Cool star’s seven rocky planets offer chance of life

Astronomers have discovered seven Earth-sized rocky planets circling a star that lies right in our cosmic back yard – and some could be home to life.

TRAPPIST-1 planet
An artist imagines the view from just above the surface of one of the rocky worlds orbiting TRAPPIST-1, with other planets in the distance. Image credit: ESO/M. Kornmesser/spaceengine.org

The major find, announced by NASA tonight, was made by an international team of scientists. It is a record for the number of Earth-like, potentially habitable worlds in one star system.

The astronomers are excited because at least three of the rocky planets are at just the right temperature to be covered with oceans (though no one knows if they actually are). There could be liquid water on the others too – vital for life as we know it.

The Hubble Space Telescope is already searching for atmospheres around the planets as a first step to check them out for clues that they are inhabited.

Rocky worlds have been detected before, but never so many of similar size to Earth in one star system. The new finds were detected orbiting a star called TRAPPIST-1 which lies just 39 light-years away in the constellation of Aquarius.

Some of the most powerful telescopes on Earth and in space were used to observe the planets passing in front of the star. Though they are too small to be seen individually, they gave themselves away by blotting out a little of the star’s light. From these observations of fading starlight, astronomers were able to measure the planets’ sizes and temperatures, and to weigh them up.

The planets do not have names yet, but have simply been labelled TRAPPIST-1b, c, d, e, f, g and h. They all lie closer to their home star than our own inner planet Mercury is from the Sun. However, the star is a red dwarf that is much cooler than the Sun, and so at least six of the new worlds enjoy similar conditions that could make them habitable.

Rocky planets in TRAPPIST-1 system
A highly speculative illustration of how the seven planets orbiting TRAPPIST-1 might appear, alongside some images of the rocky planets in our Solar System. Data about the size and orbital periods of all the planets is also shown. Image credit: NASA

Dr Amaury Triaud, of the Institute of Astronomy, Cambridge, in the UK, helped lead the discovery team. He told Skymania today: “There have been announcements before of planets that might be habitable, but we didn’t have enough information to be sure that they were rocky, and so Earthlike.

“The difference this time is that we have the radius, the mass and an indication of the approximate temperatures of each of the planets, and will be able to study them in greater detail.

“We will be able to probe their atmospheres remotely from Earth, find out their climate, and eventually determine the chemical composition of the atmospheres. Life leaves a chemical trace within the atmosphere, so we hope one day to detect that too.

“Here we have the right target to start the search for life. It is the first one we have really, but instead of just one planet we have seven!”

Dr Triaud said that Hubble will monitor the rocky planets as they pass in front of their star – known as a transit – and detect the starlight shining through their atmospheres to help learn about them. And NASA’s giant new James Webb Space Telescope, due to launch next year, will be powerful enough to check them out in more detail.

“Hubble needs to capture about 10 transits for each of the planets to start distinguishing whether they are more Venus-like or Earth-like. We won’t know right away their composition but we will already gain very insightful information.

“And then with the James Webb Space Telescope, which is due to be launched in April or March next year, we will have the capacity to go much deeper, and measure the abundance of individual molecules. From the cocktail of molecules detected, we will deduce whether there is active biology on the surface – life in other words – or whether the planets are barren.”

Light curves for TRAPPIST-1
This diagram, plotting the star’s brightness against the passage of time, shows how its light was seen to fade as each planet passed across it, as observed with NASA’s Spitzer Space Telescope. Image credit: ESO/M. Gillon et al.

Dr Triaud cautioned that conditions might also make life less likely. He said: “The planets are very Earth-like in many aspects – temperature, size, composition, radius – but they are very different in other aspects.

“They are so close to their star that they would probably show a permanent day side and a permanent night side, known as tidal locking. The impact of this on the climate has been theoretically studied but not empirically verified, so it is something we have to check.”

He added: “I’m convinced that if there is life there, we will have the chance to detect it within the next decade. So we could soon have an answer to one of the oldest philosophical questions – are we alone in the Universe – and that is quite incredible.

“For me this discovery is extremely exciting. I’m doing research because I want to find out how frequently life has formed in the Universe, and to be at such an advanced stage in our understanding of this, and to participate in the adventure, is great.

“I’d thought the discovery of life might come when I retire, or maybe on my deathbed, but now we have a real shot at finding it during our careers.”

The discovery team was led by Michaël Gillon of the STAR Institute at the University of Liège in Belgium. He said: “This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth!”

Early in 2016, Michaël Gillon announced the discovery of three planets orbiting TRAPPIST-1, which has only 8 per cent the mass of the Sun and is just slightly bigger than Jupiter.

Since then, observations with NASA’s Spitzer Space Telescope, plus telescopes in Chile, Morocco, the Canary Islands, Hawaii and South Africa, have confirmed the existence of at least seven planets and taken their vital statistics.

The planets are so close to their star that they zip round it in just a few days. The closest, TRAPPIST-1b, has a “year” that is only 1.51 days long, while the most distant, TRAPPIST1-h, takes around 20 days to make one orbit.

The planets’ temperatures can be compared to those in own inner Solar System. In particular, the planets TRAPPIST-1c, d and f get a similar amount of heat from their star as Venus, Earth and Mars do from our sun.

But if, as scientist believe, the planets are “tidally locked”, that means they will keep the same side always facing their home star, rather than spinning like the Earth. If so, then there will probably be extreme differences in temperature between the side with perpetual day and that with eternal night.

TRAPPIST-1 is less than a tenth the size of the Sun and a little larger than the biggest planet in our solar system, Jupiter. The ultracool red dwarf is estimated to be greater than 500 million years old, compared to the Sun’s age of 4.6 billion years.

The announcement of the new planets comes soon after the news that planets have been “filmed” orbiting in another alien solar system.


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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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