So read the headlines based on a NASA press release about the most recent “habitable planet” discovery. Here’s what really happened:
NASA’s press release actually contained more circumspect language in claiming to have “confirmed the first near-Earth-size planet in the ‘habitable zone’ around a Sun-like star.” NASA did label Kepler-452b (the exoplanet under scrutiny) as Earth’s bigger, older cousin, but clearly stated that the find represents an important milestone in the journey toward locating another Earth. The discovery of Kepler-452b demonstrates for the first time that scientists have the capacity to detect something akin to Earth orbiting around a star like the Sun. Previous claims of potentially habitable planets involved objects closer in size to Earth, but these were orbiting M dwarfs—stars much smaller than the Sun.
According to an article published in the Astronomical Journal, the exoplanet in question orbits a G2 star with an estimated age of 6 billion years and a surface temperature of 5,757 K (kelvin). For comparison, our Sun is a G2 star, is 4.6 billion years old, and has a surface temperature of 5,778 K.1 The exoplanet’s diameter measures 60 percent larger than Earth’s and orbits in a period of 385 days. Given the luminosity of its host star, the authors note that Kepler-452b always resided in the habitable zone (the more optimistic range) and will continue to do so for the next 3 billion years. The larger diameter of Kepler-452b puts it in the super-Earth category, but research indicates it has a better than even chance of being rocky.
While recognizing the importance of this find in the search for an Earth twin, a couple of caveats are in order regarding the potential habitability of Kepler-452b. Many super-Earths end up as giant gas balls depending on the environment in which they formed. According to the paper, this one has just under a 50 percent chance of accumulating so much gas that it has no rocky surface. Even if a rocky surface exists, other research shows objects like Kepler-452b usually acquire dense atmospheres with pressures that prevent life on the planet.2 Even if Kepler-452b contains a rocky surface and a thin atmosphere, the development of biospheres may require so many rare events that it never happens.
Scientists continue to make steady, and sometimes spectacular, progress toward finding Earth-like planets orbiting stars other than the Sun. As they do so, they draw closer to answering the ultimate question of whether life exists beyond the confines of Earth. The more they discover exoplanets that resemble Earth, the greater our tools become to evaluate the rarity of Earth’s capacity to host abundant and diverse life.