Sun-like stars represent just 15 per cent of all stars in the Milky Way. More than half of those, in turn, exist in binary star systems that have also been disregarded as being too different from the conditions present in the solar system. The search for Earth twins therefore covers a nearly insignificant fraction of all the outcomes in nature.
“How frequently is life found elsewhere?” ask the research teams at the University of Cambridge and the University of Liège in Belgium. This simple change of words means that we should also be investigating planetary systems unlike the solar system. It would be disappointing and surprising if Earth were the only template for habitability in the Universe.
“No matter what we find by studying planets orbiting ultra-cool dwarfs, we cannot lose. We can only learn. If we manage to identify the presence of life on a planet similar to those in the TRAPPIST-1 system, then we can start measuring how frequently biology emerges in the universe. We could have the first clues of extraterrestrial biology in a decade!”
In March of 2017, the research team reported that a nearby star, called TRAPPIST-1A, is orbited by seven planets similar in size and mass to Earth. All seven planets are temperate, meaning that under the right atmospheric and geologic conditions, they could sustain liquid water. Three of the planets show particular potential for habitability, receiving about as much energy from their star as the Earth receives from the Sun.
Once we reset the goal to measuring the total frequency of biology, ultra-cool dwarfs become an obvious target. Half the stars in the Milky Way have masses less than one-quarter of the Sun’s. The preliminary results suggest that rocky worlds are common orbiting low-mass stars, including ultra-cool dwarf system, possibly more so than in orbit around Sun-like stars. Ultra-cool dwarfs also open a much easier route to detecting and studying temperate, Earth-like planets.
The scientific advantages of ultra-cool dwarfs come from their stellar properties, from how we identify exoplanets, and from how we expect to investigate their atmospheres. The TRAPPIST-1 planets were found as they passed in front of their star, events known as transits. When the planet transits, it casts a shadow whose depth tells us how much of the stellar surface is being hidden by the planet; the bigger the planet, the deeper the shadow. Because ultra-cool dwarfs are so small, the transit of an Earth-sized planet in front of TRAPPIST-1A is approximately 80 times as prominent as an equivalent transit against a much larger, Sun-like star.
During a transit, any gases in the planet’s atmosphere change the appearance of starlight streaming through. Around ultra-cool dwarfs, the atmospheric signature is boosted by about a factor of 80. The atmospheric composition of the TRAPPIST-1 planets will be detectable using current and upcoming facilities, such as the James Webb Space Telescope launching in 2018, unlike the decades of technological development needed to study an Earth twin.
Artist’s view of planets transiting a red dwarf star in the TRAPPIST-1 system. Credit: NASA, ESA, and STScI
Extracting a reliable atmospheric signal requires observing dozens of transits. Here, too, systems such as TRAPPIST-1 have huge advantages. Around tiny ultra-cool dwarfs, transits of temperate planets happen once every few days to every couple of weeks, instead of once a year for a planet exactly like Earth.
Astronomers, the authors wrote, have already begun investigating the compositions of giant planets around other stars, detecting molecules such as water, carbon monoxide, methane, and hydrogen cyanide. With the discovery of the TRAPPIST-1 system, we can extend those explorations to Earth-sized planets. Their first efforts will be to characterize the greenhouse gas content of atmosphere, and assess whether the surface conditions are conducive for liquid water. Then we will seek out signs of biologically produced gases, analogous to ways that living organisms have transformed the composition of Earth’s atmosphere.
Claiming a discovery of life will be hard. We cannot rely on the detection of a single gas but instead will need to detect several, and will need to measure their relative abundances. In addition, we will have to be extremely wary of false positives. For instance, repeated stellar flares could build up oxygen in an atmosphere without the presence of life.
The richness of the TRAPPIST-1 system is an important asset, because we can compare its planets to one another. All seven planets originated from the same nebular chemistry; they share a similar history of receiving flares and meteoritic impacts. Weeding out false positives will be much easier here than in planetary systems containing only one or two temperate, potentially Earth-like worlds.
More important, TRAPPIST-1 is not a one-off discovery. Ultra-cool dwarf stars are so common that there could be numerous other similar systems close to us in the galaxy. The TRAPPIST (Transiting Planets and Planetesimals Small Telescopes) facility we used to find the TRAPPIST-1 planets was just the prototype of a more ambitious planet survey called SPECULOOS (Search for habitable Planets Eclipsing Ultra-Cool Stars), which has already begun operations.
We expect to find many more Earth-sized, rocky planets around dwarf stars within the next five years. With this sample in hand, we will explore the many climates of such worlds. The solar system contains two: Venus and Earth. How many different types of environments will we discover?
Using SPECULOOS, we will also begin to address the many objections scientists have raised about the habitability of planets around ultra-cool dwarfs. One argument is that such planets will be tidally locked, meaning that they have permanent day and night sides. Planets orbiting in close proximity around small stars could excite each other’s orbits, leading to major instabilities. Ultra-cool dwarf stars frequently flare up, emitting ultraviolet and X-rays that might vaporize a planet’s oceans into space.
The red areas in HK Tau star system at the top of the page represent material moving away from Earth and the blue indicates material moving toward us. The two binary stars in this system, which is located approximately 450 light-years from Earth in the constellation Taurus, are less than 5 million years old and separated by about 58 billion kilometers, or 13 times the distance of Neptune from the Sun.
The Daily Galaxy via University of Cambridge
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