By Kenneth Chang
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 biological signs of alien life outside of the solar system.
The planets orbit a dwarf star named Trappist-1, about 40 light years, or about 235 trillion miles, from Earth. That is quite close, 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 – planets around stars other than the sun – 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,” said Michael Gillon, an astronomer at the University of Liege in Belgium and the leader of an international team that has been observing Trappist-1.
They could even discover convincing evidence of aliens.
“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 to thrive and releases gases similar to that we have on Earth, then we will know.”
The findings appear Wednesday in the journal Nature.
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 Telescope, scheduled to launch next year, will peer at the infrared wavelengths of light, ideal for studying the dimmer longer wavelength light coming from 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, gaining an understanding of the constraints for when life can exist or cannot would also be important new knowledge.
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.
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 Chile’s Atacama Desert that 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 of heat radiating from the sun. Trappist is a shortening of Transiting Planets and Planetesimals Small Telescope.
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 difficult 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.
Trappist-1 periodically dimmed slightly, 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 NASA’s Kepler Space Telescope.
Kepler observed Trappist-1 nearly around the clock for 20 days, capturing 34 transits. Together with the ground observations, the scientists calculated not three planets, but seven. The planets are too small and too close to the star to be photographed directly.
If placed within our solar system, all seven would lie within the orbit of Mercury, and they circle the star quickly. The innermost completes an orbit in just 1.5 days. The farthest one completes an orbit in 12.35 days. That makes the planetary system more like the moons of Jupiter than a larger planetary system like our solar system.
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 six planets orbit in the star’s “habitable zone,” where the planets could be awash in 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, Triaud said. (That would still be brighter than the moon at night.) But the star would be far bigger. On Trappist-1f, the fourth 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,” 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,” Triaud said.
If observations reveal oxygen in a planet’s atmosphere, that could point to photosynthesis of plants – but that would not be conclusive. But oxygen together with methane, ozone and carbon dioxide, particularly in certain proportions, “would tell us there is life with 99 percent confidence,” 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.