The sensational discovery of seven Earth sized rocky planets in orbit around an ultracool dwarf star 40 light years away, was followed up with intense observation campaigns to better understand these alien worlds. Because of these observations, astronomers know more about the TRAPPIST-1 system than any other planetary system, apart from our own Solar System. All the seven known planets in the system are in orbit closer to the host star than Mercury is to the sun. Three of the planets, TRAPPIST-1 e,f and g are all in the habitable zone of the star, which is at just the right distance for liquid water to exist. The planets are among the most likely exoplanets to harbour life.
The follow up observations studied the density and composition of the planets, with one of the aims being to further investigate the habitability of the planets. NASA’s Kepler and Spitzer telescope observed the system, with the latter dedicating over 500 hours in a program. The European Southern Observatory (ESO) also observed the system through its nearly complete SPECULOOS facility.
All the data was churned in complex computer models. The computer models used the newly measured orbital periods of the planets, along with their masses, the planetary radii and a number of other factors. The lead scientist in the team, Simon Grimm at the University of Bern in Switzerland, described the effort as solving a “35-dimensional problem”. The team worked through most of 2017 to develop the new techniques and run simulations.
Grimm explains discovering the masses as, “The TRAPPIST-1 planets are so close together that they interfere with each other gravitationaly, so the times when they pass in front of the star shift slightly. These shifts depend on the planets’ masses, their distances and other orbital parameters. With a computer model, we simulate the planets’ orbits until the calculated transits agree with the observed values, and hence derive the planetary masses.”
The simulations suggest that there is water on the TRAPPIST-1 planets. A lot of it. The measurement of the density and masses suggest that the planets have a lot of volatile material, which is most likely to be water. Only 0.02 percent of the Earth’s mass is made up of water. On some of the TRAPPIST-1 planets, the water could make up as much as 5 percent of the masses. That is 250 times more water than the oceans of the Earth. This water could exist as ice, water or gas, depending on the amount of sunlight that the planet receives from TRAPPIST-1.
TRAPPIST-1 b, the innermost planet in the system, is most exposed to the energy from the host star. The atmosphere is most likely thicker than the atmosphere of the Earth. TRAPPIST-1 c also has a rocky core, and an atmosphere thicker than the Earth, but not as thick as the atmosphere on TRAPPIST-1 b. TRAPPIST-1 d is the least massive among all the known planets in the system, at about only 30 percent the mass of the Earth. Researchers are unsure if the planet has a thick atmosphere, an ocean or if it is covered in ice.
TRAPPIST-1 e is the rockiest planet among all the ones in the system, and is actually denser than the Earth. Researchers believe this could be because of a dense iron planetary core. The density also indicates that it may not have an atmosphere, ocean or ice layer. However, there is still the potential for some water to exist on the surface. Overall, TRAPPIST-1 e is the planet most similar to Earth in terms of size, composition, and the amount of energy received from the host star. TRAPPIST-1 f
TRAPPIST-1 f, g and h are so far from the host star that any surface water is likely to be frozen solid. The thin wispy atmospheres on these planets are likely to contain the heavier molecules found in the Earth’s atmosphere, such as carbon dioxide. Caroline Dorn, co-author of the study from the University of Zurich in Switzerland says, “it is interesting that the densest planets are not the ones that are the closest to the star, and that the colder planets cannot harbor thick atmospheres.” The graph below plots the density of the planets in the TRAPPIST-1 system against the amount of energy received from the star. The inner worlds from our own solar system are in there for comparison.
The findings were supported by findings from the Hubble Space Telescope, which indicated a lack of hydrogen in the atmospheres of TRAPPIST-1 d, e and f, providing additional evidence that the small worlds are rocky. The planets are likely to be tidally locked to the host stars. This means that one side enjoys a perpetual day, another endures a neverending night, with a twilight zone in between where any observer on the surface can watch a sunset frozen in time forever. The other planets of the system would all be visible in the sky, some close enough for the observer to make out the details on their surfaces. NASA has previously found evidence that the system is older than the Solar System, allowing more time for life to emerge. The TRAPPIST-1 system is between 5.4 and 9.8 billion years old, while our Solar System is only about 4.5 billion years old.
The scientific understanding of this interesting system will continue to improve, and resolve with greater granularity with continued studies. It remains a prime candidate for future observational campaigns, including for the James Webb Space Telescope that NASA has developed as a successor to the Hubble Space Telescope. The SPECULOOS facility by ESO is purpose built to find other rocky planets in orbit around ultracool dwarf stars in the celestial neighbourhood.