The existence of a solar system containing seven planets approximately the same size as Earth was discovered by a team of researchers, according to a paper published in Nature Feb. 23.
Out of the planets, three orbit a parent star, known as TRAPPIST-1, at a distance known as the “habitable zone.” The habitable zone describes the perfect distance between a planet and a parent star that allows for the planet to sustain life.
If a planet is located too close to a parent star, its oceans will boil, creating thick clouds of vapor in the atmosphere. This atmosphere traps solar heat sourced from the planet star, raising surface temperatures and ultimately making the planet uninhabitable. TRAPPIST-1 has a cool temperature and radiates significantly less heat than the Earth’s sun, making the area surrounding it a habitable zone.
TRAPPIST-1 is located approximately 40 light years away from Earth, or about 44 million years away at the cruising speed of a commercial passenger jet.
It is a dim and small celestial body located in the constellation of Aquarius, making the solar system one of over a dozen exoplanet systems located there. It is TRAPPIST-1’s dimness and relatively cool temperature that allows the planets to fit into tight orbits around the star.
The constellation is also home to Gliese 876, a red dwarf star orbited by four planets and calculated to be approximately 15 light years away from Earth. Scientists have discovered nearly 3,500 of these exoplanets in total.
The discovery that three of the planets orbit TRAPPIST-1 within the habitable zone implies that they may have the perfect surface temperature to contain liquid surface water and possibly sustain life.
"This is the first time that so many Earth-sized planets are found around the same star," said lead researcher Michael Gillon from the University of Liege in Belgium.
The research team utilized data from NASA’s Spitzer Space Telescope, Transiting Planets and Planetesimals Small Telescope-South in Chile and other assorted, ground-based telescopes to note the masses, orbits and sizes of some of the planets. More specifically, scientists were able to determine the architecture and rocky terrain of the planets by observing how the planets’ motions blocked sections of TRAPPIST-1’s light.
Although scientists are not yet certain of whether the planets currently sustain life, they noted that there is a lot of time for organisms to come into existence and evolve. TRAPPIST-1 is approximately 500 million years old, yet its estimated life span is approximately 10 trillion years. In comparison, the Earth’s sun is already nearly halfway through its lifespan of 10 billion years.
According to astronomer Ignas Snellen, who works within the Dutch Leiden Observatory, TRAPPIST-1’s relatively long lifespan can be at least partially attributed to the slow rate at which it burns hydrogen, giving it more than enough time to allow for life to evolve.
"I think that we've made a crucial step towards finding if there is life out there," said University of Cambridge astronomer and paper co-author Amaury Triaud.
As for actually standing on the surface of a TRAPPIST-1 system planet, an observer would find himself or herself staying in seemingly relentless twilight. TRAPPIST-1’s dimness results in low amounts of natural lighting on its surrounding planets.
According to Triaud, TRAPPIST-1’s infrared wavelengths would be enough to warm the surface air of the planets, but not enough to make the skies brighter than the Earth’s skies immediately following a sunset. Due to the tightness of the planets’ orbits, an observer's view of the six planets while standing on one planet would be unique.
"If you were on the surface of one of these planets you would have a wonderful view of the other planets," said Gillon. "You wouldn't see them like we see Venus or Mars, like dots of light. You would see them really as we see the moon … You would see the structures on these worlds."
Although the TRAPPIST-1 planets orbit quickly around their parent star—a full year on most of the planets equates to two weeks on Earth—the days of the planets are incredibly long. Constant gravitational pulling as a result of the planets’ tight orbit routes leads to irregular planetary transit and possible tidal locking. This means that one side of each planet in the system will almost always be facing toward TRAPPIST-1.
It is still unclear if these seemingly eternal days will lead to the TRAPPIST-1 system being deemed incompatible with life, or if the day-length will be suitable.