NASA’s Kepler spacecraft, first launched in 2009, has now found more than 2,000 potential exoplanets. Although only a fraction of these have been definitively confirmed to be planets, the excitement and interest around these discoveries has been palatable. One of the planets to pique the interest of the public and scientists alike was a terrestrial, rocky planet 620 light years away around a Sun-like star dubbed Kepler-22b. At the time of its discovery in May 2009 it was the smallest and first planet to be found comfortably within the habitable zone of its host star with a radius just 2.4 times that of Earth. Since then it has been usurped of its status by smaller planets but it’s discovery has been one of the most important in the grand scale of planet hunting.
“Kepler-22b was for Kepler the first planet to be discovered in a so-called habitable zone where the temperature at the surface would be okay for liquid water to exist,” said Steve Howell, Deputy Project Scientist for the Kepler Mission. “It’s probably something more like Neptune in terms of having a pretty thick atmosphere and perhaps a solid part underneath, and we wouldn’t think that would be a place people could live. It kind of spurred us on, though, that there clearly are planets that orbit stars at the right distance and this one isn’t too far off being an Earth analogue and being the right size and temperature.”
Kepler-22b’s detection came as somewhat of a surprise, as its first transit was found just three days after the telescope became operational in 2009. As is the case with the transit method, through which a stars dip in brightness is measured as the planet passes in front relative to our view from Earth, its status as a planet could not be confirmed until a third transit was observed, which it was in late 2010.
Indeed, while Kepler-22b itself might not be thought to host life as we know it, it was the first time that a planet more similar in size and temperature to Earth than Jupiter had been found. “You know, it was a surprise in some senses, but on the other hand almost everything Kepler has found has been a surprise,” said Howell. “Before Kepler was launched, almost every other planet known in orbit around another star was a large planet like Jupiter that orbited its star very closely, perhaps with only a two or four day orbit, just incredibly close to the surface of the star. These big Jupiters would be much too hot if they were solid, or if not solid they would be big balls of gas like Jupiter.
“No other small planets were known, mostly because nobody could detect them. At the time [before Kepler’s launch] people thought our Solar System might be really rare having small planets as all the other planets we knew at the time, maybe 40 or 50 large Jupiter-like planets, were thought to be the norm. Once Kepler was launched it was clear that those are the odd balls, and solar systems with lots of small rocky planets like our own seem to be the normal case. So we’re back to feeling pretty good about our own Solar System. Thanks to Kepler-22b we now know there are small planets, and we know eventually we’ll find planets like the Earth in terms of their size and temperature. My personal prediction is that we’ll find something along the lines of one or two planets like Earth within a year. That would be very exciting.”
Despite its significance in the search for Earth-like planets, the mass and composition of Kepler-22b are still generally unknown, and as Howell explains it can be very difficult to define these characteristics with modern technology. “We can only be so accurate. Things that we can know pretty well, maybe with a 15-20% error margin or sometimes a little better, down to 5%, is the size of the planet. That’s mainly what Kepler measures.
“The mass is the hard part. Unless we know the mass then we can’t find the average density, whether its really dense like rocks or like Jupiter and gaseous, which is what we really want. In order to find that measurement of how much rock or gas it has, you need to be able to estimate or discover the mass of the planet. In many cases for these small planets we won’t know that value; Kepler cannot determine that value by itself. There are a few ways we can make additional measurements with ground based telescopes to find the mass and get the mean density and work out if it’s rocky or gaseous or a big ice ball or something. In most cases for now, though, we’re just going to have to guess that if it’s small, it’s probably rocky. We have evidence that’s true, namely that every small planet in the Solar System is rocky, so that’s the best we can do.”
Estimates so far suggest Kepler-22b has a similar mass to Neptune in our own Solar System, roughly 17 times the mass of Earth. If it is slightly smaller at 10 Earth masses then it is possible that the planet has liquid water on its surface. Indeed, if its density turns out to be similar to that of our planet’s then it’s surface gravity would be about 2.4 times our own, a more than manageable force for life forms. It’s exact composition, however, is a mystery, although its surface temperature is thought to be around 262 Kelvin (-11°C / 12°F). The planet could be rocky, liquid, gaseous or a mixture of the three.
But while the Kepler telescope must rely on indirect methods, namely the transit method, to find exoplanets, Howell highlights bigger and better telescopes that will become operational in the future that might be able to directly image exoplanets. “Kepler’s biggest discovery in terms of the future of planet work is that small planets are everywhere, and because of that it means that small planets also probably exist orbiting stars that are very close to our Sun. Kepler looks at stars really far away, and the idea is to look at a lot of stars because before it was launched you didn’t know how many stars had planets, so you needed to look at 100,000 stars simultaneously. Now that we know that many stars have planets there’s going to be future missions like the European Extremely Large Telescope and James Webb Space Telescope, amongst others, that could directly image planets. They know they can look at nearby stars and there’s a chance those stars will have nearby planets, and those will be the ones that are imaged because they will be close enough to the Earth that they can separate bright light from the star and the little bit of dim light that comes from the planets by reflected light or infrared. The Kepler planets are the tip of the iceberg, there’s a lot of them out there, now let’s go find some we can get images of.”
But that’s not to say planets like Kepler-22b will just be thrown away, however. “Kepler-22b probably will be revisited by future telescopes,” explained Howell. “They’ll be able to get a picture and there’ll be a dot [representing the planet]. Even more importantly, we’ll be able to study that planet. We’ll be able to tell you if the planet has an atmosphere and the composition of that atmosphere, that’ll be very exciting.”
So, when does Howell think we’ll discover an exact Earth replica? “I think we’ll find many more candidates in the coming year. I think it’ll be another decade before we have planets identified at nearby stars where someone gets an image and tells us that planet has an atmosphere with oxygen and it’s made out of rock like the Earth. I think that’ll be another decade.” And will we be able to directly image one of these Earth replicas? “I absolutely think we’ll be able to snap one of these, yes.”
Images courtesy of NASA, Ames, JPL-Caltech, Robert Hurt, Kepler and SSC.
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