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-tenth 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.
During 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.
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 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 Spiter 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 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 fartherest 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.
Because the planets are so close to a cool star, their surface could be at the right temperatures to have water flow, considered one of 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.
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.
