It might be in dire straits, but that hasn’t stopped the exoplanet-hungry Kepler Space Telescope from using its reams of data to tell us something new: that about twenty percent of Sun-like stars in our Galaxy – the Milky Way – have Earth-sized planets that could be favourable for life.
The finding which is the result of the now-crippled telescope joining forces with the W.M. Keck Observatory, suggests that when we look up at the thousands of stars in the night sky, the nearest Sun-like star with an Earth-sized world tangoing in an orbit around it could only be a short 12 light-years away and can be seen with the naked eye.
“That is amazing,” says Erik Petigura who led the analysis of the space and ground telescope’s data and of whom is affiliated with the University of California, Berkeley.
With a better idea of the number of Sun-like stars (which Petigura stresses are smaller and cooler K-type stars but the result can still be applied to our Sun’s G-type) likely to host a planet similar to Earth, helps NASA’s future missions immeasurably. “[This is number] is really important, because successor missions to Kepler will try to take an actual picture of a planet and the size of the telescope they have to build depends on how close the nearest Earth-size planets are,” says Andrew Howard of the Institute for Astronomy at the University of Hawaii whose institution teamed up with UC Berkeley for the study. “An abundance of planets orbiting nearby stars simplifies such follow-up missions.”
However, while Petigura and Howard dub these planets as Earth-sized and in Earth-like orbits or, in the habitable zone of their stars – the distance where it’s not too hot nor too cold, but conditions are just right – the pair insist that we should err on the side of caution when considering if these planets are hospitable for life as we know it. Something that’s echoed by planet hunter Geoffrey Marcy, who was also part of the team.
“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive,” Marcy, who is also at UC Berkeley, warns. “Others may have rocky surfaces that could harbour liquid water suitable for living organisms. We don’t know what range of planet types and their environments are suitable for life.”
It wasn’t that long ago, however, that Marcy and his team’s previous work provided hope that habitable zone planets are rocky worlds similar in size to Earth after reporting an Earth-sized exoplanet with similar density and very likely to be composed of rock and iron, just like our very own planet.
However there’s a snag – this world sizzles at a scorching 2,000 Kelvin – way too hot for life as we know it. But Howard isn’t put off.
“This gives us some confidence that when we look out into the habitable zone [of a star], there maybe Earth-size, rocky planets,” he says.
Over four years, a whopping 150,000 stars were photographed every 30 minutes leaving multi-tasking Kepler with more than 3,000 planet candidates for its efforts. However with such a great number, scientists found that they were faced with a whole zoo of planets bigger than Earth. These worlds ranged from giants with thick atmospheres akin to Neptune to gas giants like our gaseous neighbour, Jupiter as well as worlds so close to their stars that they were being cooked to scorching temperatures.
With so many planets to sort through to sieve out the Earth-sized worlds, this called for the 10-metre telescopes of the Keck Observatory at the summit of Mauna Kea, Hawaii. With the High-Resolution Echelle Spectrometer (HIRES), the team were able to get their hands on as many stars as possible – namely 42,000 stars that are similar to our own Sun or cooler and smaller. Using the transit method – an exoplanet detection technique where a distant world passes across the face of its star – the team were able to measure each world’s size, allowing them to focus on Earth-diameter planets. They returned 603 candidates.
However, narrowing down the results further, only ten of these worlds were around one to two times the diameter of Earth and orbiting their star at a distance that suggests a comfortable temperature suitable for life. Had the team missed out some worlds that fit the bill?
To find out, Petigura injected fake planets into Kepler’s data. He saw it as a test for his new planet-hunting software which would tell him which worlds – fake or real – his algorithm could find.
“What we’re doing is taking a census of extrasolar planets, but we can’t knock on every door,” he admits. “Only after injecting these fake planets and measuring how many we actually found, could we really pin down the number of real planets that we missed.”
Now including their “neglected” planets, the team found that only a small portion of worlds are orientated in such a way that, from Earth, they pass in front of their host star. This brings forth an estimate that around twenty percent of all stars like our Sun held in our Galaxy have Earth-sized worlds in their habitable zones.
“Until now, no one knew exactly how common potentially habitable planets were around Sun-like stars in the Galaxy,” Marcy concludes.
Images courtesy of Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.