40 light years from Earth sits an ultracool dwarf star formerly known as 2MASS J23062928-0502285. Today, it’s called TRAPPIST-1 after the telescope used to find the trio of planets orbiting the star. And the star system just shot to the top of the list for potentially habitable planets.

Why? The proximity to Earth makes follow-up observations much easier. Plus, the qualities of the dwarf star and its planets are intriguing.

TRAPPIST-1 is about one-eighth the size of our sun, and much cooler. Last fall, scientists noticed the star’s infrared signal would fade at regular intervals. Objects passing in front of the star were the most obvious culprits. Additional observations confirmed what scientists suspected. Three planets were orbiting the star.

The two closest planets are about the size of Earth and Venus. One orbits the star every 1.5 days while the other orbits every 2.4 days. Nailing down the orbit of the third planet is a little trickier. Scientists know it takes longer than 4 days, but less than 73 days to orbit.

Despite the closer orbits, the two innermost stars receive just four and two times the amount of radiation compared to what the Earth receives from the sun. The third planet may receive less radiation than the Earth.

There is one problem, though. While the star is much cooler than the sun, the three planets are likely tidally locked. That means one side of the planet is permanently exposed to the star while the other sits in perpetual darkness.

Tidally locked planets aren’t usually good candidates for life. Temperatures on the planet range from scorching hot on the day-side to incredibly cold on the night-side.

Preliminary observations did identify a “sweet spot” on the western side of the two innermost planets that sits on the day-side, but with cooler temperatures.

Thanks to their proximity to Earth, follow-up observations should tell us much about these worlds.

“These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited,” co-author Julien de Wit says. “All of these things are achievable, and within reach now. This is a jackpot for the field.”

The challenging search for habitable planets

The search for alien life, especially outside our solar system, is notoriously difficult. The Kepler Space Telescope has found hundreds of exoplanets, but many of them are way too hot to support life as we know it. Most exoplanet searches focus on bright, sun-like planets.

But because these stars are so bright, scientists can’t see any signals from the planets. They know the planets are there, but they have trouble studying their atmospheres.

What scientists need to find are planets around cooler stars. And that’s where the TRAPPIST telescope comes in.

Trappist telescope

TRAPPIST telescope. Credit: ESO

TRAPPIST, or TRAnsiting Planets and PlanetesImals Small Telescope, is a proof of concept. Today’s exoplanet search missions aren’t designed to find planets around cool, dwarf stars.

“That means they can’t detect planets around such stars,” says de Wit. “So you have to design a completely different survey using special instruments and detectors – it’s a risk.”

Scientists also needed a little luck with their search. Because TRAPPIST is a proof of concept, scientists couldn’t look at thousands of stars like Kepler. They looked at just 60 stars.

“It’s a few of them that you’re spending time on, one at a time,” said de Wit. “And one paid off.”

What’s next?

The next generation of telescopes will tease out details of the planets’ atmospheres and search for signs of life.

Telescopes like the James Webb Space Telescope will offer much more insight into the atmospheric properties of these worlds. The trio of planets aren’t perfect, but they do make compelling targets for additional study.

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