin their lives in groups. Most stars, including our Sun, are born in small, benign groups that quickly fall apart.

Others form in huge, dense swarms that survive for billions of years as stellar clusters.

Within such rich and dense clusters, stars jostle for room with thousands of neighbors while strong radiation and harsh stellar winds scour interstellar space, stripping planet-forming materials from nearby stars.

It would thus seem an unlikely place to find alien worlds.

Yet 3,000 light-years from Earth, in the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars.

The discovery, published in the journal Nature, shows that planets can develop even in crowded clusters jam-packed with stars.

"Old clusters represent a stellar environment much different than the birthplace of the Sun and other planet-hosting field stars," says lead author Soren Meibom of the Harvard-Smithsonian Center for Astrophysics (CfA).

"And we thought maybe planets couldn't easily form and survive in the stressful environments of dense clusters, in part because for a long time we couldn't find them."

The two new alien worlds appeared in data from NASA's Kepler spacecraft. Kepler hunts for planets that transit, or cross in front of, their host stars.

During a transit, the star dims by an amount that depends on the size of the planet, allowing the size to be determined.

Kepler-66b and Kepler-67b are both less than three times the size of Earth, or about three-fourths the size of Neptune (mini-Neptunes).

Of the more than 850 known planets beyond our solar system, only four - all similar to or greater than Jupiter in mass - were found in clusters.

Kepler-66b and -67b are the smallest planets to be found in a star cluster, and the first cluster planets seen to transit their host stars, which enables the measurement of their sizes.

Meibom and his colleagues have measured the age of NGC 6811 to be one billion years. Kepler-66b and Kepler-67b therefore join a small group of planets with precisely determined ages, distances, and sizes.

Considering the number of stars observed by Kepler in NGC 6811, the detection of two such planets implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars (stars not within a cluster or association) in the Milky Way galaxy.

"These planets are cosmic extremophiles," says Meibom. "Finding them shows that small planets can form and survive for at least a billion years, even in a chaotic and hostile environment."

More information: Paper: DOI: 10.1038/nature12279