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We have found a lot of exoplanets outside of our Solar System.
We’ve found at least thousands.
But until recently, it didn’t look like we’d know whether those exoplanets
are habitable until we could send a space probe
or satellite to collect more information.
Now we have LOUPE, the Lunar Observatory
for Unresolved Polarimetry of the Earth.
It’s a powerful camera that scientists plan to
send to the Moon and take pictures of Earth in 2022.
They will use our planet to create a template of what
they might see on potentially habitable exoplanets.
And that moon’s eye view will take us one step closer
to finding planets like ours without leaving Earth!
If certain conditions on Earth have made it possible for it to support human life,
then it makes sense to look for similar things on exoplanets.
Based on what we know about Earth, we might expect habitable exoplanets
to have bodies of water, land masses,
an atmosphere with clouds and oxygen, and vegetation.
LOUPE will take images of Earth as if it were an exoplanet and send them
to scientists so they can gather information about those qualities.
They’ll be able to use these pictures to see sunlight being reflected in
different ways over time by clouds, gases, and plants on Earth
to make a template of its rotation, weather patterns, and seasons.
It’s been tough to gather this kind of information about exoplanets from light
in the past because it’s hard to separate a
star’s glare from the exoplanet’s polarization data.
That’s the direction of the light waves coming from the exoplanet.
From what we have learned so far, exoplanets are very diverse.
Some have surfaces of lava, while others are puffy with styrofoam-like densities.
And getting more polarization data with LOUPE will help tell us even more!
Like, if we want to know about an exoplanet’s atmosphere,
the direction of the light can give us the information we need.
Some of the light we see in images of exoplanets is starlight that’s reflecting off of
the exoplanet, and some is light coming straight from the star itself.
It’s usually hard to figure out the difference between all that light.
But scientists will be able to use polarization data to separate them more because
the light coming from an exoplanet is polarized, meaning the light
is coming from one unified direction,
while the starlight is unpolarized, or coming from multiple directions.
When starlight passes through the atmosphere of an exoplanet and gets scattered,
or when it’s reflected off of an exoplanet’s surface,
polarization usually increases by up to 10%.
And on some planets, light could have to pass through clouds in the atmosphere.
Polarization data can help researchers identify clouds because
they block an exoplanet’s polarized light from reaching the camera.
And if it’s a lucky exoplanet, clouds can even be
characterized by the presence of rainbows!
The rainbows we know and love on Earth generally live in the “Roy G. Biv” family.
This is the result of sunlight hitting water droplets in the air
and refracting and reflecting off of them at different angles.
But if clouds are made of different gases, the rainbow spectrum shifts.
Using this data, we’ll be able to figure out what the clouds are made of
and where the clouds are on the exoplanet.
And speaking of gas, the most important habitability signal might be
how much oxygen there is on an exoplanet.
LOUPE images will be able to help scientists estimate the amount of oxygen
by combining cloud and water data.
Oxygen and hydrogen make water, so if there’s not much water,
but there is oxygen, it’s likely that the oxygen is in the atmosphere.
And that could signal that there are plants creating oxygen on the exoplanet.
Below the atmosphere, we can also figure out
if the exoplanet has continents and oceans.
We can learn if there are continents on an exoplanet because
light can become unpolarized when it reflects off of rough surfaces like rock.
Oceans, on the other hand, reflect light at
consistent angles that are specific to water.
When there’s enough wind to create waves, the pattern of
this reflection of light becomes broader.
The faster the wind, the broader the light pattern.
Using this knowledge, scientists will even be able to study
whether there are white caps and the direction of the waves.
We can also identify oceans by color alone.
The angle of light coming from an exoplanet with an ocean
makes it appear blue, white, and red.
And colors can also tell us about vegetation!
Plants here on Earth usually have chlorophyll, a pigment that gives them
a green color, and it also happens to help capture light for the plants to use
along with carbon dioxide to make energy and oxygen.
So any plants on exoplanets that convert light into oxygen
might have something like green chlorophyll, too.
But it’s even more conclusive to get data on an exoplanet’s “vegetation red edge,”
which is the light that’s close to but just outside
the range of red light that humans can see.
Scientists think that plants on Earth might reflect green light and near-red light
because it’s inefficient to use it for photosynthesis.
Since this is the part of the light spectrum where chlorophyll and the things
made by chlorophyll reflect light, oxygen-producing plants on exoplanets
could reflect light in a similar way.
Since we know what kinds of gases are in Earth’s atmosphere and where its
continents, oceans, and vegetation are, scientists can overlay
what LOUPE captures about Earth with the knowledge we already have.
Then they’ll have a template for what it would look like
to take pictures of similar things on exoplanets.
With LOUPE, we’ll be able to figure out the likelihood
of exoplanet habitability from a distance.
We won’t have to send a satellite or space probe light years away to an exoplanet.
All of the benefits of interplanetary travel from the comfort of our own planet.
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