Though astronomers have long known that other stars in the Milky Way galaxy have planets orbiting them, they weren’t actually able to see such exoplanets until just a couple of decades ago. Now they have confirmed the existence of more than 3,400, according to the latest estimates. Last year, scientists announced the discovery of an Earth-like planet orbiting Proxima Centauri, the closest star to us (4.24 light years away). Now, the discovery of seven Earth-like planets around just one star—the dwarf star Trappist-1, part of the constellation Aquarius—suggests exoplanets may be even more common than originally thought.
Initially, scientists searching for life outside our solar system focused on planets that may be orbiting bright stars (similar to our sun). But such stars give out so much light that it is often difficult to find their planets. Trappist-1, on the other hand, is what astronomers call an “ultracool dwarf”: It has only about 1/12th the mass of the sun, and its surface temperature is 4,150 degrees F° (compared with 10,000 degrees on the sun).
Back in 2015, a team of researchers led by Michaël Gillon, an astronomer at the University of Liege in Belgium, were observing Trappist-1 through the
European Southern Observatory’s Transiting Planets and Planetesimals Small Telescope (or TRAPPIST), the robotic telescope in Chile for which the dwarf star is named. They noticed periodic dips in the star’s brightness at regular intervals, caused by planets crossing between the star and Earth and blocking some of its light. Based on their observations, the researchers concluded Trappist-1 had three rocky planets orbiting it.
But when they looked more closely, using a larger telescope on the ground, Gillon’s team confirmed that what they thought was one exoplanet was actually three different ones crossing between their star and Earth at the same time. They quickly requested the use of NASA’s Spitzer Space Telescope, launched in 2003, which orbits behind Earth. Using Spitzer, Gillon and his colleagues watched Trappist-1 nonstop for 20 days (or 34 transits). In combination with ground observations, the scientists have now concluded that a total of seven planets orbit the dwarf star. They published their findings yesterday in the journal Nature.
Though the planets in the Trappist-1 system are all about the same size as Earth, they orbit in extremely tight circles around their star, compared to the planets in our solar system. Scientists have determined that the six inner planets are in what’s known as orbital resonance, which means their orbital periods are related to each other by a ratio of whole numbers. In this way, the system is more similar to the moons of Jupiter than to a larger planetary system like our solar system.
The orbital resonance suggests that the planets formed farther out from Trappist-1 and then moved inward. This makes it more likely that they could contain some form of water, as volatile compounds (molecules that easily turn to gas) like water are usually concentrated further out in star systems. The fourth, fifth and six planets orbit in the star’s habitable zone, or “Goldilocks” zone, so they are particularly likely to have liquid water on their surface.
So far, scientists can only speculate about the planets’ atmospheres. Because the Trappist-1 system is so close to us, however, they’ll soon have the opportunity to observe them in much greater detail. Telescopes on the ground, as well as the Hubble Space Telescope, are already trained on the system, and will be able to make out some molecules in the planets’ atmospheres. The system is also within the range of the James Webb Telescope, scheduled for launch next year, which will be able to detect both atmospheric components and thermal emissions from the planets.
There’s reason to doubt we’ll be seeing alien life on these planets anytime soon, however. Because they’re so close to each other, and to their star, the planets may be tidally locked, like Earth’s moon. This means one side of each planet always faces the star, while the other is always in darkness. This would cause far more dramatic weather patterns than we see on Earth, including big temperature swings and strong winds.
But even if no life is found on any of the seven Trappist-1 exoplanets, the system will still be hugely exciting for scientists to investigate. At this point, they’re only speculating about what parameters might define “habitability” when it comes to life on other planets. Now, for the first time ever, the search for the conditions in which life can or cannot exist on planets outside our solar system is not just a matter of speculation, but of observation.