Scientists working with telescopes at the European Southern Observatory and NASA have announced a remarkable new discovery: An entire system of Earth-sized planets. Moreover, according to density measurements of the planets the six innermost are Earth-like rocky worlds, three of which lie in the star’s habitable zone.
The unexpected discovery, reported last week in the journal Nature, represents the first time astronomers have detected so many terrestrial planets orbiting a single star. It seems more and more likely that the Milky Way is teeming with worlds that resemble our own rocky home. Researchers say the newly discovered solar system is now the best laboratory for studying distant worlds.
“Before this, if you wanted to study terrestrial planets, we had only four of them and they were all in our solar system,” said lead author Michaël Gillon, an exoplanet researcher at the University of Liège in Belgium. “Now we have seven Earth-sized planets to expand our understanding. Yes, we have the possibility to find water and life. But even if we don't, whatever we find will be super-interesting.”
The planets orbit a red dwarf named Trappist-1, a small, faint star in the constellation of Aquarius, perhaps appropriately the water-bearer, at a distance of 39 light years from Earth. Though it would still be a long way for humans to travel, it's practically next door when you consider that the Milky Way galaxy alone is 100,000 light-years across.
Only marginally larger than Jupiter, Trappist-1 shines with a feeble light about 2,000 times fainter than our Sun. Its intriguing new planets circle tightly around it; the closest takes just a day and a half to complete an orbit and the most distant takes about 20 days. Three of them receive the same amount of heat as Venus, Earth and Mars, putting them in “the habitable zone,” the region where it's thought life can thrive.
The researchers call these worlds “Earthlike,” though it’s a generous term. The planets of the Trappist-1 system do resemble Earth in terms of size, mass and the energy they receive from their star, but there's a lot that makes our planet livable besides being a warm rock. Astronomers are already focusing on whether the Trappist-1 bodies have atmospheres and if they hold water, methane, oxygen and carbon dioxide — the molecules that scientists consider “biosignatures,” or signs of life.
The Hubble telescope could detect methane and water in the alien air, but both can be produced without life. More complex and convincing molecular signatures might be spotted by Nasa’s James Webb Space Telescope, which is due to launch next year, and other instruments, such as the Giant Magellan Telescope, a ground-based observatory due to switch on in 2023.
There is, however, only so much that can be done from afar, and we may never be 100% sure until we go there. Meanwhile, scientists are also trying to get a better look at Proxima b, a rocky world that was discovered orbiting Proxima Centauri, our Sun's nearest neighbor ar a distance of just 4 light years.
The conditions on planets so close to dwarf stars, which are known to release fierce bursts of x-rays and ultraviolet light, might not be the most conducive for life. The planets' proximity to the star and one another also means that they are probably tidally locked, like Earth's moon. One side of each planet always faces the sun; the other is stuck in constant darkness. This would make for a dramatic temperature gradient that could generate powerful winds — not exactly an earthling's idea of a cozy home.
But when the Sun goes out in a few billion years, Trappist-1 will still be an infant star. It burns hydrogen so slowly that it will last another 10 trillion years. That is more than 700 times longer than the universe has existed, so there is plenty of time yet for life to evolve.
Whatever secrets it may harbor, the Trappist-1 system would surely be a sight to behold. Though the star is small, its nearness to the planets means that, from their perspective, it appears about three times as large as our Sun. Its dim, salmon-colored glow bathes the planets in warmth and paints their skies with the crimson hues of a perpetual sunset.
“The spectacle would be beautiful,” said Amaury Triaud, one of the scientists involved in the research. “Every now and then you’d see another planet, about as big as another moon in the sky.”
Gillon and his colleagues have been interested in Trappist-1 since late 2015. Using the European Southern Observatory's Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, they sensed small dips in the star's brightness at regular intervals. These dips were caused by planets transiting — crossing between the star and Earth — and blocking some of its light. Last May, the scientists published their discovery: three rocky bodies, dubbed Trappist-1b -1c and -1d, orbited the small star, they said.
But right around the time the study was published, Gillon noticed that Trappist-1d was behaving oddly. When he went to get a closer look with the Very Large Telescope, the ESO's gigantic observatory in South America's Atacama Desert, he realized that the dip in brightness he thought came from 1d was actually caused by three planets, all transiting at the same time.
This happens only once every three years, said Julien de Wit, a planetary scientist at the Massachusetts Institute of Technology and a co-author on the study. “The chance of catching it is less than one in a thousand,” he explained. “It's funny because it’s such a huge paper with amazing results, and we got it from sheer luck.”
Next the team hurried to request time at the Spitzer Space Telescope, whose Earth-trailing orbit around the sun offered an uninterrupted view of Trappist-1 and its companions. During 20 days with the Spitzer telescope, the team witnessed 34 transits. These observations revealed that instead of three planets, Trappist-1 had seven, renamed Trappist-1b through -h in order of their distance from the star. Planets e, f and g are the ones positioned in Trappist-1's habitable zone.
The scientists also determined that the six inner planets are locked in an orbital resonance, which suggests that they formed farther out from their sun and then migrated inward. This makes it more likely that they will contain water in some form, since water and other volatile compounds (molecules that readily turn to gas) tend to concentrate on the outer edges of solar systems.
Gillon and his colleagues plan to seek out similar solar systems with a new project, Search for Habitable Planets Eclipsing Ultracool Stars, or SPECULOOS. Like Trappist beer, speculoos cookies are a Belgian delicacy. To remain in style, his next effort will have to be called FRIES.
Image: Trappist-1 line-up (NASA/JPL-Caltech)