The red dwarf has seven alien planets the size of Earth, giving TRAPPIST-1 more potentially habitable worlds than any other star we know about.

Key takeaways

  • TRAPPIST-1 hosts seven Earth-sized planets, with six likely rocky and all potentially having liquid water.
  • These planets orbit in the habitable zone of their star, where conditions may support liquid water—essential for life as we know it.
  • NASA’s Spitzer and Hubble telescopes are among those focused on TRAPPIST-1, aiming to study each planet in depth for signs of life.
  • TRAPPIST-1 is an ultracool dwarf star, smaller and cooler than our Sun, yet hosts a planetary system reminiscent of Jupiter’s moons.

The most promising site to look for life outside of our solar system has just become much more appealing.

A team of researchers using ESO’s Transiting Planets and Planetesimals tiny Telescope, or TRAPPIST, found three planets circling a tiny, faint red dwarf star 39 light-years distant. All three exoplanets were around the size of Earth and were in the so-called “Goldilocks Zone,” where temperatures may range between 0 and 100 degrees Celsius—ideal conditions for liquid water and, perhaps, life.

The team, led by Michaël Gillon from the STAR Institute at the University of Liège in Belgium, enthusiastically pointed more observatories toward TRAPPIST-1, including NASA’s Spitzer Space Telescope and the ESO’s Very Large Telescope. According to a research published in Nature, TRAPPIST-1 features seven Earth-sized planets, six of which are likely rocky, and all seven might potentially host liquid water.

This chart shows, on the top row, artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.

“All of these planets are the best targets found so far to search for signs of life in the next decade, and it is remarkable that they are all transiting the same star,” co-author and MIT planetary scientist Julien de Wit told Popular Mechanics in an email. “This means that the system will allow us to study each planet in great depth, providing for the first time a rich perspective on a different planetary system than ours.”

A Solar System That Looks Like Jupiter

TRAPPIST-1 is considered an ultracool dwarf star. This faint little star has just 8% of the mass of the sun, and its temperature is predicted to be roughly 2,550 Kelvin, compared to 3,800 Kelvin for other red dwarfs and a scorching 5,800 Kelvin for our sun. In reality, TRAPPIST-1 is just slightly larger than Jupiter.

Fortunately, the seven planets around TRAPPIST-1 are significantly closer to their host star than we are. The orbits of the star’s inner planets mirror Jupiter’s Galilean moons. All seven are significantly closer to TRAPPIST-1 than Mercury is to the sun, so they receive roughly the same amount of energy and heat as Earth.

The planets also pass relatively close together as they circle. The space agency says: “If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth’s sky.”

The measured orbital periods of the planets, compared with those of Jupiter\’s Galilean moons and the four inner planets of the Solar System. The sizes of all the objects are approximately to scale.

Some planets are better positioned to accommodate liquid water than others. TRAPPIST-1A is the name given to the star, and the planets are 1b, 1c, 1d, 1e, 1f, 1g, and 1h, in order of innermost to outermost. TRAPPIST-1b, 1c, and 1d are likely too near and heated for water. TRAPPIST-1h is possibly too far and too cold. TRAPPIST-1e, 1f, and 1g, on the other hand, are perfectly situated in the Goldilocks Zone. For all we know, these are true paradises, complete with rolling waters and huge woods. (Probably not, but not impossible.)

However, we know very little about these planets, except than estimations of their size, distance from the host star, and orbital periods. We can identify exoplanets when they transit their host stars, which means they pass in front of the star from our perspective and block part of the light that reaches our telescopes. We can determine the presence of planets and even their size based on the subsequent decrease in the brightness of the stars.

Red dwarfs, such as TRAPPIST-1, are ideal stars to seek since they are already faint, making any change in brightness easier to notice. Planets circling dwarf stars often have short orbital periods, making the complete trip around in weeks or even days, thus there is plenty of chance to observe them moving across the star’s face. (The TRAPPIST-1 planets orbit their star every 1.5–20 days).

Red dwarfs are also the most numerous and long-lived stars in our galaxy. Red dwarfs account for an estimated 85 percent of the Milky Way’s 100 billion stars. TRAPPIST-1 will continue to slowly burn hydrogen for another 10 trillion years, 700 times longer than the universe’s whole history—more than enough time for life to establish itself and evolve.

Artist\’s impression of view from planet in the TRAPPIST-1 planetary system.

These universes, as fascinating as they are, have a few characteristics that bring their habitability into doubt. For starters, the planets are most likely tidally locked with TRAPPIST-1, which means that the same face of the planet is constantly facing the star, similar to how one side of the moon always faces the Earth. If this is the case, one half of each planet sees constant sunshine while the other side is trapped in an eternal night.

Another problem is that red dwarfs may be extremely active, with stellar eruptions, flares, and coronal mass ejections hitting surrounding planets with radiation. For this reason, a recent research calls the habitability of Proxima b, the nearest exoplanet to Earth, into question. However, TRAPPIST-1 is a colder star than Proxima Centauri, therefore its planets may not be bombarded with high-energy particles as frequently.

“To the best of our knowledge, TRAPPIST-1 appears particularly quiet,” remarks de Wit.

TRAPPIST-1 may, in fact, be a peaceful, life-giving star. But it’s over 40 light-years away, very tiny, and quite faint. How do we know for sure?

Probing the Atmospheres of Exoplanets

Breakthrough Starshot is a developing concept to send microscopic nanoprobes toward the Alpha Centauri system, the nearest stars to humanity. With a network of massive lasers on Earth, it may be able to accelerate a probe to around 20% the speed of light by repeatedly striking a reflecting surface with focused beams of light. This technique, known as photonic propulsion, may enable humanity to reach Alpha Centauri in 20 or 30 years.

But we won’t be traveling to TRAPPIST-1 anytime soon. It is approximately eight times further distant than Alpha Centauri. Even if we could launch a probe at relativistic speeds, it would take two years to reach its destination, and the vast distance makes it improbable that it would ever arrive. Even if it did, detecting a signal from a tiny nanoprobe 40 light-years away would be next to impossible. For the time being, we’ll have to study the TRAPPIST system at home.

ESO/M. Kornmesser Artist\’s impression of view from distant planet in the TRAPPIST-1 planetary system.

We can do more with our telescopes than you might think, especially the JWST. By imaging the light that passes through an exoplanet’s atmosphere, we can look for gaps in the electromagnetic spectrum—wavelengths of light that are absorbed by the presence of specific elements. This technique, called absorption spectroscopy, can tell us the composition of a planet’s atmosphere, even 40 light-years away.

“There is a lot to be done with what we already have and what we are about to have!” argues de Wit in reference to telescopes. “With observations of this system taken by Hubble last May, we have already ruled out the presence of puffy, hydrogen-dominated atmospheres around the two innermost planets, which means that they are not ‘mini-Neptunes’ that would be uninhabitable, but are terrestrial like Mercury, Venus, Earth and Mars.”

The launch of the James Webb Space Telescope (JWST) next year, together with the completion of the Giant Magellan Telescope (GMT) in Chile’s high Atacama Desert, will provide us with a sharper image of these exoplanets than ever before. James Webb’s 18 gold-plated mirrors will identify objects 16 times fainter than Hubble, while the massive GMT, with seven 15-ton mirrors, could be able to image exoplanets directly rather than seeking for dips in a star’s light. These two telescopes, together with others like the European Extremely Large Telescope, will reveal exactly what atmospheres the TRAPPIST-1 planets have.

“Over the next two years, we are hoping to leverage Hubble’s capabilities to search for the presence of water- or methane-dominated atmospheres,” de Wit explains. “In the future, upcoming observatories like the James Webb Space Telescope will help us constrain the planets’ atmospheric composition, temperature, and pressure profiles—all essential information for determining the surface conditions possible over their globes.”

This material will not give conclusive proof, but it may provide significant visual evidence if one of these planets does sustain life. Astronomers will look for “biosignatures,” such as high quantities of oxygen, which may be caused by photosynthesis. However, abundant oxygen alone does not indicate the presence of life.

“We need more than just O2,” says de Wit. “Biosignatures can appear in many forms from complex molecules like CFCs chlorofluorocarbons or mixes of molecules such as H2O, O2/O3, CO2 or CH4. JWST may not provide sufficient evidence to prove the presence of a biomass by itself, but it will inform us on the habitability of the planets.”

Though the TRAPPIST-1 system is too far for a probe, it is very close on a cosmic scale. With planets round their sun every few days, we will have hundreds of opportunities to examine these fascinating worlds via our ever-improving telescopes. Furthermore, many astronomers believe TRAPPIST-1 may not be an oddity at all, but rather one of millions of neighboring red dwarfs and other stars with large planetary systems.

It’s difficult not to believe that one of those planets contains rich alien gardens, and that with continuing scientific advancements like these, we’ll discover them.

NASA/JPL-Caltech The TRAPPIST-1 star, an ultracool dwarf, and seven Earth-size planets orbiting it.
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