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It seems that 2017 is expected to become an annus mirabilis (latin: miraculous year) in science, particularly in physics. It's only a few weeks ago that the news on the discovery of metallic hydrogen broke out; now we're presented with the possibility of life existing on another star system.
Hopefully this trend keeps up! |
"The prediction was that the protoplanetary disk around such small stars would be too thin to make planets. The theories for exoplanets are based on very few observations, I didn’t believe the theorists. I decided to follow my intuition."
Michaël Gillon, Institut d'Astrophysique et de Géophysique Université de Liège, 2017
In May 2016, Michael Gillon and his team announced the discovery of three Earth-sized exoplanets around TRAPPIST-1, an ultra cool M-dwarf star, using the small TRAPPIST telescope at ESO-La Silla, Chile. It was an exciting discovery—yet on that day no one could possibly have imagined that less than a year later they would make another significant discovery involving the same system. But here we are: on 22 February 2017, they announced in Nature the discovery of seven potentially habitable Earth-like worlds.
The star, named TRAPPIST-1, is a fairly inconspicuous star in our Milky Way. Small (8% the mass of the sun) and cold (half the temperature of the sun), it is a member of an ultra-cool dwarf population that represents 15% of the star population of our galaxy. In 2016, Gillon and his team detected the transit (i.e., the shadow of a planet passing between its host star and us) of three exoplanets at the inner edge of the habitable zone of their star.
Energized and excited by this discovery, the team requested and received additional telescope time to follow up on this system during the second half of 2016. The NASA Spitzer telescope is one of the facilities they selected for an ambitious program that called for monitoring the TRAPPIST-1 system almost continuously for twenty days. Spitzer and other ground-based telescopes allowed the team to detect thirty-four transits, more than they had anticipated, suggesting the existence of additional exoplanets in the system.
After an careful analysis, the data revealed the presence of seven Earth-sized exoplanets (named TRAPPIST 1b, c, d, e, f, g, and h) in orbit around this M-type star. Because their orbital periods are short (less than twelve days for planets b to g), several transits were detected during the campaign. Accurately measurements of those tiny events (0.6% dimming of the star) provide a wealth of information about the planets’ orbits, sizes, and even masses by measuring precisely the timings of the transits which are perturbed by the gravity of other planets.
It’s very tight; the most distant planet (h) is at 0.06 AU (the standard astronomical unit, 1 AU equals the average distance between the center of the Sun and the center of the Earth) from its star. The closest one is at 0.01 AU. For comparison, Mercury orbits at 0.39 AU from our sun. This is NOT equivalent to our solar system, but more a minuscule version of it, comparable in size/mass ratio to Jupiter and its Galilean Moons.
Similar to our solar system, these exoplanets travel circular orbits all of which go in the same direction. They probably formed more than 500 million years ago together with their star.
The depth of the transits provides a measurement of the radius of the planets, which are similar to Earth (b, c, e, f, g) or Mars (d, h) so they range from 75% to 110% the size of Earth.
The data are accurate enough to detect mutual gravitational effects, so we can infer the mass of the first six planets, and hence their density. Those are rocky worlds that range in density from 60 and 117% that of Earth.
Considering the amount of energy they receive from the star, these planets might have a temperature suitable for the presence of liquid water on their surface (e,f,g) if we assume a similar atmosphere than Earth.
Last year, astronomers announced the discovery of a potentially terrestrial planet around Proxima Centauri, located only 4.2 light-years away from us. Today, this group reveals the existence of seven potentially habitable worlds around a small star that could live forever. The field of exoplanet is without doubt booming, and it is not over.
A team led by Emmanuel Jehin, co-author of the paper, is currently building Speculoos, a TRAPPIST telescope “on steroids”, made of four, 1-meter robotic telescopes installed at Cerro Paranal. With this telescope, the team will survey 10 times more red dwarfs than TRAPPIST did. So we could expect the discovery of a dozen systems similar to this one soon. We will have the opportunity to explore the diversity of atmospheres and climates on Earth-like worlds, and potentially life out there.
The discovery of these strange new worlds where life could flourish is the beginning of an exciting time for astronomers and biologists. A time when we are beginning to see the unimaginable, a time when life as we don’t know it can be imaged and studied. We will probably need to build complex and expensive instruments to understand those worlds—but what an astonishing, awe-inspiring, life-changing prize to win, don’t you think?
The star, named TRAPPIST-1, is a fairly inconspicuous star in our Milky Way. Small (8% the mass of the sun) and cold (half the temperature of the sun), it is a member of an ultra-cool dwarf population that represents 15% of the star population of our galaxy. In 2016, Gillon and his team detected the transit (i.e., the shadow of a planet passing between its host star and us) of three exoplanets at the inner edge of the habitable zone of their star.
Energized and excited by this discovery, the team requested and received additional telescope time to follow up on this system during the second half of 2016. The NASA Spitzer telescope is one of the facilities they selected for an ambitious program that called for monitoring the TRAPPIST-1 system almost continuously for twenty days. Spitzer and other ground-based telescopes allowed the team to detect thirty-four transits, more than they had anticipated, suggesting the existence of additional exoplanets in the system.
After an careful analysis, the data revealed the presence of seven Earth-sized exoplanets (named TRAPPIST 1b, c, d, e, f, g, and h) in orbit around this M-type star. Because their orbital periods are short (less than twelve days for planets b to g), several transits were detected during the campaign. Accurately measurements of those tiny events (0.6% dimming of the star) provide a wealth of information about the planets’ orbits, sizes, and even masses by measuring precisely the timings of the transits which are perturbed by the gravity of other planets.
It’s very tight; the most distant planet (h) is at 0.06 AU (the standard astronomical unit, 1 AU equals the average distance between the center of the Sun and the center of the Earth) from its star. The closest one is at 0.01 AU. For comparison, Mercury orbits at 0.39 AU from our sun. This is NOT equivalent to our solar system, but more a minuscule version of it, comparable in size/mass ratio to Jupiter and its Galilean Moons.
Similar to our solar system, these exoplanets travel circular orbits all of which go in the same direction. They probably formed more than 500 million years ago together with their star.
The depth of the transits provides a measurement of the radius of the planets, which are similar to Earth (b, c, e, f, g) or Mars (d, h) so they range from 75% to 110% the size of Earth.
The data are accurate enough to detect mutual gravitational effects, so we can infer the mass of the first six planets, and hence their density. Those are rocky worlds that range in density from 60 and 117% that of Earth.
Considering the amount of energy they receive from the star, these planets might have a temperature suitable for the presence of liquid water on their surface (e,f,g) if we assume a similar atmosphere than Earth.
Last year, astronomers announced the discovery of a potentially terrestrial planet around Proxima Centauri, located only 4.2 light-years away from us. Today, this group reveals the existence of seven potentially habitable worlds around a small star that could live forever. The field of exoplanet is without doubt booming, and it is not over.
A team led by Emmanuel Jehin, co-author of the paper, is currently building Speculoos, a TRAPPIST telescope “on steroids”, made of four, 1-meter robotic telescopes installed at Cerro Paranal. With this telescope, the team will survey 10 times more red dwarfs than TRAPPIST did. So we could expect the discovery of a dozen systems similar to this one soon. We will have the opportunity to explore the diversity of atmospheres and climates on Earth-like worlds, and potentially life out there.
The discovery of these strange new worlds where life could flourish is the beginning of an exciting time for astronomers and biologists. A time when we are beginning to see the unimaginable, a time when life as we don’t know it can be imaged and studied. We will probably need to build complex and expensive instruments to understand those worlds—but what an astonishing, awe-inspiring, life-changing prize to win, don’t you think?
Ponder this
Why are the planets so close together, with relatively short orbital periods?
Considering the short orbital periods of the planets, how would this affect seasonality, or the day/night cycle? How would it affect life on the planet (assuming it exists)?
Discuss
How would this discovery affect our society on Earth? We had always thought that we are alone in this universe, special, exceptional; will this affect our views on ourselves, and our place in the universe? How and why?
Further readings
TRAPPIST-1, planetary system website.
"Largest batch of Earth-size, habitable zone planets", at NASA's Exoplanet Exploration programme.
Michael Gillon, hero of the hour
