by Chris Larson | March 2017
The discovery of planets orbiting stars other than the Sun was news 20 years ago, but is not any longer. Even the discovery of Earth-like planets is no longer remarkable, since a fair number have been recorded in recent years. But the discovery of a relatively close star orbited by seven planets, all to some degree Earth-like, and all orbiting at a distance from the star that might allow for the presence of liquid water—that is unprecedented, and certainly newsworthy. This discovery was announced in a study appearing in Nature in February 2017.
The discovery of planets orbiting stars other than the Sun was news 20 years ago, but is not any longer. Even the discovery of Earth-like planets is no longer remarkable, since a fair number have been recorded in recent years. But the discovery of a relatively close star orbited by seven planets, all to some degree Earth-like, and all orbiting at a distance from the star that might allow for the presence of liquid water—that is unprecedented, and certainly newsworthy. This discovery was announced in a study appearing in Nature in February 2017.
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The discovery of Earth-like planets is no longer remarkable, since a fair number have been recorded in recent years. But the latest discovery of a relatively close star orbited by seven planets, all to some degree Earth-like, and all orbiting at a distance from the star that might allow for the presence of liquid water—that is unprecedented.
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The title of the Nature paper gets right to the point of what the astronomers found: "Seven Temperate Terrestrial Planets around the nearby Ultracool Dwarf Star TRAPPIST-1." Michael Gillon was the lead author of the study, which was conducted by a team with members from a number of countries, including the U.S., Belgium, France and Britain.
Ultra-Cool
TRAPPIST-1 is located about 12 parsecs, or 39 light-years, away from the Sun. It is a great deal smaller, cooler and dimmer than the Sun. It is also much less massive—it has about 8% of the Sun's mass. It is about the size of Jupiter, although it is denser: it has roughly 80 times the mass of Jupiter. The low core temperature means that it is barely able to fuse hydrogen into helium, and does so at a low rate. The low fusion rate translates into a long lifetime, much longer than that of the Sun and of stars of roughly the same size as the Sun or larger.
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TRAPPIST-1 is located about 12 parsecs, or 39 light-years, away from the Sun. It is a great deal smaller, cooler and dimmer than the Sun. It is also much less massive—it has about 8% of the Sun's mass. It is about the size of Jupiter, although it is denser.
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TRAPPIST is an acronym for the TRAnsiting Planet and Planetesimals Small Telescope. The telescope that discovered the planets circling TRAPPIST-1 is about 60 centimeters (24 inches) in diameter and is located in Chile. After the initial find by TRAPPIST, further observations were made by a number of other telescopes, including the Spitzer Space Telescope.
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TRAPPIST, the telescope that discovered the planets circling TRAPPIST-1, is about 60 centimeters (24 inches) in diameter and is located in Chile. The initial survey by TRAPPIST, and a subsequent closer look by other telescopes, yielded enough information to deduce the existence of seven Earth-like plants that included data on orbital periods, distance to TRAPPIST-1, planet radius and planet mass.
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TRAPPIST focuses on ultra-cool stars, stars with less than 15% of the mass of the Sun. There are a number of reasons for concentrating on these stars, the researchers point out:
• Ultra-cool stars are the most common stars in our galaxy, and thus learning about the planets that orbit them means presumably learning about the most common planets in our galaxy.
• These stars are small—and detection by the transit photometry method of planets orbiting small stars is much easier. This method measures the light emitted by a star; when a planet passes in front of the star (more precisely, when a planet passes between the star and the Earth—a passage referred to as a "transit"), the light output falls. On a website about TRAPPIST-1 curated by the researchers, they note that "the small size of the host star means the transit signals produced by Earth-sized planets are 80x more pronounced compared to similar planets transiting a Sun-like star (the signals from the atmosphere are also enhanced by 80 times)."
• Current technology allows researchers to determine the radius and massof an Earth-like planet with an Earth-like temperature if the planet orbits an ultra-cool dwarf star, but not when it is orbiting a star like the Sun.
• Planets orbiting ultra-cool dwarf stars share a number of characteristics with Earth, but differ in significant respects as well: they differ in the amount of radiation received, and presumably also always have the same side turned to their star. This mix of familiar and unfamiliar aspects should enlarge our understanding of how different factors combine to create climate and other phenomena.
Near Resonance
The initial survey by TRAPPIST enabled the researchers to identify three planets circling the star; the subsequent closer look by a suite of telescopes yielded enough information to deduce the existence of four more. The six inner planets, the researchers observe in their paper, "form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers." (An orbital period is how long it takes a planet to complete an orbit—for Earth this is 365 days, roughly, but since these planets are much closer to a much smaller star, their orbital periods are much shorter.) The ratios mean that the planets will have regularly recurring times when they are close enough to each other to have a significant gravitational interaction, and this can cause small changes in the transit times for the planet. The masses of the planets can be estimated as a result.
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TRAPPIST-1 is small, and the detection of planets by transit photometry—measuring the light emitted by a star—was continuously monitored by the Spitzer Space Telescope and ground-based data collectors from September 19, 2016 to October 10, 2016. When a planet passed between the star and the Earth, the light output fell. These observations enabled researchers to establish the relative positions of the planets' orbital phases.
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The research team observed that accepted theory on how planets form suggests that the TRAPPIST-1 planets formed further from their star than they presently are, and then migrated inwards. This is what is thought to have happened with the moons of Jupiter, for example. If the TRAPPIST-1 planets formed where it is thought they did, their composition would reflect what was found in that part of the disc of materials around the star, and this in turn would mean that they probably have lower densities than Earth.
Wait Till Next Year
Investigations into the TRAPPIST-1 system are in a sense just beginning. The James Webb Space Telescope is supposed to be launched into orbit in 2018, and if it is and the telescope performs as planned, we should be able to obtain information on what sort of atmospheres the planets have and get a better idea of their temperatures. This would allow the researchers to delve into the climates of the planets.
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Investigations into the TRAPPIST-1 system are in a sense just beginning. The James Webb Space Telescope is supposed to be launched into orbit in 2018, and along with other new astronomical tools, should enable researchers to gather additional information on the atmosphere and habitability of the planets.
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Is there, or was there ever, any liquid water on any of the planets? We don't know yet, but the possibility certainly can't be ruled out.
Other new astronomical tools will be coming on line in addition to the Webb telescope; in particular, a bigger and better version of TRAPPIST, called the Search for Planets EClipsing ULtra-COOl Stars (SPECULOOS), currently under construction in Chile. The new facility will be able to home in on many more ultracool stars than TRAPPIST.
NASA Jet Propulsion Lab
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The reason for the excitement about the TRAPPIST-1 discovery is that it suggests that Earth-like planets are abundant in our galaxy. Increasingly, the odds seem to indicate that Earth is not alone in the universe.
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The reason for the excitement about the TRAPPIST-1 discovery is that it suggests that Earth-like planets are abundant in our galaxy, the Milky Way. This can't be regarded as proven, since conceivably the discovery of seven such planets orbiting TRAPPIST-1 could be a lucky accident. But, increasingly, the odds seem to indicate that Earth is not alone; and if you are hoping to find life elsewhere in the universe, that must be regarded as good news, since to date Earth is the only place we know of where life has evolved.
Discussion Questions
Earth and the other planets in the Solar System orbit in a fairly small plane or slice of space, called the ecliptic plane. You can visualize this as a CD with, at the center, a small ball representing the Sun. Suppose someone living on a planet orbiting another star looked at the Sun—would they see the Earth transiting in front of the Sun? What would such an observation depend on?
Journal Abstracts and Articles
(Researchers' own descriptions of their work, summary or full-text, on scientific journal websites.)
Gillon, Michael, et al. "Seven Temperate Terrestrial Planets around the nearby Ultracool Dwarf Star TRAPPIST-1." Nature (Feb. 23, 2017) [accessed March 1, 2017]: http://www.nature.com/nature/journal/v542/n7642/full/nature21360.html.
Bibliography
Clery, Daniel. "Seven Potentially Habitable Earth-Sized Planets Spied around Tiny Nearby Star." Science (Feb. 22, 2017) [accessed March 2, 2017]: http://www.sciencemag.org/news/2017/02/seven-potentially-habitable-earth-sized-planets-spied-around-tiny-nearby-star.
"TRAPPIST-1" Website curated by TRAPPIST-1 researchers [accessed March 2, 2017]: http://www.trappist.one/.
Gillon, Michael, et al. "Seven Temperate Terrestrial Planets around the nearby Ultracool Dwarf Star TRAPPIST-1." Nature (Feb. 23, 2017) [accessed March 1, 2017]: http://www.nature.com/nature/journal/v542/n7642/full/nature21360.html.
Snellen, Ignas A. G. "Astronomy: Earth's Seven Sisters." Nature (Feb. 23, 2017) [accessed March 1, 2017]: http://www.nature.com/nature/journal/v542/n7642/full/542421a.html.
Keywords
TRAPPIST, TRAPPIST-1, TRAnsiting Planet and Planetesimals Small Telescope , SPECULOOS, Search for Planets EClipsing ULtra-COOl Stars, dwarf star, terrestrial planets, Michael Gillon
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