A super-earth discovered by NASA’s Kepler Space Telescope and orbiting an older version of our Sun, is tantalising astronomers as potentially being the closest thing to a world like Earth.
“Kepler-452b is the first small planet [we have found] in the habitable zone of a star like our own Sun,” says Jon Jenkins, Kepler’s lead data analysis scientist at NASA’s Ames Research Center.
The newly-discovered exoplanet orbits its star, which is 1,400 light years away, every 385 days, meaning that its year is only 20 days longer than our own. This orbital period places it at only five percent further from its star than Earth is from the Sun, slap bang in the middle of its star’s habitable zone, where temperatures should be suitable for liquid water to exist, on the condition that an atmosphere is present around the planet.
Interestingly, Kepler-452b provides a look into the future of our own Earth. Its star is a ‘G2-type’ star, which is nearly identical to our own Sun, but because it is 1.5 billion years older, there are a couple of critical differences. As stars age they grow hotter and brighter and, in 1.5 billion years, our Sun will have grown so hot that it will instigate a runaway greenhouse effect in Earth’s atmosphere, evaporating the oceans and rendering our planet an inhospitable desert. If Kepler-452b is rocky and has oceans, it could be in danger of being about to enter this runaway greenhouse stage.
“It’s maybe good that this planet is a bit further from its star than we are,” says Jenkins. The extra distance – it orbits 157.1 million kilometres from its star, compared to Earth’s 149.6 million kilometres – will reduce the heat it receives, perhaps staving off the runaway greenhouse stage for another 500 million years.
Unfortunately, there are two caveats to this story. The first is that we have no way of knowing for sure what Kepler-452b is like. Its mass is an estimated five times the mass of Earth, which would make Kepler-452b a ‘super-earth’, but we have no example of a super-earth in our Solar System and geologists are still struggling to model what their interiors are like. For example, would such a world contain too much internal heat for plate tectonics to occur, which are vital for the carbon-silicate cycle that helps regulate the ice ages, warm periods and long-term climate in general on Earth? The Kepler team consulted with planetary geologist Bill Moore of Hampton University who, based on geological models, predicted that Kepler-452b could be covered with copious volcanism. At this stage, however, this is just an educated guess. Even confirming its mass from observations of the Doppler shift its gravity incurs in its parent star is impossible with current technology because the star is too faint.
Similarly, we have no way of knowing yet if Kepler-452b has an atmosphere or water. Future missions that could study exoplanet atmospheres, such as the James Webb Space Telescope and the Ariel mission (http://ariel-spacemission.eu), which is under consideration by the European Space Agency, will look to make Kepler-452b a priority target.
The other problem is the true nature of Kepler-452b. “There’s a better than even chance that Kepler-452b is rocky,” says Jenkins, but not all planetary scientists agree. Some suspect that, when a planet’s mass is over one and a half times the diameter of Earth, it is more likely to be a gaseous, water-rich ‘mini-neptune’ (see the pre-print of a new paper by Leslie Rogers of the California Institute of Technology that describes this hypothesis http://arxiv.org/abs/1407.4457). Again, follow-up observations with the next generation of planet-studying telescopes will be required to determine whether this is the case.
Kepler-452b is just one of more than 500 potential new planets just announced by the Kepler team to add to the 4,175 planetary candidates already found by the planet-hunting space telescope.