Two centuries ago, the poet William Blake wrote about seeing a world in a grain of sand. Now, by peering at a dim red star that is the equivalent of a cosmic sand grain, astronomers have found not one world, but seven – all similar in size and mass to our own.
"This is very promising for the search for life beyond our solar system," said Michael Gillon, an astrophysicist with the University of Liege in Belgium who led the team that made the find.
The discovery, published Wednesday in the journal Nature, marks the first time that so many terrestrial planets – planets that are primarily made of rock, like Earth – have turned up in one solar system.
Three of the planets fall within a temperature range that should allow water to exist on their surfaces – a placement that is sometimes referred to as "the Goldilocks zone" because it is neither too hot nor too cold. Given the right sort of atmosphere, water could also be present on three of the others.
Such a multiplicity of worlds in the same system improves the odds that at least one of them will have the right combination of factors to support life, said Sara Seager, a Canadian astrophysicist and MIT professor who specializes in characterizing the conditions on alien worlds but who was not a member of the discovery team.
"In this planetary system, Goldilocks has many sisters," Dr. Seager said during a press briefing held in Washington, D.C. to discuss the findings.
The planets circle around a red dwarf star that the team designated TRAPPIST-1, after the Transiting Planets and Planetesimals Small Telescope, which was involved in the find. The star, which is only one-tenth the sun's diameter and only a fraction of its brightness, is too faint to be seen with the unaided eye. It is located 39 light-years away from our own solar system in the direction of the constellation Aquarius.
Several ground-based observatories were involved in detecting the planets, together with NASA's Spitzer Space Telescope, which is sensitive to the infrared wavelengths where TRAPPIST-1 shines brightest. None of the planets have been imaged directly, but their number and relative sizes were deduced from the amount of starlight they block as they cross in front of the star at regular intervals – the result of a lucky alignment of the distant solar system's orientation with respect to Earth.
These frequent crossings, called transits, will be a boon to astronomers hoping to learn more about the newfound planets. Because the planets are backlit by the star whenever they transit, it means astronomers may be able to use the star's light as a probe to determine whether any of the planets have atmospheres and, if so, what gasses they contain.
"Maybe the most exciting thing here is that these seven planets are very well-suited for detailed atmospheric study," said Dr. Gillon.
Some gasses, including oxygen – which is abundant in Earth's atmosphere because of the presence of plants on land and in the oceans – would be an indicator that life may exist there.
While astronomers have detected thousands of planets beyond our own solar system over the past two decades, this find has generated much more interest than usual.
Alan Boss, a researcher at the Carnegie Institution in Washington who studies planet formation, said the finding was consistent with data emerging from NASA's Kepler spacecraft, which surveyed solar systems around a larger number of stars.
"It seems, as you go to lower and lower mass stars, you get even more and more of the smaller mass planets, which is a little surprising," Dr. Boss said.
If the trend is real, it would mean that Earth-sized planets are extremely abundant in our galaxy because cool red dwarf stars such as TRAPPIST-1 vastly outnumber stars like our sun.
The prospect of so many potentially habitable planets so close to one another is fodder for countless science-fiction scenarios. Unlike our solar system, where even the nearest planets appear as bright stars, the worlds around TRAPPIST-1 would appear to each other more like the moon does in Earth's night sky.
At such close quarters, Dr. Gillon and his colleagues point out, the mutual gravitational pull between planets in such systems could play an important role in how they evolve and behave. As an extreme example, the interaction between Jupiter's four large moons generates enough energy to cause volcanoes to erupt on the surface of the innermost moon, Io.
"It fires the imagination as to what could be going on in a system like this," said Jon Willis, an associate professor of astronomy at the University of Victoria and author of All These Worlds are Yours: The Scientific Search for Alien Life.
"We're looking at nature now not just on our planet but beyond our planet, and we're finding that at almost every step, it's richer and more varied than we dared imagine."
How far is TRAPPIST-1, really?
TRAPPIST-1 is an ultra-cool dwarf star located 39.13 light-years away in the constellation Aquarius. A light-year is the distance light travels in 365.25 days. The speed of light in a vacuum is 299,792,458 metres per second.
Follow Ivan Semeniuk on Twitter: @IvanSemeniuk
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