Ultracool dwarf hosts three planets – and they might be habitable

Our best bet for finding life outside our Solar System lies with a recently spotted planetary trio orbiting a cool, dim star 40 light-years away. Belinda Smith reports.

This artist’s impression shows an imagined view from close to one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth. One of the inner planets is seen in transit across the disc of its tiny and dim parent star. – ESO / M. Kornmesser

In a triple treat for exoplanet seekers, a Jupiter-sized star just 40 light-years away has been found playing host to three rocky planets with sizes and temperatures similar to Earth. They are, the researchers say, are our best bet for finding life outside our Solar System.

Astronomers from Belgium, India, the US and UK trained the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile onto the nearby ultracool dwarf star TRAPPIST-1 and saw three tiny planets orbiting it closely, perhaps within the solar system's "habitable zone". The planetary trio was published in Nature.

"So far, the existence of such ‘red worlds’ orbiting ultracool dwarf stars was purely theoretical," says study co-author Emmanuël Jehin from the University of Liège in Belgium.

"But now we have not just one lonely planet around such a faint red star but a complete system of three planets!"

Lead author Michaël Gillon, also from the University of Liège, adds: "If we want to find life elsewhere in the Universe, this is where we should start to look."

Discovered in the Aquarius constellation in 2000, TRAPPIST-1 (technically known as 2MASS J23062928-0502285) is what astronomers call an ultracool brown dwarf star.

As their name suggests, ultracool brown dwarfs not as hot or as big as our Sun – TRAPPIST-1 is around 8% our Sun's mass and only 0.05% its brightness.

This picture shows the Sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only around a tenth the Sun's diameter and is much redder. – ESO

Ultracool dwarfs are common in our part of the Universe – they comprise around 15% of the stars in the Milky Way. TRAPPIST was designed by Gillon and Jehin to specifically monitor nearby brown dwarfs in the infrared because they're too faint to be seen in optical light, and look for orbiting planets.

From September to December 2015, the team watched as TRAPPIST-1's infrared signal dipped slightly at regular intervals, suggesting that something – or things – passed in front of the star, blocking some of its light. (This type of exoplanet detection is called the transit method.)

Follow-up measurements using larger telescopes confirmed not one, but three Earth-sized planets orbiting the star. Two made the trip around the star in 1.5 days and 2.4 days, placing them very close to the star, and potentially in the way of too much radiation, even from an ultracool dwarf.

But, even if the two innermost planets do cop a lot of heat, there may be regions that could sustain life. Objects orbiting that closely and quickly are usually tidally locked – in other words, one side faces the star at all times, like we see the same side of the Moon always faces Earth.

So while the always-daytime side of the inner two planets might be too hot, and the shaded side which never sees radiation too cold, the edges between the two may be just right.

The third planet's orbit wasn't so easily calculated. It travels around its star once every 4.5 to 73 days. It may be in the habitable zone but it could also simply be too cold being that far away.

So what's next? The objective is clear, says Julien de Wit from the Massachusetts Institute of Technology and part of the research team: "Now we have to investigate if they’re habitable.

“We will investigate what kind of atmosphere they have, and then will search for biomarkers and signs of life."

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Belinda Smith is a science and technology journalist in Melbourne, Australia.