Earlier in this series we talked about astrobiology, and we mentioned that we have discovered hundreds
of exoplanets, which are planets outside of our solar system.
Now let’s get a closer look at a particularly fascinating system of exoplanets, those belonging
to the star TRAPPIST-1.
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.
And thus we conclude our exploration of the Trappist system, with two excellent candidates
for habitable worlds, those being Trappist-1d and 1e.
With so many potentially habitable planets such as these in the galaxy, the odds that
one or more of them actually harbor complex life seems quite reasonable, and perhaps we
will see some evidence of this in the near future.