How do we know it rains IRON on WASP-76b?

How do we know it rains IRON on WASP-76b?


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. So, if you enjoyed this video and want to
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