In Search of Earth 2.0
NASA’s Kepler spacecraft is a prolific hunter of planets outside of our Solar System. Since it launched in 2009, Kepler has confirmed more than 1000 of these exoplanets and catalogued thousands of more candidates. The menagerie includes planets with densities greater than iron and lower than Styrofoam; planets smaller than Mercury and bigger than Jupiter; planets with one sun, two suns, and even four suns!
The number and variety of exoplanets is exhilarating to researchers, but in some ways, the collection leaves something to be desired. Lay people and scientists alike share a longing to find one particular type of world—a world like Earth—that speaks to us in a way that “hot Jupiters” and multiple star systems cannot.
We long for a world whose very existence cries “We are not alone.”
Kepler has just found the closest analog to the Earth-Sun system.
In July 2015, NASA announced the discovery of Kepler-452b, an exoplanet a little over 1½ times wider than Earth orbiting a G2-type star similar to our own Sun. Moreover, Kepler-452b’s orbit is located in the “Goldilocks Zone” of its star system where the temperature is just right for liquid water to exist. This is also known as the “habitable zone” because water is required for life as we know it.
“We can think of Kepler-452b as an older, bigger cousin to Earth," says Jon Jenkins of NASA's Ames Research Center, who led the team that discovered Kepler-452b. "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.”
Kepler-452b isn’t the first exoplanet found in a habitable zone. Indeed, about a dozen such worlds in this size range have been discovered so far -- that is, 10 to 15 planets between one-half and twice the diameter of Earth, depending on how the habitable zone is defined and allowing for uncertainties in measured planetary sizes.
Among those dozen, Kepler-452b fires the imagination most because it is the closest analog to the Earth-sun system discovered to date: a planet only a little over one-and-a-half times the diameter of Earth orbiting within the habitable zone of a star very much like our own Sun.
A year on Kepler-452b is 385 days long, only a few weeks longer than a year on Earth. The planet is only 5 percent farther from its parent star than Earth is from the Sun. This extra distance is mitigated by extra sunlight. Kepler-452b’s parent star is 20 percent brighter and has a diameter 10 percent larger than the Sun. The similarities are remarkable, indeed.
Says John Grunsfeld, Associate Administrator of NASA’s Science Mission Directorate at the agency’s headquarters in Washington, DC. “This exciting result brings us one step closer to finding an Earth 2.0."
Kepler-452b whispers the possibility, that we are not alone. How much longer before a new discovery shouts it out?
For updates from the planet hunt, stay tuned to science.nasa.gov