February 22, 2017

from PHYS Website

This artist's impression

shows the view from the surface of one of the planets

in the TRAPPIST-1 system.

At least seven planets orbit this ultra cool dwarf star

40 light-years from Earth

and they are all roughly the same size as the Earth.

They are at the right distances from their star

for liquid water to exist on the surfaces

of several of them.

Credit: ESO/M. Kornmesser

Astronomers have found a system of seven Earth-sized planets (exoplanets) just 40 light-years away.


They were detected as they passed in front of their parent star, the dwarf star TRAPPIST-1. Three of them lie in the habitable zone and could harbor water, increasing the possibility that the system could play host to life.


It has both the largest number of Earth-sized planets yet found and the largest number of worlds that could support liquid water.

Astronomers using,

  • the TRAPPIST-South telescope at ESO's La Silla Observatory

  • the Very Large Telescope (VLT) at Paranal

  • the NASA Spitzer Space Telescope,

...as well as other telescopes around the world, have now confirmed the existence of at least seven small planets orbiting the cool red dwarf star TRAPPIST-1.


All the planets, labeled TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their parent star, have sizes similar to Earth.

Dips in the star's light output caused by each of the seven planets passing in front of it (astronomy) - events known as transits - allowed the astronomers to infer information about their sizes, compositions and orbits.


They found that at least the inner six planets are comparable in both size and temperature to the Earth.

Lead author MichaŽl Gillon of the STAR Institute at the University of LiŤge in Belgium is delighted by the findings:

"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!"


This artist's concept shows

what the TRAPPIST-1 planetary system may look like,

based on available data about the planets'

diameters, masses and distances from the host star.

Credit: NASA-JPL/Caltech

With just 8% the mass of the Sun, TRAPPIST-1 is very small in stellar terms - only marginally bigger than the planet Jupiter - and though nearby in the constellation Aquarius (The Water Carrier), it appears very dim.


Astronomers expected that such dwarf stars might host many Earth-sized planets in tight orbits, making them promising targets in the hunt for extraterrestrial life, but TRAPPIST-1 is the first such system to be found.

Co-author Amaury Triaud expands:

"The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun. Planets would need to be in far closer orbits than we see in the Solar System if there is to be surface water.


Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1!"

The team determined that all the planets in the system are similar in size to Earth and Venus in the Solar System, or slightly smaller.


The density measurements suggest that at least the innermost six are probably rocky in composition.


Imagine standing on the surface

of the exoplanet TRAPPIST-1f.

This artist's concept is one interpretation

of what it could look like.

Credit: NASA/JPL-Caltech

The planetary orbits are not much larger than that of Jupiter's Galilean moon system, and much smaller than the orbit of Mercury in the Solar System.


However, TRAPPIST-1's small size and low temperature mean that the energy input to its planets is similar to that received by the inner planets in our Solar System.


TRAPPIST-1c, d and f receive similar amounts of energy to Venus, Earth and Mars, respectively.

All seven planets discovered in the system could potentially have liquid water on their surfaces, though their orbital distances make some of them more likely candidates than others.


Climate models suggest the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces.


The orbital distance of the system's outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbor liquid water - assuming no alternative heating processes are occurring.


TRAPPIST-1e, f, and g, however, represent the holy grail for planet-hunting astronomers, as they orbit in the star's habitable zone.


These new discoveries make the TRAPPIST-1 system a very important target for future study.


The NASA/ESA Hubble Space Telescope is already being used to search for atmospheres around the planets and team member EmmanuŽl Jehin is excited about the future possibilities:

"With the upcoming generation of telescopes, such as ESO's European Extremely Large Telescope and the NASA/ESA/CSA James Webb Space Telescope , we will soon be able to search for water and perhaps even evidence of life on these worlds."

This research was presented in a paper entitled "Seven Temperate Terrestrial Planets Around the Nearby Ultracool Dwarf Star TRAPPIST-1", by M. Gillon et al., to appear in the journal Nature. doi:10.1038/nature21360






These Seven Alien Worlds

...could help Explain how Planets Form
by Alexandra Witze
22 February 2017

from Nature Website




An artist's illustration

shows two Earth-sized planets

spinning across the face of

an M dwarf star called TRAPPIST-1.



The Earth-sized astronomical bounty

circles a dim star

that flew under the radar

of exoplanet researchers.


Seven alien, Earth-sized worlds bask in the cool, red light of their parent star.


The planetary menagerie exists around a star overlooked by other exoplanet hunters, although it is just 12 parsecs (39 light years) from Earth.

Astronomers have found other seven-planet systems before, but this is the first to have so many Earth-sized worlds.


All of them orbit at the right distance to possibly have liquid water somewhere on their surfaces.

"To have this system of seven is really incredible," says Elisa Quintana, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.


"You can imagine how many nearby stars might harbor lots and lots of planets."

Some of the planets were announced last year, but the authors debuted five newfound ones in a paper published on 22 February in Nature.1


Because the system is so close to Earth, astronomers can study the planets' atmospheres relatively easily.


That could reveal an astonishing diversity of worlds, ranging in composition from rocky to icy.

"This system is going to be one of the best laboratories we have for understanding the evolution of small planets," says Zachory Berta-Thompson, an astronomer at the University of Colorado Boulder.

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.

"These small stars had been completely overlooked," says MichaŽl Gillon, an astronomer at the University of LiŤge in Belgium.




Magnificent seven


MichaŽl Gillon leads the TRAPPIST collaboration, which hunts for planets using two 60-centimetre telescopes:

  • one in Chile

  • one in Morocco

They look for the faint dimming of a star's light that occurs when a planet moves across its face.


The team initially reported three planets around the star, known as TRAPPIST-1, last May.2


The team had caught only two glimpses of one of those planets, so they followed up on the faint signals with other telescopes. That process included 20 consecutive days when NASA's Spitzer Space Telescope stared at the star.


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, 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.


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.




An artist's illustration of

what TRAPPIST-1's seven planets

might look like.




Closing in

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.

"This is a Rosetta stone with seven different languages - seven different planets that can provide us with completely different perspectives on planet formation," adds team member Julien de Wit, a data scientist at MIT.

Although at least some fraction of each planet could harbor liquid water, it doesn't necessarily follow that they are habitable.


TRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have.3


And the worlds are likely locked into orbits where the same hemisphere always faces the star, rendering them permanently half-lit and half-dark. That would make it much more challenging for life to thrive.

Other researchers are already using the Hubble Space Telescope to hunt for atmospheres on the TRAPPIST-1 planets.


Kepler is also observing the system and will gather data that can better pin down the planetary masses, says Courtney Dressing, an astronomer at the California Institute of Technology in Pasadena.


And the TRAPPIST team is building four new 1-metre-diameter telescopes in Chile to continue the work.

"For all the worlds that we see in science fiction, these are even more extraordinary," says Hannah Wakeford, an exoplanet scientist at Goddard.


  1. Gillon, M. et al. Nature (2017) - Seven Temperate Terrestrial Planets Around the Nearby Ultracool Dwarf Star TRAPPIST-1 - Some of the planets were announced last year, but the authors debuted five newfound ones in a paper published on 22 February in Nature

  2. Gillon, M. et al. Nature 533, 221Ė224 (2016) - Temperate Earth-sized Planets transiting a nearby Ultracool Dwarf Star - The team initially reported three planets around the star, known as TRAPPIST-1, last May

  3. Wheatley, P. J., Louden, T., Bourrier, V., Ehrenreich, D. & Gillon, M. Mon. Not. R. Astron. Soc. Lett. 465, L74ĖL78 (2017) - Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1 - ÖTRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have