Three nearby, Earth-like exoplanets have just increased our chances of finding life beyond Earth. The three exoplanets circle around a very dim star that lies relatively close to us, and are all within or near the “habitable zone” (the area where the star’s light could support extraterrestrial life).
An international team of astronomers, including astrophysicists from UC San Diego, and astronomers from MIT and the University of Liège in Belgium, detected the three planets orbiting an ultracool dwarf star, which is just 40 light years from Earth. These are the first planets ever to be discovered in the “habitable zone” of such a tiny and dim star.
The researchers said that the sizes and temperatures of these worlds are similar to those of Earth and Venus, and are the best targets so far to be found in the search for life outside the solar system. Adam Burgasser, a professor of physics at UC San Diego’s Center for Astrophysics and Space Sciences and a key participant of the international team, said in a statement:
“The kind of planets we’ve found are very exciting from the perspective of searching for life in the universe beyond Earth.”
The discovery was made using TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), which is a 60-centimeter telescope operated by the University of Liège, based in Chile. It was designed to focus on 60 nearby dwarf stars (very small, cool stars that are so faint they are invisible to optical telescopes).
The astronomers led by Michaël Gillon of the University of Liège, Belgium, traced the star’s light at infrared wavelengths (most of the light it emits is in the infrared) over 62 nights. After analyzing the changes in brightness of the starlight, they discovered three periodic dips as the planets traverse its face, casting shadows.
The ultracool dwarf star known as TRAPPIST-1 is a Jupiter-sized star that is one-eighth the size of our sun and is significantly cooler. The two innermost planets complete a single orbit once every 1.5 and 2.4 days. However, they receive only four and two times the amount of radiation, respectively, as Earth receives from the sun, according to MIT News.
The third planet may orbit the star in anywhere from four to 73 days, and may receive even less radiation than Earth. Given their size and proximity to their ultracool star, all three planets may have regions with temperatures well below 400 kelvins, within a range that is suitable for sustaining liquid water and life, MIT added. Gillon explained:
“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun.
“The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System.”
Julien de Wit, co-author and a postdoc in the Department of Earth, Atmospheric, and Planetary Sciences, believes because the system is only 40 light years from Earth, scientists should soon be able to study the planets’ atmospheric compositions. Not only that, we should be able to assess their habitability and whether life does actually exists within this planetary system. De Wit said:
“These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited.
“All of these things are achievable, and within reach now. This is a jackpot for the field.”
The two innermost planets would likely be tidally locked (one face always toward the star and the other always dark). This could limit the circulation of water and atmosphere, where they may be trapped on the cold, dark side, although astronomers speculated that life could still exist in the perpetual dawn on the day side’s western edge.
However, the tidal forces may be keeping the planet surface warm even on the dark side; the down side of this is that the same forces could also drive strong geothermal activity which could make the surfaces unstable for life, Burgasser said:
“Fortunately, we may be able to answer these questions in the near future, as the geometry of the system makes it likely that we will be able to detect the atmospheric gases of these planets in the next decade with the launch of the James Webb Space Telescope.
“This facility will allow us to search for biogenic gases — oxygen or methane for example — that would firmly indicate the presence of life, or search for other gas species that would tell us about the planets’ compositions, geothermal activity and evolutionary history.
“We expect intense scrutiny of this system by our colleagues, given its remarkable properties.”