Trappist-1 is a red dwarf star that is just a bit under 40 light years away from earth, in the region we refer to as the constellation Aquarius, so it’s actually very close to us as far as stars go, and this star has seven, count them, seven roughly earth-sized planets in very tight orbits, which are shown here with sizes roughly to scale, but orbits not to scale. We will get to these planets in a moment, first let’s look at the star itself first. Trappist-1 burns very cool, with a surface temperature of only about 2,500 Kelvin, or less than half as hot as our sun. And being a red dwarf, it’s very tiny.
Here we can see a size comparison with our own sun, which is about ten times larger and more massive. In fact, Trappist-1 is barely bigger than the planet Jupiter, although much more massive, since Jupiter is predominately made of gas. We can also compare its seven planets, which are given letters B through H, to other objects in our solar system. As we can see, they are comparable in size to the inner terrestrial planets, especially earth. While these planets do not orbit their star as tightly as the Galilean moons orbit Jupiter, they orbit their star much more closely than even Mercury, the innermost planet in our solar system.
As is shown here, the entire Trappist-1 system fits in this very narrow distance well within the orbit of Mercury, which is enlarged by a factor of 25 here so that we can see the relative orbits of the seven planets. These were discovered by the method we described previously involving the transit of planets in front of their stars, which slightly diminishes the amount of light we can see from that star, in specific quantitative ways. As we can see, the farther away each planet is from the parent star, the longer it will take to complete the transit, and we can also determine the orbital periods of each planet with this technique.
So let’s take a look at these planets in order of increasing orbital radius, starting off with Trappist-1b. This planet is almost identical to earth in mass though just slightly larger, and orbits less than two million kilometers from its star, which is about one one-hundredth of an astronomical unit, or one percent of earth’s orbital radius. This is so close that it has a year of around one and a half earth days. So every day and a half on earth, this planet goes once around its star.
This planet is rocky, and is shown by spectroscopy to have a thick atmosphere like that of Venus. This combined with the proximity to its star means is surface temperature is exceptionally high, hotter still than Venus, and likely uninhabitable. Moving on to Trappist-1c, this is the most massive planet of the system, though still only slightly heavier than earth. Again, we expect a thick Venus-style atmosphere, though not quite as thick as that of 1b, while still remaining rather uninhabitable, due to the close proximity to its star. One year on this planet lasts around two and a half earth days. Next up is Trappist-1d. This is the tiniest of the lot, less than a third as massive as the earth, and around three quarters the size. But this one has an orbital radius of a little over three million kilometers, which puts it at the inner edge of this star’s habitable zone.
Even though it is still very close to its star compared to planets in our solar system, the star is so dim that this planet gets about the same amount of light as earth does, and it seems to have a surface temperature similar to earth’s as well. It may also have liquid water, much more than earth in fact, and despite the inhospitable atmosphere, it remains one of the most earth-like exoplanets that we know of, with a similarity index of 0.91. One year here lasts four earth days. Moving on we get to Trappist-1e.
This one is almost as big as earth but only three quarters the mass. This has a density almost identical to earth’s, indicating a solid rocky surface, differentiating it from the other Trappist planets that seem to be covered in either a steamy atmosphere, a global ocean, or an ice shell. Existing squarely in the habitable zone of this star, there could be liquid water on its surface, and much like 1d, this is one of the most earth-like worlds we have found. A year here lasts around six earth days. Next up is Trappist-1f. This one is almost identical to earth in mass and size, and orbits near the outer edge of the habitable zone. With a fairly thick atmosphere, this planet still exhibits somewhat habitable temperatures, and appears to harbor water, though it is unclear as to whether it is predominately liquid or gas. A year on this planet is around nine earth days.
Now we get to Trappist-1g. This one is just a little heavier and a little bigger than earth, though it is a bit less dense. Given that we are now at the outer limit or even somewhat outside of the habitable zone, at around 0.047 astronomical units, this is more likely to be an icy planet, though depending on the precise composition and thickness of the atmosphere, there could be liquid water as well. A year here is a little more than twelve earth days. And finally we get to Trappist-1h, the outermost planet in this system. This one is a third the mass of the earth and three quarters the size, with a density similar to Mars. Calculations indicate that water is here, likely in the form of an ice shell, given a temperature that is similar to the south pole on earth.
Even though this is the outermost planet, we are still at a distance from Trappist-1 that is only about six percent of the distance from the sun to the earth, and one year on this planet lasts around nineteen earth days. So how common is this type of system? Well red dwarf stars are actually exceptionally common. In fact, they are the most common type of star, with a range in mass of 0.08 and 0.6 solar masses. About 70 percent of all stars fall into this range. They can be a little tricky to observe due to their low luminosity, but they tend to produce a number of planets within their habitable zone, which usually span from roughly one tenth to two tenths of an astronomical unit. However, there are complications that come with a set of planets orbiting this closely to a star.
First, being so close to a star results in excessive bombardment with radiation, which likely contributes to the inhabitability of innermost planets. Second, planets such as these have a tendency to become tidally locked, with one side facing the star at all times, just like earth’s moon has one face to earth at all times. This makes one side very hot and the other very cold, neither of which are ideal for life. But if planets have a particular atmospheric composition, this can allow for heat to be distributed to the colder areas, which could increase the habitability. And even if one face is too hot and the other face is too cold, it still leaves open the possibility of a band of habitability encircling the planet around the twilight portion, which we call the terminator line, and it’s not impossible that life could have evolved specifically in these regions of such a planet.
Planetary systems such as these also exhibit a fascinating phenomenon called orbital resonance, whereby their orbital periods are in particular whole number ratios, due to their gravitational influence on one another. In particular the Trappist planets form a Laplace resonance chain, with two orbits for 1h equaling 3 orbits for 1g, and then going inwards, 4, 6, 9, 15, and 24 orbits, which yields smaller integer ratios for each pair of planets. This phenomenon enhances the stability of such a system, given that the planets are incredibly close to one another.