I’m sure we’ve all heard of exoplanets where some pretty bizarre stuff goes on, like
evaporating exoplanets, whose orbits have taken them too close to their parent star.
Or a planet orbiting a pulsar, likely to be composed of crystalline carbon, which would
be similar, but denser than diamond.
However, a newly discovered exoplanet called WASP-76b, discovered using ESO’s VLT, has
been found to have a very special characteristic: on this planet, temperatures are so hot that
instead of water rain, it rains molten iron.
How can this be?
And how can researchers know this from an object 640 light years away?
Exoplanets are pretty difficult to detect.
Most exoplanets are only observed when the planet’s orbit is aligned just right so
that it transits in front of its parent star from our perspective.
Any whose orbits don’t align with our perspective are exceptionally hard to discover, so chances
are that there’s a lot more out there in our relatively near vicinity that we don’t
know about.
Exoplanets that are very close to their stars, with very small orbits, are the easiest to
discover, because we can see a very clear pattern on the star’s light curve over a
short period of time.
WASP-76b is one such planet.
Plus, it is huge, way bigger than Jupiter, and combine this with the fact that it only
takes 1.8 Earth days to make one orbit, it made it comparatively easy to detect.
But detecting the presence of an exoplanet is one thing, how do astronomers know anything
about its physical characteristics?
Interestingly, the first thing astronomers do is find out the physical characteristics
of the parent star, WASP-76.
The distance to the star is first determined, and then the star is classified based on its
brightness and colour.
Knowing the distance helps us determine how bright it is, and we measure its colour simply
by observing it, which helps us determine how hot it is.
If the star is on the main sequence, then this chart also helps us know the radius and
mass of the star, as they all tend to follow a pattern.
Once we have that information, we can determine the characteristics of the orbiting planet
itself.
Knowing the mass and radius of the star means we can measure the mass of the orbiting planet
using some clever equations, based on the Law of Universal Gravitation.
As it happens, WASP-76b is a super-Jupiter, way bigger than our Jupiter.
That means that although it is massive, this mass is spread out across a large volume,
likely making it a gas giant.
Orbiting this close to the star means the planet is probably tidally locked, only one
side faces the star at any given time.
Also, due to the proximity of the planet, it orbits within the star’s atmosphere,
the physics of which we really don’t understand yet.
However, the star facing side will be extremely hot, estimated to be around 2,400°c, easily
hot enough to vaporise metals.
Models go on to suggest the night side is about 1500°c, still blisteringly hot, but
much cooler.
To really find out what WASP-76b is made of though, we need to go back to the light curve
of the transit.
Scientists look for differences in the light when the planet passes in front of the star,
as light from the star will shine through the planet’s atmosphere.
Certain atoms block certain wavelengths of light, so any reduced wavelengths help us
know what is in the atmosphere.
This is known as spectroscopy.
For WASP-76b, the biggest surprise that scientists detected was an abundance of iron in the atmosphere!
Based on what we know about the planet so far, it seems like iron exposed to the day
side of the planet is vaporised, where it is transported through strong wind processes
to the terminator line between the day and the night side of the planet.
Here, the temperature is low enough for the iron the cool and condense, producing iron
raindrops which fall deeper into the atmosphere.
By the time the wind has reached the “morning” side of the planet, iron can no longer be
detected.
This remarkable measurement, taken by the ESPRESSO instrument on ESO’s VLT, is the
first time variations have been spotted like this on an ultra-hot gas giant.
Although it probably won’t be the last time!
So, there we have it.
How it can rain iron on WASP-76b.
I have no doubt we’ll be hearing about more astonishing exoplanet discoveries in the future,
there are a lot of missions that have either launched or will be launched in the not too
distant future, helping us get a better understanding of the universe around us.
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