In a much-anticipated press conference on 22 February 2017, NASA announced the discovery of seven “Earth-sized” planets orbiting a single star 40 light years away. Three are “firmly” located in the habitable zone:

The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system. All of these seven planets could have liquid water – key to life as we know it – under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.

The findings were published in the journal Nature, released in concert with the press conference. NASA monitored this star, named TRAPPIST-1 after the Chilean ground telescope that discovered it and two of the seven planets, for 20 continuous days using their Spitzer Space Telescope. By observing tiny but periodic dips in light from the star, scientists were able to confirm the existence, size, and orbit of seven “Earth-sized” planets, as reported in a feature accompanying the article:

The resulting data revealed that what the scientists thought was a single planet was actually four that orbit their star roughly every 4, 6, 9 and 12 days.

Those four joined the two innermost planets [already discovered] by TRAPPIST and confirmed by the Hubble Space telescope], which whirl around the star once every 1.5 days and 2.4 days. The team also caught a hint of a seventh, more distant planet.

[Lead author Michaël] Gillon says that the six inner planets probably formed farther away from their star and then migrated inward. Now, they are so close to each other that their gravitational fields interact, nudging one another in ways that enabled the team to estimate each planet’s mass. They range from around 0.4 to 1.4 times the mass of the Earth.

While the sheer number of terrestrial planets in or near the habitable zone is newsworthy in its own right, this system has the added advantage of being close enough to Earth to enable scientists to gather even more detailed information, including the basic composition of the planets’ atmospheres. Speaking in a Reddit AMA (“ask me anything”) session hosted by NASA, Natalie Batalha, project scientist for the prolific Kepler Telescope, said:

Kepler taught us that temperate (i.e. Habitable Zone), terrestrial-sized planets are relatively common in the galaxy. The name of the game now is to find those near enough for atmospheric characterization. Of the few dozen Habitable Zone planets that have been detected to date, most are hundreds of light-years away whereas TRAPPIST-1 is just 40 light-years away.

In fact, NASA had already developed a telescope capable of characterizing the atmospheres of exoplanets, the James Webb Telescope, and it is scheduled to launch in 2018. NASA has signalled that they will be using it to investigate this system:

Spitzer, Hubble, and Kepler will help astronomers plan for follow-up studies using NASA’s upcoming James Webb Space Telescope, launching in 2018. With much greater sensitivity, Webb will be able to detect the chemical fingerprints of water, methane, oxygen, ozone, and other components of a planet’s atmosphere. Webb also will analyze planets’ temperatures and surface pressures – key factors in assessing their habitability.

While the scientists behind the study state that climate models allow for the possibility of liquid water on all seven planets, questions concerning any of these planets’ habitability are complicated by unknowns about the planet’s composition, their star, and its evolutionary history.

TRAPPIST-1—an M class star or red dwarf—is a smaller and cooler kind of star compared to our own. That means that the habitable zone is extremely close to the star itself and that these seven planets are very close together:

All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun. The planets also are very close to each other. If a person were 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.

One major implication of this close proximity, though, is that it very likely means that the planets:

May also be tidally locked to their star, which means the same side of the planet is always facing the star, therefore each side is either perpetual day or night. This could mean they have weather patterns totally unlike those on Earth, such as strong winds blowing from the day side to the night side, and extreme temperature changes.

Tidal locking or not, M class stars come with a host of other issues as well, answered NASA Goddard Space Flight Center astrobiologist Giada Arney on Reddit:

Red dwarfs (also called M dwarfs to astronomers) have a number of complications as host stars. M dwarfs flare frequently and can emit a lot of high energy radiation that can batter their planets’ atmospheres. On top of that, when M dwarfs are young, they are much, much brighter during a period called the pre-main sequence. What this means is that planets currently in the habitable zone of M dwarfs were actually interior to the habitable zone during the long pre-main sequence phase and could now be desiccated and potentially Venus-like despite being in the habitable zone today.

One way of getting around this challenge is if the planets migrated into the habitable zone from farther out after the super luminous pre-main sequence phase ended — which is possible!

Penn State University astronomer James Kastings, who was not involved in this study, told us that in terms of habitability, “there’s a list of problems with M star planets about as long as my arm, but [that] doesn’t mean that there’s not ways around them.” For him and many others in the field though, questions about habitability are only one reason to be excited by the find.

Reporting in Nature, Alexandra Witze wrote of these planets:

The arrangement of so many Earth-sized planets so close together will be a bonanza for researchers who are working to compare how worlds evolve. Venus and Earth started out in similar conditions, but ended up in two highly different states; uninhabitable Venus is now choked under a dense blanket of clouds. The TRAPPIST-1 system probably has a similar variety of worlds. “If one of these planets hosts life and the adjacent one doesn’t, why not?” asks Sarah Ballard, an astronomer at the Massachusetts Institute of Technology (MIT) in Cambridge.

The discovery also demonstrates the importance of looking at M class stars in future exoplanet surveys, she wrote, and is vindicates those who have already begun that work:

It’s also vindication for astronomers who hunt for planets around the cool, dim stars known as M dwarfs. These are the most common type of star in the Milky Way, but many exoplanet searches have focused instead on bigger and brighter stars that more closely resemble the Sun. Even NASA’s Kepler space telescope, which found most of the more than 4,700 planetary candidates known so far, turned to M dwarfs only recently.

While the James Webb Telescope will undoubtedly bring more details to light about these alien worlds, Arney told Reddit that we shouldn’t count on seeing any detailed images of the planets anytime soon:

[The James Webb Telescope] will observe planets transiting their host stars. […] From these transits, we can observe how gases in the planet’s atmosphere interact with starlight passing through the atmosphere. Unfortunately, this technique doesn’t allow us to see the surfaces of exoplanets. To do that, we’ll need farther future technology that may become available in the coming decades that will allow us to block out the star’s light and observe the planets directly.

However, the TRAPPIST-1 planets, being so close to their host star, would likely be tricky to directly observe in this way. These starlight suppression technologies fail once you get too close to the star, and so these types of observations would be extremely difficult. Other planetary systems orbiting hotter stars may be detectable with these technologies.

“I’m hoping this helps light the fire for building these big space telescopes to look for other such planets,” Kastings told us.


NASA   “NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star”
    22 February 2017.

Gillon, Michael et al.   “Seven Temperate Terrestrial Planets Around the Nearby Ultracool Dwarf Star TRAPPIST-1.”
    Nature.   22 February 2017.

Witze, Alexandra.   “These Seven Alien Worlds Could Help Explain How Planets Form.”
    Nature.   22 February 2017.   “We’re NASA Scientists & Exoplanet Experts. Ask Us Anything About Today’s Announcement of Seven Earth-Size Planets Orbiting TRAPPIST-1!”
    22 February 2017.