Meet Kepler 452b
A Potentially Habitable Earth-like World
On a teleconference Thursday, members of the Kepler space observatory team announced the discovery of Kepler 452b, an Earth-like planet that orbits a Sun-like star in its habitable zone, some 1,400 light years away in the constellation Cygnus.
Artist's concept of Kepler 452b, a new Earth-like exoplanet confirmed on Thursday. Credit: NASA Ames/JPL-Caltech/T. Pyle
The Kepler spacecraft, which orbits the Sun at a slightly greater distance than Earth, has been searching for exoplanets (planets orbiting a star other than our Sun) since 2009, and today marks the release of its seventh catalog, which contains 4,696 exoplanet "candidates" identified over that time.
We already knew that some resided in the habitable zone of their star. This zone is defined as the region around a star where, based on that star's energy output, liquid water could exist on the surface of a planet; outside the habitable zone it would freeze, and inside the habitable zone it would boil away. Planets that reside in the habitable zone are thought to be good candidates for harboring life, since life as we understand it right now requires liquid water.
We've also known that some of Kepler's exoplanet candidates have been roughly Earth-sized, though these are not as easy to detect as larger exoplanets, for reasons discussed below. This is the first time, however, that a near Earth-sized planet has been discovered that orbits a star with similar properties to our Sun, in the habitable zone of that star. This makes Kepler 452b, in a sense, the most "Earth-like" planet we've discovered. And since Earth is the only planet that we know for sure can support life, Kepler 452b will be an object of great interest.
Of course, 452b isn't exactly like the Earth, but even in considering the differences, it's remarkable how familiar a planet it is. Kepler 452b orbits its star at a distance of 1.05 AU (astronomical units), compared to Earth's 1 AU distance from the Sun (this is how AU is defined, in fact). At that distance, it takes 385 days to complete one orbit, compared to Earth's 365. It's larger than Earth, with a radius approximately 60% larger. If it is rocky, which the team suspects it to be, this would mean it would be approximately 5 times more massive than the Earth. These two factors would combine to make 452b's surface gravity about twice as strong as what we're used to here. This would be markedly uncomfortable for a human, but, as far as we know, that amount doesn't prohibit the development of life.
Diagram comparing Kepler 452b's orbit (middle) with those of the Earth and other planets in our solar system (bottom). The habitable zones of the stars are shaded in green. Credit: NASA/JPL-CalTech/R. Hurt
In a way, 452b is a sort of preview of an older Earth. 452b and its parent star are estimated to be about 6 billion years old, compared to the Earth and Sun's 4.5 billion year age. As stars like our sun age, they become brighter, larger, and emit more energy. This means 452b's somewhat older star causes it to receive about 10% more energy than the Earth does from the Sun. If the Earth were receiving as much energy as 452b now is, it would be on the verge of a "runaway greenhouse effect." This is a sort of feedback loop where increased surface temperature puts more water vapor into a planet's atmosphere, which causes more trapping of energy from the star and hence a higher surface temperature, which puts still more water vapor into the atmosphere, and so on until the planet is blistering hot and unsuitable for life. But 452b's greater size prevents it from being at that dangerous point just yet.
This animation released by NASA shows the relative locations of the Earth and 452b in their stars' habitable zones over time. Note that the planets themselves aren't actually changing location; the movement represents the habitable zones changing over time as the parent stars' energy output increases. The Earth and 452b's current locations along this timeline of development are marked by the blue and orange dashed lines, respectively.
Note that, even though 452b is about 2.5 billion years further along, it's still inside the "conservative" habitable zone, that is, the set of conditions where astronomers are certain that liquid water could exist on the surface (the "optimistic" habitable zone loosens the limits on those conditions a bit). 452b will be an obvious target for future observations, perhaps like those of the James Webb Space Telescope, which will launch in 2018 and attempt to analyze exoplanets for composition. It's hoped that the presence of life on exoplanets could be inferred from compositions that are different than would be expected due to formation and geology alone.
How Kepler Finds Exoplanets
The Kepler spacecraft identifies these exoplanet candidates by closely observing the light from distance stars. If a star has a planet, and that planet passes in front of its star from Kepler's perspective (called a "transit"), the light that we see from that star should dim, ever so slightly, as the planet blocks it. NASA illustrated the effect with this animation:
Animation of observing the transit of a planet in front of its parent star. As it passes in front of the planet, the light that we detect from the star dims very slightly. Credit: NASA Ames and Dana Berry
Kepler keeps careful track of the light emitted from thousands of distant stars in one patch of our sky, looking for these tell-tale drops in brightness. A legitimate exoplanet will produce multiple, consistent dips in brightness as it orbits around its star, so data indicating these are flagged as "candidate" exoplanets, to be reviewed further by scientists or, more recently, automated software that can take a closer look. Following this, other observatories and space telescopes can look for those same dips in brightness to confirm the candidate as a planet.
Illustration of the small part of the sky that Kepler observes for exoplanets. Credit: NASA
This is how Kepler has identified nearly 5,000 exoplanet candidates, and that's by analyzing just a small portion of the sky. Further uniform analysis of the full data set will likely lead to refinements of this estimate, but Kepler's data suggests that stars in our galaxy have an average of 1.6 planets orbiting them. A 2014 study based on Kepler data indicates that 22% of stars in our galaxy have an Earth-sized planet in their habitable zone. If these figures are accurate, our galaxy alone may harbor 40 billion potentially habitable planets, and that's going only by the conditions for life as we know them. Keep in mind, our observable universe is estimated to be home to 225 billion galaxies, some smaller than our Milky Way, some larger.
With just a cursory observation of one part of the sky, Kepler has massively advanced our knowledge of planets beyond our solar system. We now know they are very common, outnumbering the stars, and that planets with Earth-like characteristics are out there, in large numbers. Now that we know how to identify the best candidates for life outside our solar system, future space-based observatories like the James Webb Space Telescope will be able to look for the tell-tale signs that life actually exists on these planets.