Earth’s Seven Sisters

The search for life in the Universe is one of the driving forces behind modern astronomy, inspiring a search out to the stars with the hope of glimpsing something that proves we, as a species, are not alone.

In 2010, a group of astronomers led by Michaël Gillon at the University of Liege in Belgium set out to study small, dim stars called red dwarfs in the local area of our galaxy. They used the Transiting Planets and Planetesimals Small Telescope–South (TRAPPIST) telescope at the European Southern Observatory’s La Silla facility in Chile to spot planets around these tiny stellar objects.

The search proved successful, and one star in particular caught the eye of the astronomers. In early 2016, the group announced in the journal Nature the exciting discovery of three Earth-sized worlds orbiting TRAPPIST-1, just under 40 light-years away — practically in our cosmic backyard. As a particularly small and dim star, TRAPPIST-1 is classed an ultracool dwarf. This was the first detection of any planets around this kind of star, opening up exciting new possibilities of finding life beyond the Solar System. Inspired by the find, the astronomers intensified their search of the system.

In February, the astronomers struck gold again, with the incredible discovery of seven Earth-sized worlds orbiting TRAPPIST-1 — the most ever discovered in a single system. With three the right distance from the star to potentially support oceans of water, the rich planetary system could be humanity’s best hope yet of finding extraterrestrial life.

Planets and transits

Exoplanets — worlds outside our Solar System — have been known to astronomers for years, with the number continuing to grow alongside the improvement of telescope technology. These searches have tended to focus on large planets, such as hot Jupiters, which are easier to detect. Recently, however, astronomers have gained the fine observational tools required to survey the sky and detect much smaller, Earth-sized planets.

One of the methods used to spot planets around other stars is known as transit photometry. Much like a Venus transit across the Sun that can be seen by eagle-eyed astronomers here on Earth, planets can pass between their star and us and obscure a tiny amount of its light. By studying both the amount and characteristics of the dip in light, astronomers can learn not only of the presence of these planets, but also infer a great deal about their size, orbits and composition.

Stars on the Main Sequence, such as our Sun, are so brilliantly luminous that looking for dips in light due to transiting planets is nothing short of a needle in a haystack. Gillon and his Belgian-led TRAPPIST team therefore chose to focus their search on smaller stars because a greater proportion of their disc is obscured by transiting planets. This makes it easier to detect the holy grail of smaller, Earth-sized worlds.


Unlike our Sun, in which a million Earths could fit with room to spare, TRAPPIST-1 is tiny — at least in stellar terms. In fact, it’s only slightly larger than Jupiter and barely massive enough to support nuclear fusion, the process through which stars are powered. The ultracool part of its name come from its relatively low effective temperature, suspected to only be around 2500C.

Despite their diminutive size, stars like TRAPPIST-1, also named 2MASS J23062928-0502285 after its catalogue and sky coordinates, are a promising target for planet-hunting astronomers, because the environment in which they form means they could play host to a large number of planets that are similar to the size of Earth. They are also very common in the Milky Way, making them an exciting target for future investigation of potentially habitable worlds.

Seven sisters

Following the initial discovery in 2016 of three Earth-sized planets around TRAPPIST-1, the astronomers were inspired to study the system in even more detail, turning some of the world’s leading telescopes on the star. A combination of ground- and space-based facilities, including NASA’s Spitzer Space Telescope, watched carefully for the characteristic dips in TRAPPIST-1’s light as its planets transited in front. These careful observations paid off for the astronomers; what they had thought was three was actually at least seven planets, and all of them were roughly the size of Earth.

The team were able to tease even more information from the telescope data. The orbits of the planets around their star are very close, with all seven comfortably sitting inside the equivalent of Mercury’s orbit around the Sun. Fortunately for any life on these worlds, TRAPPIST-1’s low energy output means that the planets are not blasted with lethal radiation, despite their proximity.

Not only are they Earth-sized, but the innermost six in the system are likely made of rock too. Perhaps the final basic ingredient for life, then, is water. And thankfully, these worlds could have an abundance.

Jehin and his team ran climate models for the atmospheres of the planets, with some exciting results. The three innermost planets, TRAPPIST-1b, c and d are simply too close to their parent star to support anything more than isolated pockets of water. Information about the most distant planet, TRAPPIST-1h, is the most scant, but astronomers say it is likely to be an icy world if water is present. TRAPPIST-1e, f and g, however, have the potential to host vast oceans of water on their surfaces, according to the team.

Three of the planets in the system could therefore be rocky, Earth-sized temperate worlds with low levels of irradiation and oceans of water. It’s no surprise that planet-hunting astronomers are excited by this discovery.

The future

Telescopes are getting better all the time. With the next generation of observing facilities, such as ESO’s European Extremely Large Telescope and the NASA/ESA James Webb Space Telescope — both looking to start surveying the cosmos within the next decade — the possibilities for astronomical discovery are truly astounding.

Star systems like TRAPPIST-1 present key targets for these telescopes, as they will have the capability to detect water in the atmospheres of the planets. Though it is perhaps a long-shot, the presence of life also leaves its unique biomarkers in an atmosphere, which could potentially be detectable in the starlight that passes through them on the way to Earth.

There is no denying that this discovery has truly excited the field of exoplanet hunting. Not only have astronomers discovered seven Earth-sized worlds in a single system, the type of star that hosts them is very common in the Milky Way. There could be a plethora of potentially habitable worlds dotted throughout our galaxy — it’s just up to us to find them.

Image credit: NASA

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