lpetrich
Contributor
On 2017 February 22, NASA announced that the star TRAPPIST-1 has 7 planets (NASA Telescope Reveals Record-Breaking Exoplanet Discovery | NASA). These planets were found by observing their transits or eclipses of their star. They were first found by a team at the University of Liège in Belgium with the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the La Silla Observatory in Chile. They were then observed using the Spitzer Space Telescope, the Very Large Telescope at Paranal Observatory in Chile, and the Kepler Space Telescope in its K2 mission.
At arxiv.org:
[1703.01424] Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
[1703.04166] A terrestrial-sized exoplanet at the snow line of TRAPPIST-1
[1704.02957] Limits on the Stability of TRAPPIST-1
[1704.04290] Updated Masses for the TRAPPIST-1 Planets
The star,
TRAPPIST-1, is a very low-mass red dwarf, at about the lower limit for nuclear fusion in its core. It is about 1/9 the Sun's size, close to that of Jupiter, and its overall luminosity is about 1/2000 the Sun's. Its visible-light luminosity is even less.
The planets are currently named, in order from their star, b, c, d, e, f, g, and h, as is typical of exoplanets.
The planets orbit very close to the star, about 0.01 to 0.06 times the Earth's distance. Their orbits are nearly circular with an approximately geometric, Bode-like distribution of orbit sizes. They are close enough so that one can resolve some details of them from each other. Their star has even greater angular size, however, from 1d at the farthest planet to 5.5d at the closest planet.
These planets are likely tidally locked, so that one side of them always faces their star. Something like the Moon from the Earth and most of the larger moons from their planets.
The planets' masses have been measured with a rather unusual technique: transit timing variations. The planets perturb each others' motions, and that makes them alternately fast and slow. Furthermore, the TRAPPIST-1 planets are in near-resonances, with their orbit periods' ratios being close to small-number fractions. This makes amplified TTV's, though the amplified ones vary relatively slowly. That is nevertheless enough to come up with estimates of the planets' masses.
For the TRAPPIST-1 planets, both their sizes and their masses are fairly close to the Earth's size and mass, meaning that their densities are not far off either. But with the most recent TTV masses, the densities of planets e, f, g, and h are low enough to be consistent with their being largely water. Planet b is on the watery side of being all-rock, planet d straddles the all-rock line, and planet c is likely rock-iron, like the Earth itself.
Also interesting about them is that some of the planets are within their star's habitable zone, with temperatures just right for liquid water. Judging from how much starlight they get, planet b is in between Mercury and Venus, c is like Venus, d is like the Earth, e is between the Earth and Mars, f is a little beyond Mars, g is like Vesta, and h is like Ceres. So d, e, and maybe f are within TRAPPIST-1's habitable zone.
The Solar System has plenty of largely-water worlds, though most of their water is frozen. Worlds like most of the larger outer-planet moons and many of the Kuiper-Belt Objects. So it may have been that TRAPPIST-1's planets formed farther out and then spiraled in. That would explain the resonances -- they would be from the outer planets pushing the inner planets inward.
Water worlds have an interesting habitability conundrum. It's nowadays thought that the likely site for the origin of life is hydrothermal vents. These have some advantages over the open ocean like chemical disequilibria and abundant catalytic sites on the vent rocks. But a few hundred kilometers down in a planetary ocean, the water will freeze into high-pressure forms of ice called Ice VI and Ice VII. Thus, hydrothermal vents will get surrounded by ice, though they may be able to create liquid-water chimneys in the ice.
Finally, TRAPPIST-1 -- a site called trappist.one
At arxiv.org:
[1703.01424] Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
[1703.04166] A terrestrial-sized exoplanet at the snow line of TRAPPIST-1
[1704.02957] Limits on the Stability of TRAPPIST-1
[1704.04290] Updated Masses for the TRAPPIST-1 Planets
The star,
TRAPPIST-1, is a very low-mass red dwarf, at about the lower limit for nuclear fusion in its core. It is about 1/9 the Sun's size, close to that of Jupiter, and its overall luminosity is about 1/2000 the Sun's. Its visible-light luminosity is even less.The planets are currently named, in order from their star, b, c, d, e, f, g, and h, as is typical of exoplanets.
The planets orbit very close to the star, about 0.01 to 0.06 times the Earth's distance. Their orbits are nearly circular with an approximately geometric, Bode-like distribution of orbit sizes. They are close enough so that one can resolve some details of them from each other. Their star has even greater angular size, however, from 1d at the farthest planet to 5.5d at the closest planet.
These planets are likely tidally locked, so that one side of them always faces their star. Something like the Moon from the Earth and most of the larger moons from their planets.
The planets' masses have been measured with a rather unusual technique: transit timing variations. The planets perturb each others' motions, and that makes them alternately fast and slow. Furthermore, the TRAPPIST-1 planets are in near-resonances, with their orbit periods' ratios being close to small-number fractions. This makes amplified TTV's, though the amplified ones vary relatively slowly. That is nevertheless enough to come up with estimates of the planets' masses.
For the TRAPPIST-1 planets, both their sizes and their masses are fairly close to the Earth's size and mass, meaning that their densities are not far off either. But with the most recent TTV masses, the densities of planets e, f, g, and h are low enough to be consistent with their being largely water. Planet b is on the watery side of being all-rock, planet d straddles the all-rock line, and planet c is likely rock-iron, like the Earth itself.
Also interesting about them is that some of the planets are within their star's habitable zone, with temperatures just right for liquid water. Judging from how much starlight they get, planet b is in between Mercury and Venus, c is like Venus, d is like the Earth, e is between the Earth and Mars, f is a little beyond Mars, g is like Vesta, and h is like Ceres. So d, e, and maybe f are within TRAPPIST-1's habitable zone.
The Solar System has plenty of largely-water worlds, though most of their water is frozen. Worlds like most of the larger outer-planet moons and many of the Kuiper-Belt Objects. So it may have been that TRAPPIST-1's planets formed farther out and then spiraled in. That would explain the resonances -- they would be from the outer planets pushing the inner planets inward.
Water worlds have an interesting habitability conundrum. It's nowadays thought that the likely site for the origin of life is hydrothermal vents. These have some advantages over the open ocean like chemical disequilibria and abundant catalytic sites on the vent rocks. But a few hundred kilometers down in a planetary ocean, the water will freeze into high-pressure forms of ice called Ice VI and Ice VII. Thus, hydrothermal vents will get surrounded by ice, though they may be able to create liquid-water chimneys in the ice.
Finally, TRAPPIST-1 -- a site called trappist.one
