Kepler discovers "Earth's bigger, older cousin"

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Artist's concept of Kepler-425b(Credit: NASA Ames/JPL-Caltech/T. Pyle)

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The odds of finding a habitable planet outside of our Solar System got a significant boost today, as NASA announced the discovery of the most Earthlike world orbiting the most Sunlike star yet. Named Kepler-425b, the new world located 1,400 light-years away in the constellation Cygnus was detected by the Kepler space telescope. It has been characterized by the space agency as "Earth's bigger, older cousin."

According to NASA, planet Kepler-425b is remarkable because it combines a number of factors that make it the most likely candidate for habitability found outside the Solar System so far. It's not only roughly Earth-sized, but it also orbits in the habitable zone of a sun-like star.

To date, the Kepler mission has confirmed the existence of 1,030 exoplanets, but so far the search for a planet similar enough to Earth to support life has been an exercise in extremely rough approximations. The technique used by the unmanned Kepler space telescope (launched in 2009) to detect exoplanets relies on measuring the dip in a star's light as a planet passes in front of it. This tends to favor finding large planets orbiting very close to their stars, which means that the easiest ones to find have been very hot super-Jupiters.

Another problem is that even if a small rocky planet like Earth is found, it has to be in its star's habitable zone. That is, its distance from its star is such that the possible surface temperature could allow liquid to exist, so it can't be too close or too far away.

However, one particular sticking point until today is the kind of star these planets revolve around. The most common type of stars in our galaxy are red dwarfs, which are K or M types. Since they're so numerous, it's no surprise that most exoplanets are found orbiting them, but these are small, old, and cold stars, which poses many problems for the habitability of their planets.

Earth, on the other hand, orbits a young, medium-sized G2 star, which is relatively warm and stable with a lifespan long enough for life to establish itself. Since so little is known about other star systems, and ours is the only one known to have life, scientists tend to favor sun-like stars as the best candidates for habitable planets.

These are the reasons why Kepler-425b has NASA so excited. According to NASA scientists at today's press conference, they believe from computer models that it's a rocky planet 60 percent larger than Earth with five times the mass and a surface gravity twice that of ours. It may have active volcanoes, and an atmosphere that may be thicker than Earth's. Equally important, its 385-day orbit is similar to that of Earth and is only five percent farther from its star – putting it smack in the habitable zone.

But the clincher is that the planet's star, Kepler-425, is also a G2-type. It's 6 billion years old, which makes it older than the Sun by 1.5 billion years, is 10 percent bigger and 20 percent brighter, but has the same temperature, though it emits more energy. Put this all together and Kepler-425b is the best chance yet for an exoplanet where life could exist. However, NASA does point out that the composition of the atmosphere and other factors have yet to be determined, so don't pack your bags yet.

NASA says that the Kepler data was confirmed by ground observations from the University of Texas at Austin's McDonald Observatory, the Fred Lawrence Whipple Observatory on Mt. Hopkins, Arizona, and the W. M. Keck Observatory atop Mauna Kea in Hawaii.

"We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment," says Jon Jenkins, Kepler data analysis lead at NASA's Ames Research Center in Moffett Field, California. "It’s awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star; longer than Earth. That’s substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet."

NASA also announced today that the latest Kepler findings have produced 521 new planetary candidates. This brings the total to 4,696, with 12 Earthlike candidates orbiting in their stars' habitable zones.

The recent discoveries have been accepted for publication in The Astronomical Journal.

The video below outlines the movement of Kepler-425b and the Earth through their respective habitable zones over time.


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