Published: 23 February 2017
Not just one, but seven Earth-size planets that could potentially harbor life have been identified orbiting a tiny star not too far away, offering the first realistic opportunity to search for signs of alien life outside the solar system.
The planets orbit a dwarf star named Trappist-1, about 40 light-years, or 235 trillion miles, from Earth. That is quite close in cosmic terms, and by happy accident, the orientation of the orbits of the seven planets allows them to be studied in great detail.
One or more of the exoplanets in this new system could be at the right temperature to be awash in oceans of water, astronomers said, based on the distance of the planets from the dwarf star.
“This is the first time so many planets of this kind are found around the same star,” Michael Gillon, an astronomer at the University of Liege in Belgium and the leader of an international team that has been observing Trappist-1, said during a telephone news conference organized by the journal Nature, which published the findings on Wednesday.
Scientists could even discover compelling evidence of aliens.
Cool red dwarfs are the most common type of star, so astronomers are likely to find more planetary systems like that around Trappist-1 in the coming years.
“You can just imagine how many worlds are out there that have a shot to becoming a habitable ecosystem,” Thomas Zurbuchen, associate administrator of NASA’s science mission directorate, said during a NASA news conference on Wednesday. “Are we alone out there? We’re making a step forward with this — a leap forward, in fact — towards answering that question.”
Telescopes on the ground now and the Hubble Space Telescope in orbit will be able to discern some of the molecules in the planetary atmospheres. The James Webb Space Telescope, scheduled to launch next year, will peer at the infrared wavelengths of light, ideal for studying Trappist-1.
Comparisons among the different conditions of the seven will also be revealing.
“The Trappist-1 planets make the search for life in the galaxy imminent,” said Sara Seager, an astronomer at the Massachusetts Institute of Technology who was not a member of the research team. “For the first time ever, we don’t have to speculate. We just have to wait and then make very careful observations and see what is in the atmospheres of the Trappist planets.”
Even if the planets all turn out to be lifeless, scientists will have learned more about what keeps life from flourishing.
Astronomers always knew other stars must have planets, but until a couple of decades ago, they had not been able to spot them. Now they have confirmed more than 3,400, according to the Open Exoplanet Catalog. (An exoplanet is a planet around a star other than the sun.)
The authors of the Nature paper include Didier Queloz, one of the astronomers who discovered in 1995 the first known exoplanet around a sunlike star.
While the Trappist planets are about the size of Earth — give or take 25 percent in diameter — the star is very different from our sun.
Trappist-1, named after a robotic telescope in the Atacama Desert of Chile that the astronomers initially used to study the star, is what astronomers call an “ultracool dwarf,” with only one-twelfth the mass of the sun and a surface temperature of 4,150 degrees Fahrenheit, much cooler than the 10,000 degrees radiating from the sun. Trappist is a shortening of Transiting Planets and Planetesimals Small Telescope.
During the NASA news conference, Dr. Gillon gave a simple analogy: If our sun were the size of a basketball, Trappist-1 would be a golf ball.
Until the last few years, scientists looking for life elsewhere in the galaxy have focused on finding Earth-size planets around sun-like stars. But it is hard to pick out the light of a planet from the glare of a bright star. Small dim dwarfs are much easier to study.
Last year, astronomers announced the discovery of an Earth-size planet around Proxima Centauri, the closest star at 4.24 light-years away. That discovery was made using a different technique that does not allow for study of the atmosphere.
“I think that we have made a crucial step toward finding if there is life out there,” said Amaury H. M. J. Triaud, an astronomer at the University of Cambridge in England and another member of the research team. “Here, if life managed
Trappist-1 periodically dimmed noticeably, indicating that a planet might be passing in front of the star, blocking part of the light. From the shape of the dips, the astronomers calculate the size of the planet.
Trappist-1’s light dipped so many times that the astronomers concluded, in research reported last year, that there were at least three planets around the star. Telescopes from around the world then also observed Trappist-1, as did the Spitzer Space Telescope of NASA.
Spitzer observed Trappist-1 nearly around the clock for 20 days, capturing 34 transits. Together with the ground observations, it let the scientists calculate not three planets, but seven. The planets are too small and too close to the star to be photographed directly.
All seven are very close to the dwarf star, circling more quickly than the planets in our solar system. The innermost completes an orbit in just 1.5 days. The farthest one completes an orbit in about 20 days. That makes the planetary system more like the moons of Jupiter than a larger planetary system like our solar system.
“They form a very compact system,” Dr. Gillon said, “the planets being pulled close to each other and very close to the star.”
In addition, the orbital periods of the inner six suggest that the planets formed farther away from the star and then were all gradually pulled inward, Dr. Gillon said.
Because the planets are so close to a cool star, their surfaces could be at the right temperatures to have water flow, considered one of the essential ingredients for life.
The fourth, fifth and sixth planets orbit in the star’s “habitable zone,” where the planets could sport oceans. So far that is just speculation, but by measuring which wavelengths of light are blocked by the planet, scientists will be able to figure out what gases float in the atmospheres of the seven planets.
So far, they have confirmed for the two innermost planets that they are not enveloped in hydrogen. That means they are rocky like Earth, ruling out the possibility that they were mini-Neptune gas planets that are prevalent around many other stars.
Because the planets are so close to Trappist-1, they have quite likely become “gravitationally locked” to the star, always with one side of the planets facing the star, much as it is always the same side of Earth’s moon facing Earth. That would mean one side would be warmer, but an atmosphere would distribute heat, and the scientists said that would not be an insurmountable obstacle for life.
For a person standing on one of the planets, it would be a dim environment, with perhaps only about one two-hundredth the light that we see from the sun on Earth, Dr. Triaud said. (That would still be brighter than the moon at night.) The star would be far bigger. On Trappist-1f, the fifth planet, the star would be three times as wide as the sun seen from Earth.
As for the color of the star, “we had a debate about that,” Dr. Triaud said.
Some of the scientists expected a deep red, but with most of the star’s light emitted at infrared wavelengths and out of view of human eyes, perhaps a person would “see something more salmon-y,” Dr. Triaud said.
NASA released a poster illustrating what the sky of the fourth planet might look like.
If observations reveal oxygen in a planet’s atmosphere, that could point to photosynthesis of plants — although not conclusively. But oxygen together with methane, ozone and carbon dioxide, particularly in certain proportions, “would tell us that there is life with 99 percent confidence,” Dr. Gillon said.
Astronomers expect that a few decades of technological advances are needed before similar observations can be made of Earthlike planets around larger, brighter sunlike stars.
Dr. Triaud said that if there is life around Trappist-1, “then it’s good we didn’t wait too long.”
“If there isn’t, then we have learned something quite deep about where life can emerge,” he continued.
The discovery might also mean that scientists who have been searching for radio signals from alien civilizations might also have been searching in the wrong places if most habitable planets orbit dwarfs, which live far longer than larger stars like the sun.
The SETI Institute in Mountain View, Calif., is using the Allen Telescope Array, a group of 42 radio dishes in California, to scrutinize 20,000 red dwarfs. “This result is kind of a justification for that project,” said Seth Shostak, an astronomer at the institute.
“If you’re looking for complex biology — intelligent aliens that might take a long time to evolve from pond scum — older could be better,” Dr. Shostak said. “It seems a good bet that the majority of clever beings populating the universe look up to see a dim, reddish sun hanging in their sky. And at least they wouldn’t have to worry about sun block.”
An earlier version of this article named the wrong telescope that is trained on the Trappist-1 dwarf star. It is the Spitzer Space Telescope, not the Kepler. The article also misstated how many days it takes for the planet farthest from Trappist-1 to orbit the star. It is about 20 days, not 12.35.