The Biggest Alien Planet Discoveries of 2014

This past year was a banner one for the field of exoplanet science, with the tally of known alien worlds doubling to nearly 2,000 by the end of 2014.

Here's a look at the top exoplanet discoveries of 2014, from the first potentially habitable Earth-size world to a staggering haul of 715 newly announced alien planets:

'Earth's cousin'

In April, scientists announced the discovery of Kepler-186f, the first known Earth-size planet that resides in its star's "habitable zone" — the range of distances that could support the existence of liquid water on a world's surface. [10 Exoplanets That Could Host Alien Life]

As its name suggests, Kepler-186f was found by NASA's prolific Kepler space telescope. The planet lies 490 light-years from Earth and is just 10 percent wider than our home world. Kepler-186f is not the elusive "Earth twin" that astronomers have long sought; the planet circles a red dwarf, a star smaller and dimmer than the sun. But Kepler-186f is a member of the family nonetheless, with its discoverers characterizing it as an "Earth cousin."

A habitable world next door?

Kepler-186f isn't the only planet found last year that might be capable of supporting life. A world called Gliese 832c is also potentially habitable — and it lies just 16 light-years away, a mere stone's throw considering the vast scale of the universe.

Astronomers found Gliese 832c, which also orbits a red dwarf, using three different ground-based instruments. The exoplanet is a "super Earth" at least five times as massive as Earth, its discoverers say. While Gliese 832c may be habitable, it could also resemble scorching-hot Venus, whose thick atmosphere has led to a runaway greenhouse effect.
Artist's concept of the potentially habitable super Earth Gliese 832c, against a background of a stellar nebula.

715 newfound exoplanets

Exoplanet discoveries usually come in drips and drops, but in February, the Kepler team unleashed a torrent: Researchers announced the spacecraft had spotted 715 new alien worlds, nearly doubling the known population in one fell swoop.

More than 90 percent of the newfound planets are smaller than Neptune, and four of them are habitable-zone worlds less than 2.5 times the size of Earth, scientists said.

Researchers confirmed this huge haul of Kepler planets using a technique called "validation by multiplicity," which relies on probability and statistics rather than additional observations by other telescopes.

'The Godzilla of Earths'

Another headliner from 2014 is Kepler-10c, a planet about 17 times more massive than Earth. Such hefty worlds were thought to be primarily gaseous, but Kepler-10c is rocky.

Kepler-10c is therefore the first known member of a new class of exoplanets, the "mega-Earths." This "Godzilla of Earths," as one of its discoverers described Kepler-10c, orbits a sunlike star that lies about 560 light-years from Earth.

Gas dwarfs

Just as rocky planets can apparently be much larger than previously thought, gaseous worlds can be surprisingly small. That's the conclusion of another 2014 study, which laid out the classification of "gas dwarf" exoplanets.

After studying more than 600 newfound Kepler planets, the researchers determined that worlds less than 1.7 times the size of Earth are likely to be rocky, while those at least 3.9 times bigger than our planet are gaseous. Most worlds between these two extremes are probably "gas dwarfs," planets with rocky cores and thick hydrogen-helium atmospheres that never grew to the size of Saturn, Jupiter and other gas giants, the study found.

The first exomoon?

Astronomers may have detected the moon of an alien planet for the first time in 2014, but we'll never know for sure.

The team used a technique called gravitational microlensing, which notes how a foreground object's gravity warps the light from a distant star when it passes in front of the star from Earth's perspective. The researchers saw one lensing event caused by a foreground object that could be one of two things: a free-flying "rogue planet" with a rocky exomoon, or a small star that hosts a planet about 18 times more massive than Earth.

Unfortunately, there's no way to follow up on the find, because microlensing events are random encounters. So the search for the first confirmed exomoon continues.

The first exoplanet of Kepler's new mission

Kepler's original exoplanet hunt ground to a halt in May 2013, when the second of its four orientation-maintaining reaction wheels failed. But the Kepler team devised a way to stabilize the observatory using sunlight pressure, and in May 2014, NASA approved a new, two-year mission for the spacecraft called K2, during which it has been hunting for alien planets, supernova explosions and other cosmic phenomena.

The first K2 exoplanet is now in the books. In December, researchers announced that Kepler had discovered a world called HIP 116454b, which is about 2.5 times bigger than Earth and lies 160 light-years away.

However successful K2 turns out to be, the new mission won't approach the exoplanet tally Kepler racked up during its pre-glitch operations. Kepler's original mission netted nearly 4,200 planet candidates, nearly 1,000 of which have been confirmed to date. Kepler scientists expect about 90 percent of the candidates will turn out to be bona fide planets.

The oldest potentially habitable alien planet

Also this year, astronomers announced the discovery of Kapteyn b, a super Earth that orbits in the habitable zone of a red dwarf located just 13 light-years away from our solar system.

Kapteyn b is 11.5 billion years old, making it the most ancient known planet that may be capable of supporting life. To put that age into perspective: Earth is less than 4.6 billion years old, while the universe itself was born 13.8 billion years ago. So if life took root early in Kapteyn b's history, it has had a very long time to evolve.
Artist's concept of the potentially habitable world Kapteyn b with the globular cluster Omega Centauri in the background. Kapteyn b lies just 13 light-years from Earth.

Planets around every star?

Another 2014 study suggests that virtually every red dwarf in the Milky Way galaxy hosts at least one planet — and that at least 25 percent of these small, dim stars in the sun's own neighborhood host habitable-zone worlds.

That translates to a lot of life-friendly real estate; red dwarfs make up at least 70 percent of the galaxy's 100 billion or so stars.

The team arrived at these conclusions after analyzing observations made by two instruments in Chile — the High Accuracy Radial velocity Planet Searcher (HARPS) and the Ultraviolet and Visual Echelle Spectrograph (UVES). The results bolster previous findings made by researchers who looked at Kepler data, indicating that the Milky Way is teeming with billions of planets.

Earth-like worlds in two-star systems?

This year, for the first time, astronomers found a rocky planet in an Earth-like orbit around a single star in a two-star system.

The world, known as OGLE-2013-BLG-0341LBb, lies about 3,000 light-years from Earth and is likely too cold to support life as we know it (it circles a red dwarf). And it's not the first planet to be spotted in a two-star system, or the first one known to circle just one of a binary's two stars.

But the discovery of OGLE-2013-BLG-0341LBb is significant, showing that rocky planets can form relatively far from their host stars even in two-star systems, researchers said. (Previously, it was thought that a nearby companion star might disrupt the planet-forming disk too much for this to occur.) Its existence suggests that habitable planets may be more common than scientists had supposed; half of all Milky Way stars exist in binary systems.

NASA's Kepler Reborn, Makes First Exoplanet Find of New Mission

NASA's planet-hunting Kepler spacecraft makes a comeback with the discovery of the first exoplanet found using its new mission -- K2.
The discovery was made when astronomers and engineers devised an ingenious way to repurpose Kepler for the K2 mission and continue its search of the cosmos for other worlds.
"Last summer, the possibility of a scientifically productive mission for Kepler after its reaction wheel failure in its extended mission was not part of the conversation," said Paul Hertz, NASA's astrophysics division director at the agency's headquarters in Washington. "Today, thanks to an innovative idea and lots of hard work by the NASA and Ball Aerospace team, Kepler may well deliver the first candidates for follow-up study by the James Webb Space Telescope to characterize the atmospheres of distant worlds and search for signatures of life."
Lead researcher Andrew Vanderburg, a graduate student at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, studied publicly available data collected by the spacecraft during a test of K2 in February 2014. The discovery was confirmed with measurements taken by the HARPS-North spectrograph of the Telescopio Nazionale Galileo in the Canary Islands, which captured the wobble of the star caused by the planet's gravitational tug as it orbits.
The newly confirmed planet, HIP 116454b, is 2.5 times the diameter of Earth and follows a close, nine-day orbit around a star that is smaller and cooler than our sun, making the planet too hot for life as we know it. HIP 116454b and its star are 180 light-years from Earth, toward the constellation Pisces.
Kepler's onboard camera detects planets by looking for transits -- when a distant star dims slightly as a planet crosses in front of it. The smaller the planet, the weaker the dimming, so brightness measurements must be exquisitely precise. To enable that precision, the spacecraft must maintain steady pointing. In May 2013, data collection during Kepler's extended prime mission came to an end with the failure of the second of four reaction wheels, which are used to stabilize the spacecraft.
Rather than giving up on the stalwart spacecraft, a team of scientists and engineers crafted a resourceful strategy to use pressure from sunlight as a "virtual reaction wheel" to help control the spacecraft. The resulting K2 mission promises to not only continue Kepler's planet hunt, but also to expand the search to bright nearby stars that harbor planets that can be studied in detail, to help scientists better understand their composition. K2 also will introduce new opportunities to observe star clusters, active galaxies and supernovae.
Small planets like HIP 116454b, orbiting nearby bright stars, are a scientific sweet spot for K2 as they are good prospects for follow-up ground studies to obtain mass measurements. Using K2's size measurements and ground-based mass measurements, astronomers can calculate the density of a planet to determine whether it is likely a rocky, watery or gaseous world.
"The Kepler mission showed us that planets larger in size than Earth and smaller than Neptune are common in the galaxy, yet they are absent in our solar system," said Steve Howell, Kepler/K2 project scientist at NASA's Ames Research Center in Moffett Field, California. "K2 is uniquely positioned to dramatically refine our understanding of these alien worlds and further define the boundary between rocky worlds like Earth and ice giants like Neptune."
Since the K2 mission officially began in May 2014, it has observed more than 35,000 stars and collected data on star clusters, dense star-forming regions, and several planetary objects within our own solar system. It is currently in its third campaign.
The research paper reporting this discovery has been accepted for publication in The Astrophysical Journal.