It’s a big universe, but it’s full of small planets. A group of astronomers led by Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics announced Tuesday they had found eight new planets orbiting their stars at distances compatible with liquid water, bringing the total number of potentially habitable “Goldilocks planets” to a few dozen, depending on how the habitable zone is defined.
NASA’s Kepler spacecraft, now in its fifth year of seeking out the shadows of planets circling other stars, has spotted hundreds, and more and more of these other worlds look a lot like Earth — rocky balls only slightly larger than our own home that with the right doses of starlight and water could turn out to be veritable gardens of microbial Eden.
As the ranks of these planets grow, astronomers are beginning to plan the next step in the quest to end cosmic loneliness, gauging which hold the greatest promise for life and what tools will be needed to learn about them.
On Monday, another group of astronomers said they had managed to weigh precisely a set of small planets and found that their densities and compositions almost exactly matched those of Earth. Both groups announced their findings at a meeting of the American stronomical Society in Seattle.
Alluding to the popularity of food shows and cooking apps, Courtney Dressing, also of the Harvard-Smithsonian Center for Astrophysics, said at a news conference, “I’m going to give you the recipe for a rocky planet.”
She began, “Take one cup of magnesium…”
Reviewing the history of exoplanets, Debra Fischer, a Yale astronomer, recalled that the first planet found orbiting another sun-like star, a Jupiter-like giant, was discovered 20 years ago. Before that, she recalled, astronomers worried that “maybe the Star Trek picture of the universe was not right, and there is no life anywhere else”.
She termed the progress in the last two decades “incredibly moving.”
So far, Kepler has discovered 4,175 potential planets, and 1,004 of them have been confirmed as real, according to Michele Johnson, a spokeswoman at NASA’s Ames Research Center, which operates Kepler.
Most of them, however, are hundreds of light-years away, too far for detailed study. We will probably never know any more about these particular planets than we do now, including whether anybody can or does live on them.
“We can count as many as we like,” said Sara Seager, a planet theorist at the Massachusetts Institute of Technology who was not involved in the new work, “but until we can observe the atmospheres and assess their greenhouse gas power, we don’t really know what the surface temperatures are like.”
Still, she added, “it’s heartening to have such a growing list.”
Finding Goldilocks planets closer to home will be the job of TESS, the Transiting Exoplanet Survey Satellite, scheduled to be launched in 2017. But if we want to know what the weather is like on these worlds, whether there is water or even life there, more powerful instruments will be needed.
Seager is heading a NASA study investigating the concept of a starshade, which would float in front of a space telescope and block light from a star so that its much fainter planets would be visible, the way a driver flips down a shade to block the glare of the sun.
Another group, led by Karl Stapelfeldt of NASA’s Goddard Space Flight Center, is studying the alternative method known as a coronagraph, in which the occulting disk is inside the telescope.
NASA’s James Webb Space Telescope, due for a 2018 launch, will have a coronagraph capable of seeing Jupiter-size planets, but it is too late to adapt it to a star shade.
Meanwhile, Seager and Julianne Dalcanton of the University of Washington are writing a separate report for AURA, a consortium of universities that runs observatories. The goal is have a pool of dozens of “exoEarths” to study in order to have any chance of seeing signs of life or understanding terrestrial planets, Seager said. Amassing them will require a space telescope 10 or 12 metres in diameter (the Webb will be 6.5, and the largest currently on Earth is 10).
All of this will be grist for the mill at the end of the decade when a panel of the National Academy of Sciences produces its decadal and very influential wish list for astronomy in the 2020s.
For all of Kepler’s bounty, the exact analogue of Earth, a planet the same size orbiting the same type of star as the sun — “Earth 2.0,” in astronomer parlance — has not yet been found. The most terrestrial of the new worlds announced Tuesday are a pair known as Kepler 438b and Kepler 442b, both orbiting stars slightly smaller, cooler and redder than our sun. Kepler 438b is only 12 per cent larger than Earth in diameter and has a 35-day year; Kepler 442 is a third larger than Earth and has 112-day year.
“Many of these planets have a good chance of being rocky, like Earth,” said Torres. He was reinforced in that thought by his colleagues, led by Dressing. Her group combined data from Kepler, which measures the sizes of planets, with spectrographic observations from an Italian telescope in the Canary Islands. That instrument measures planets’ masses to determine their densities, and by combining the information, Dressing’s group was able to infer the compositions of a set of small planets.
All five of the planets smaller than 1.6 times the size of Earth fell on a line consistent with Earth and Venus. Planets larger than that, Dressing and her colleagues found, were fluffier, perhaps because as planets get bigger, their mass and gravity increase, and they are better able to hang onto gas and lighter components.
The work complements and tightens studies done with a larger but less precisely measured sample of exoplanets last year by Geoffrey Marcy and his colleagues at the University of California, Berkeley; that group looks into the nature of so-called super Earths, planets bigger than ours and smaller than Neptune.
There are no planets in this range in our solar system, but according to Kepler they are common in the galaxy. Are they rocks like Earth or blobs like Neptune? The breakpoint now seems to be 1.6 times the size of Earth, according to Dressing, and it is on those planets, perhaps, that we should concentrate our search for cosmic company.
As she said in her presentation as a “test kitchen” cook, “doubling the recipe doesn’t work.”