February 24, 2017
With the biggest build-up in exoplanet research history, on February 22 NASA announced the discovery of four additional earth-sized planets orbiting a dwarf star in the constellation Aquarius. This discovery brings the total number of planets orbiting the star dubbed TRAPPIST-1 to seven, a record for any one star in NASA’s exoplanet catalog.
The star TRAPPIST-1 takes its name from the instrument that discovered the planets: TRAnsiting Planets and PlanetesImals Small Telescope–South (abbreviated TRAPPIST). The telescope is located at the La Sila observatory in Chile, the site that discovered a planet orbiting Proxima Centauri in August 2016. TRAPPIST-1, the star, is so small it isn’t visible to the naked eye and doesn’t appear in any of the bright-star catalogues commonly cited on GAILEarth.com. The European Southern Observatory (ESO), whose scientists made the discovery, originally referred to it by the tongue-twisting appellation 2MASS J23062928-0502285.
Ironically, the discovery of three planets orbiting the dwarf star was announced in May 2016, three months before the discovery of Proxima Centauri-b. But the discovery drew little media attention without the NASA PR team behind it.
As with many other exoplanet discoveries, the ESO observed periodic fluctuations in the brightness of TRAPPIST-1 using the TRAPPIST telescope. These observations suggested planets were passing between the star and Earth, dimming the star’s light. ESO followed up with observations using an 8.2 meter Very Large Telescope at Cerro Paranal, also in Chile, combined with the High Acuity Wide-field K-band Imager (HAWK-I).
The additional data confirmed that at least three planets orbit the dwarf star. Two are closer to the star than the so-called habitable zone where temperatures would be too high for liquid water to exist on the surface. Astronomers speculated that the third planet might lie within the habitable zone of the star but its precise orbit was less certain. Results of the observations were published in the journal Nature on March 2, 2016.
NASA joins the search
In the fall of 2016, NASA turned its Spitzer orbiting space telescope on TRAPPIST-1 and not only confirmed the ESO discovery but found four additional roughly earth-sized planets — a total of seven — orbiting the dwarf star. Of the four new discoveries, three orbit in the habitable zone where liquid water might exist on the surface.
The Spitzer orbiting telescope is an example of why you should keep old spacecraft running as long as possible. The telescope operates in the infra-red range of electromagnetic radiation in order to study space objects that emit radiation invisible to our eyes. To see the coldest of space objects, the radiation detectors of infra-red telescopes are cryogenically cooled to near absolute zero (4 degrees Kelvin). Engineers at NASA’s Jet Propulsion Laboratory figured correctly that they could reduce the cooling load on the detectors if they placed the telescope in an orbit far from the relatively warm Earth. So Spitzer follows the path of Earth’s orbit at a long distance.
Spitzer was launched in 2003 and ran out of liquid helium coolant in 2009. Since then it has operated at a temperature of 28 degrees K. That’s still cold enough to see a lot of infra-red radiation. And because Earth’s atmosphere is so warm, any space-based infra-red telescope is much more sensitive than one on the ground, even a cold mountain top.
TRAPPIST-1 is a relatively cool star with an effective temperature of about 2,600 K. (The Sun’s temperature is more than twice as high at 5,778 K.) So a lot of the dwarf star’s radiation is in the infra-red range. When planets pass between the Spitzer telescope and TRAPPIST-1, radiation dims much more than it would when observed by a visible-light instrument. This increased sensitivity enabled NASA astronomers to detect the additional planets and to better estimate their sizes and orbits.
Overall, Spitzer spent almost three weeks trained on the dwarf star. This wouldn’t be much time for observing a Sun-sized star, but because the TRAPPIST-1 planets orbit so close to their primary, it was enough. The remotest of the seven planets circles the star in just 20 days and the others’ orbits are much shorter, as shown in figure 2. NASA also used the Hubble telescope to observe the two inner-most planets (b and c) and found no evidence of atmospheric conditions that might preclude life on the surface. More Hubble observations of the system are planned.
NASA websites are filled with colorfully illustrated images of the exoplanets, along with possible landscapes and views of the night skies. These images captured the attention of the popular news media in a way not seen since the first days of the Space Shuttle. However, these pictures are all artists’ impressions of what the planets might look like. In reality, scientists know very little about the planets of TRAPPIST-1. Even the values of the planets’ masses and gravitation at the surface (shown in figure 2) are as yet uncertain, according to NASA scientists.
Not places you’d want to live
We can say with a high degree of confidence that almost nobody would want to live on any of the planets in the TRAPPIST-1 system. Because the planets orbit so close to the star, NASA believes they are all “tidally locked,” which means one side faces the star and the other faces away from it. There are no cycles of day and night. (The moon is tidally locked to Earth. It never shows its far side. Mercury is tidally locked to the Sun.)
Scientists are still debating what effect tidal locking might have on a planet’s atmosphere. Would it all boil away? Would perpetually strong winds blow from the dark side to the light? We don’t know, but the likelihood is very unpleasant weather. We don’t even know if the planets have atmospheres.
Because TRAPPIST-1 is so cool, the light from it is red and largely invisible infra red. Even the light side of the planet would be gloomy, and nothing remotely resembling human foodstuffs would likely grow in it.
Red dwarf stars have reputations for emitting large amounts of x-rays that are deadly to our forms of life. Solar flares might periodically waste the surfaces of nearby planets. Even if life started to evolve in the habitable-zone planets, the red dwarf might extinguish it.
TRAPPIST-1 also might not be old enough for life to have evolved on its planets. The NASA Exoplanet Archive places its age at more than 0.5 billion years. In contrast, Earth is estimated to be 4.5 billion years old.
Even though the TRAPPIST-1 planets are no candidates for future settlement, their discovery is a remarkable scientific achievement. Future observations seem likely to refine techniques for finding Earth-sized planets in other systems. And the large number of Earth-sized worlds in one system suggests that small rocky planets may be more common in our corner of the galaxy than we have previously expected.
Artists who would like to create imagined images of life on other planets might have more fun working with worlds in more Sun-like systems. We invite you to explore the worlds of GAIL Earth Branch on this website and to contribute your visions of what life on them might be like.