Earth’s bigger, older cousin

An artist’s conception of a possible appearance of the planet Kepler-452b.

ON July 23, NASA announced that its Kepler mission had found the first near-earth-size exoplanet in the “habitable zone” around a sun-like star. Called Kepler-452b, it is the smallest planet to date discovered orbiting in the habitable zone—the area around a star where the temperature is right for liquid water to form on the surface of an orbiting planet—of a G2-type star, like the sun. The confirmation of Kepler-452b brings the total number of confirmed planets to 1,030. “This exciting result brings us one step closer to finding an earth 2.0,” said John Grunsfeld of NASA.

While Kepler-452b’s mass and composition are not yet determined, previous research suggests that planets of its size have a good chance of being rocky. While Kepler-452b is larger than the earth, its 385-day orbit is only 5 per cent longer. The planet is 5 per cent farther from its parent star, Kepler-452, than the earth is from the sun. Kepler-452 is six billion years old, 1.5 billion years older than the sun; has the same temperature; is 20 per cent brighter; and has a diameter 10 per cent larger. The Kepler-452 system is located 1,400 light years away in the constellation Cygnus. The research paper on the discovery is due to be published in The Astronomical Journal.

“We can think of Kepler-452b as an older, bigger cousin to the earth, providing an opportunity to understand and reflect upon the earth’s evolving environment,” said Jon Jenkins of NASA’s Ames Research Centre who led the discovery team. “It’s awe-inspiring to consider that this planet has spent six billion years in the habitable zone of its star; longer than the earth. That’s substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet.”

“This planet is probably the most similar to the earth yet found,” McDonald Observatory astronomer Michael Endl said. Kepler data only provide the ratio of a potential planet’s size to the star’s size but not the actual size of either. So once Kepler finds a planet candidate, telescopes at the University of Texas at Austin’s McDonald Observatory and elsewhere get to work characterising the host star in as much detail as possible, Endl explained. “At around 1.5 times the earth’s radius, there seems to be a transition going on from predominantly rocky planets to planets that contain more volatiles—ices,” he added, “which would make it a mini-ice giant.” In the case of Kepler-452b, “we don’t know if it’s a big rocky planet or if it’s a mini-Neptune”. Endl also said that as a star ages and becomes brighter, the more intense radiation pushes its habitable zone farther out. Astronomers estimate how long Kepler-452b has spent in its star’s habitable zone by combining the star’s brightness and age with their measurement of the planet’s orbit. The number of planet candidates that the Kepler mission has detected so far is 4,696.