TRAPPIST-1 is a star system about 40 light years away from Earth! The star (TRAPPIST-1a) is an ultra cool red dwarf not much larger than Jupiter, though it is much more massive (as in, has more mass) and was discovered in 1999, but its first exoplanets were only found in 2015 by the TRAPPIST telescope! Since then, a total of 7 confirmed exoplanets have been found using the Spitzer Space Telescope, making it the closest system to Earth with this many planets! Despite the TRAPPIST-1 system having a similar number of planets to us, it’s very different to the Solar System we know and love! What makes it so different, and what would it be like to live there? Would life there even be possible?
What is TRAPPIST-1?
Discovered in 1999, TRAPPIST-1 (then called the very catchy ‘2MASS J23062928-0502285’ which I definitely did not just cut and paste from wikipedia…) is an ultra cool dwarf star ~40 light years from Earth. It’s small size means that it is only half as hot as our sun, which results in a solar system that is very different to our own! All of the planets we have discovered so far orbit very close to the star, which is both a benefit and a curse: the close orbits mean that they are most likely to be tidally locked (the same situation as our moon) so the same side of the planet is in perpetual day time, and the other in endless darkness. Unless the planet has a substantial atmosphere with good circulation, its surface would probably be inhospitable. (If you are interested, I could do a blog solely about tidally locked planets, as they are fascinating!). On the other hand, their proximity to their star means that 3 are considered to be in the habitable zone, as the low temperature means that this region is also incredibly close to the star! If the planets were laid out in a similar fashion to our solar system’s, not only would the star appear very small and dim, but none of the planets would be in the habitable zone, where liquid water can be found.
So what would it be like to live there?
One major difference is what the night sky would look like. Due to their proximity to each other, the planets would feature prominently in the sky, similar to how the moons of the gas giants interact and are visible from one another. In fact, Jupiter is quite a good analog for the system, as (although much more massive), TRAPPIST-1a (the star) is similar in size to it, and TRAPPIST 1’s innermost planet orbits at roughly the same distance as Jupiter’s outermost (large) moon. This planet, TRAPPIST-1b, and its neighbour, TRAPPIST-1c, are both incredibly hot, and this probably caused a runaway greenhouse effect, like Venus, and so have thick atmospheres. However, data collected from Hubble, suggests that none of the planets have a hydrogen atmosphere, which would result in a large greenhouse effect and probably end up in the planet being more of a gas giant or mini neptune than a terrestrial earth like body.
TRAPPIST-1d is on the edge of the habitable zone, and is actually very interesting. Above is an artist’s interpretation of what the surface, and sky, could look like. Of course, this is just an interpretation, based off the very limited information that we have. What do we know then? TRAPPIST-1d is a very light planet (only 0.3 Earth Masses) despite being only slightly smaller than Earth (~0.8 Earth radii). This could indicate that despite having a rocky core, it may have a thick ocean or atmosphere. If it did have an ocean, and wasn’t too hot as a result of a greenhouse effect, it could be a fairly good candidate for life!
TRAPPIST-1e has been described as the most likely to be ‘habitable’. In my opinion, we don’t have enough evidence to call any of the planets habitable, but if we did, 1e certainly is the most likely! Why? Well it is firmly within the habitable zone, and it has the most similar density to Earth. This suggests that it probably has a similar composition to Earth, a thin atmosphere and a rocky surface. Therefore it is the most likely to have accessible liquid water on its surface!
While on the topic of density… how did they actually figure out the planet’s densities?? To work out density, you need mass and volume. Volume is fairly simple to infer from the transit, as the radius can be measured. However mass is more difficult. In a solar system with just one planet, you can figure it out by the change in gravitational lensing or red shift, or other methods, but with multi-planetary systems, this can only really tell you the total mass held within all the planets. But one feature of TRAPPIST-1 is that the planets orbit very close to each other, and so exert tidal forces on each other, similar to the interaction between the earth and the moon, and the moons of the gas giants that I talk about in [this post]. From this results in the exoplanets tugging on one another, and the speed of orbit is changed as the planets interact. The amount of force exerted, and therefore the change in speed, depends on the gravitational pull of the planets, which is dependant on its mass!
Back to what life could be like there: When thinking about if life could have developed there, or even what it would be like to send manned or unmanned missions there, there is a whole extra side to this solar system that we don’t have to deal with back on Earth. TRAPPIST-1a is a red dwarf star. This type of star, which are also thought to be the most common in our galaxy, tend to be incredibly variable, and much more violent than our sun. This variability and activity could be the end of ‘habitability’, it could actually make some planets, especially 1e, more habitable! This is because during flares, the levels of ultraviolet radiation increase significantly, which although potentially threatening to early life, it could trigger reactions beneficial to life:
- Radiation can cause DNA mutations, which can be a really bad thing, but also a really good thing, as mutations are the cause of evolution!
- It can ionise water vapour, causing it to split into Hydrogen and Oxygen, which could form a thick atmosphere of oxygen, which would ultimately stop some of the most damaging radiation, allowing life to develop!
But on the whole, this radiation is believed to be detrimental to life, so the first step in searching for life around TRAPPIST-1a is to look for a thick atmosphere, ice or water layer, which could absorb the extra radiation and provide the conditions needed for life!
TRAPPIST-1f, g and h are more likely to have icy surfaces, although having a thick hydrogen atmosphere has not been ruled out for -1g. This means that they are the least likely to be habitable based on the current definition, but if it is found that other forms of life can exist at lower temperatures, then perhaps these would fall within this second, colder habitable zone!
In conclusion, we know a lot of the basic facts, but this does not translate into a deeper understanding of the system and planetary composition… yet! What makes it so exciting is how quickly our understanding has progressed. There have been many models of the system created, but all are based off of assumptions. Only with further experimentation will we be able to prove which of these models are correct, in a similar way to how models of density were proven based off real data of transit time variations! Hopefully after the launch of the James Webb Space Telescope we will be able to find more about this exciting system that has captured the attention of the media, scientists and artists alike!