Discovery of seven Earth-like planets sheds light on exoplanets

Credit: Simin Li/Art Editor Credit: Simin Li/Art Editor

If there’s one thing astronomy has proven over and over again throughout history, it’s that the Earth is less special than we think. The Earth is not the center of the universe — or even the solar system. It’s not unique in its size or material composition, and it’s not the only planet with weather, volcanoes, tectonic activity, or liquid water.

The only attribute that makes our planet one-of-a-kind is the presence of life. Yet, even this might not be the case forever. Scientists are hoping that last week’s announcement of seven Earthlike planets orbiting the star TRAPPIST-1 may put us closer than ever to discovering life elsewhere in the galaxy. The system is relatively close to Earth at 39 light-years away.

The planets of TRAPPIST-1 were discovered by researchers from the University of Liège in Belgium, confirmed last year, then announced on Feb. 22. They were spotted at Chile’s La Silla observatory using the Transiting Planets and Planetesimals Small Telescope, or TRAPPIST, which became the namesake of the star system. The planets have no names yet because the system follows typical exoplanet designation conventions where the star is referred to as TRAPPIST-1 or TRAPPIST-1A, and the planets as TRAPPIST-1b, TRAPPIST1c, etc., in order from the planet closest to the star to farthest away.

Three thousand confirmed exoplanets have been discovered since 1992. So, what makes the TRAPPIST-1 system noteworthy? All seven confirmed TRAPPIST-1 planets are terrestrial: they resemble Earth in their size and rocky composition, making their discovery highly unusual. TRAPPIST-1 has the most rocky planets of any known system, and is tied with systems Kepler-90, HD 10180, and HR 8832 for the most total confirmed exoplanets. Earth sized planets are normally hard to detect because they are relatively small. It can be difficult to tell whether fluctuations in observational data indicate the presence of actual planets or random variations in the star’s activity.

The astronomers behind the TRAPPIST project specifically targeted small stars, making small planets easier to notice. TRAPPIST-1 is a low temperature red dwarf star barely larger than Jupiter. Small, cool stars are the most common type of star, so the discovery of the TRAPPIST-1 system could imply the existence of millions more systems like it. Dwarf stars are also the longest-living stars. TRAPPIST-1 consumes energy very slowly, so it could live for an estimated 12 trillion years. For this reason, dwarf stars are hypothesized to be the most likely place for advanced alien civilizations to develop, simply because they would have enough time to evolve.

Most excitingly, three of the seven planets exist in their own star’s habitable zone, the region of the stellar system that could hypothetically sustain life. The habitable zone, sometimes called the “Goldilocks zone,” is neither too hot nor too cold for the formation of liquid water. The habitable zone in the TRAPPIST-1 system is thought to contain planets TRAPPIST1e, -f, and -g. The TRAPPIST-1 system is theoretically an ideal place for life to flourish, but there are some practical factors reducing the likelihood that we will find aliens on any of its worlds. Because TRAPPIST-1 is so cool, the habitable zone is very close. All seven planets orbit closer to the star than Mercury does to the Sun, leading astronomers to believe that all potentially habitable planets are tidally locked to the star.

Tidally locked planets are manipulated by gravity to rotate at the same rate they revolve around their star, so one hemisphere of the planet constantly faces the star, and the other constantly faces away. Both hemispheres would suffer from extreme temperatures unsuitable for life. Another probable consequence of TRAPPIST-1’s close proximity to its planets is exposure to a high amounts of X-ray and ultraviolet (XUV) radiation. Intense amounts of XUV radiation make it extremely difficult for a planet to retain liquid water, often assumed to be necessary for the development of life. At this early stage of research, it is impossible to know whether the TRAPPIST-1 system is capable of originating or sustaining life. However, several projects are already being planned to investigate further.

Examining the planetary atmospheres with large, powerful instruments like the Hubble and James Webb telescopes will allow astronomers to determine what gases are present. Some gases, like ozone and methane, are strong clues that life is present. More powerful telescopes than TRAPPIST will detect any additional planets that may be in the TRAPPIST-1 system and provide the data needed to construct accurate estimations of their mass. With accurate mass estimations, scientists will know the composition of the planets — that is, if they are almost all rock, or if they contain a significant amount of water.

Despite the overwhelming uncertainty surrounding the star system, some are already looking for signs of life there. The Search for Extra-Terrestrial Intelligence (SETI) is keeping its eyes on TRAPPIST-1 with its Allen Telescope Array, listening for alien radio transmissions. No signals have been detected yet, but SETI director Seth Shostak promises that “New observations are in the offing.”