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TRAPPIST-1 system
A diagram shows seven exoplanets orbiting an ultracool dwarf star known as TRAPPIST-1. If the planets were transported to our own solar system, they’d all lie within Mercury’s orbit. (ESO Illustration)

A second look at an exoplanet system 40 light-years from Earth has brought a bonanza for astronomers: not two, not three, but seven alien worlds – some of which could have acceptable conditions for life.

“I think that we’ve made a crucial step towards finding if there is life out there. … Before, it was indications,” said study co-author Amaury Triaud of Cambridge University’s Institute of Astronomy. “Now we have the right target.”

That claim is debatable, but in any case, the discovery suggests that there are even more planets out there than astronomers previously thought. Which is what astronomers have been saying repeatedly for the past decade.

“The solar system with its four (sub-)Earth-sized planets might be nothing out of the ordinary,” Ignas Snellen of the Leiden Observatory wrote in a commentary on the findings, published in this week’s issue of the journal Nature.

The discoveries actually started last year, when the same team of astronomers reported spotting three Earth-sized planets around an ultracool dwarf star called TRAPPIST-1, in the constellation Aquarius.

The star is only slightly bigger than Jupiter, and puts out about 0.05 percent as much light as our sun. It takes its name from the 23-inch telescope in Chile that the astronomers used to find it, known as the Transiting Planets and Planetesimals Small Telescope or TRAPPIST.

The team, led by Michaël Gillon of Belgium’s University of Liège, conducted follow-up observations with TRAPPIST and other instruments over the months that followed. They watched for repeated dimmings of the TRAPPIST-1 star as planets passed across its disk, and worked out the sizes as well as the masses of the planets by carefully monitoring the variations in the timings of the transits.

The additional data showed that the pattern of sightings they associated with the third, outermost planet was actually being caused by three separate worlds. They also detected the signatures of two farther-out planets, bringing the total to seven.

The planets are known as TRAPPIST-1 b, c, d, e, f, g and h. All of them lie within what would be the orbit of Mercury in our own solar system. But because the parent star is so dim, the e, f and g planets are in the system’s habitable zone, where it’s theoretically cool enough for water to exist in abundant liquid form.

All seven planets are roughly Earth-sized, and planets b through g appear to be rocky planets like our own. Planet h is still a question mark, however, because astronomers don’t yet have enough data to figure out its mass or its density.

Although TRAPPIST-1 isn’t close enough to visit anytime soon, the newly published findings add some extra luster to the dim bulbs of the Milky Way. Ultracool dwarfs and red dwarfs are appealing targets because they’re the most populous classes of stars in our galaxy, and because they’re so long-lived.

TRAPPIST-1 is thought to be at least a billion years old, and Gillon said it’ll probably remain stable long after our own sun has run out of gas. The same forecast applies to Proxima Centauri, the 4.8 billion-year-old red dwarf that’s only 4.2 light-years away from us and harbors at least one Earth-type planet.

There are potential drawbacks, however: For one thing, the dynamics of TRAPPIST-1’s planets suggest that they present the same face to the star all the time. That could bake one side and freeze the other, unless there’s an atmosphere thick enough to even things out.

For another thing, red dwarf stars occasionally unleash strong radiation flares that could blast away at a planet’s atmosphere. A study published this month suggested that the loss of oxygen due to such blasts could significant reduce the chances for habitability.

Gillon downplayed the radiation threat in TRAPPIST-1’s case. “It has some flares, but they are not very strong, and it is quite rare,” he said.

Planet comparison
This chart compares the basic statistics for the TRAPPIST-1 planets with similar statistics for our solar system’s rocky planets. Click on the image for a larger version. (NASA / JPL-Caltech Illustration)

The key questions, then, have to do with what’s in the atmospheres of the TRAPPIST-1 planets. Triaud said the team is already trying to figure that out, using the Hubble Space Telescope and other instruments.

“The first stage that we are doing at the moment is a reconnaissance stage, where we are trying to rule out that the planets have a large hydrogen envelope. This is to make sure that the planets are indeed Earthlike,” he explained. “This will then be followed by detailed observations to study the climate, and eventually, from the chemical information, trying to find if there is life over there.”

He hoped to get the answer to the life question “maybe within a decade.” Reaching that goal depends on building bigger and better observatories, such as NASA’s James Webb Space Telescope and the European Extremely Large Telescope.

Gillon said determining the presence of life will require not only figuring out what chemicals are in the atmosphere – such as oxygen and water vapor, carbon dioxide and methane – but also figuring out their proportions. “It’s really the combination of molecules,” he said. “Oxygen itself is not enough.”

This artist’s conception paints a speculative picture showing what it might be like to stand on the surface of the planet TRAPPIST-1f. (NASA / JPL-Caltech Illustration)

Although it’s unlikely that humans will ever stand on TRAPPIST-1f or the other planets in the star system, it’s intriguing to imagine what it’s like there. Because the planets are so tightly packed, you could travel from one world to another in a matter of days using conventional spaceflight technology. No warp drive required.

Triaud said the days wouldn’t get much brighter than, say, a sunset on Earth. “However, you will still feel quite warm, because you still receive just about as much energy from the star, which is in the infrared. That, you will feel with your skin,” he said.

It sounded as if Triaud was getting a warm glow just thinking about the scene.

“The spectacle would be beautiful,” he said, “because every now and then you would see another planet, maybe about twice as big as the moon in the sky, depending on which planet you are on.”

In addition to the TRAPPIST-South telescope in Chile, instruments contributing to the observations included TRAPPIST-North in Morocco, HAWK-I on the European Southern Observatory’s Very Large Telescope, UKIRT in Hawaii, the Liverpool and William Herschel telescopes in the Canary Islands, the SAAO telescope in South Africa and NASA’s Spitzer Space Telescope.

The Nature paper, “Seven Temperate Terrestrial Planets Around the Nearby Ultracool Dwarf Star TRAPPIST-1,” lists 30 authors including Gillon and Triaud.

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