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10 Steps to Confirm a Planet Around Another Star

10 Steps to Confirm a Planet Around Another Star

So you think you found an exoplanet – a planet around another star?
It’s not as simple as pointing a telescope to the sky and looking for a
planet that waves back. Scientists gather many observations and
carefully analyze their data before they can be even somewhat sure that
they’ve discovered new worlds.

Here are 10 things to know about finding
and confirming exoplanets.


This is an illustration of the different elements in our exoplanet
program, including ground-based observatories, like the W. M. Keck
Observatory, and space-based observatories like Hubble, Spitzer,
Kepler, TESS, James Webb Space Telescope, WFIRST and future missions.

1. Pick your tool to take a look.

The vast majority of planets around other stars have been found
through the transit method so far. This technique involves monitoring
the amount of light that a star gives off over time, and looking for
dips in brightness that may indicate an orbiting planet passing in front
of the star.

We have two specialized exoplanet-hunting telescopes scanning the
sky for new planets right now – Kepler and the Transiting Exoplanet
Survey Satellite (TESS) – and they both work this way. Other methods of
finding exoplanets include radial velocity (looking for a “wobble” in a
star’s position caused by a planet’s gravity), direct imaging (blocking
the light of the star to see the planet) and microlensing (watching for
events where a star passes in front of another star, and the gravity of
the first star acts as a lens).

Here’s more about finding exoplanets.


2. Get the data.

To find a planet, scientists need to get data from telescopes,
whether those telescopes are in space or on the ground. But telescopes
don’t capture photos of planets with nametags. Instead, telescopes
designed for the transit method show us how brightly thousands of stars
are shining over time. TESS, which launched in April and just began
collecting science data, beams its stellar observations back to Earth
through our Deep Space Network, and then scientists get to work.


3. Scan the data for planets.

Researchers combing through TESS data are looking for those transit
events that could indicate planets around other stars. If the star’s
light lessens by the same amount on a regular basis – for example,
every 10 days – this may indicate a planet with an orbital period (or
“year”) of 10 days. The standard requirement for planet candidates from
TESS is at least two transits – that is, two equal dips in brightness
from the same star.


4. Make sure the planet signature couldn’t be something else.

Not all dips in a star’s brightness are caused by transiting planets.
There may be another object – such as a companion star, a group of
asteroids, a cloud of dust or a failed star called a brown dwarf, that
makes a regular trip around the target star. There could also be
something funky going on with the telescope’s behavior, how it delivered
the data, or other “artifacts” in data that just aren’t planets.
Scientists must rule out all non-planet options to the best of their
ability before moving forward.


5. Follow up with a second detection method.

Finding the same planet candidate using two different techniques is a
strong sign that the planet exists, and is the standard for
“confirming” a planet. That’s why a vast network of ground-based
telescopes will be looking for the same planet candidates that TESS
discovers. It is also possible that TESS will spot a planet candidate
already detected by another telescope in the past. With these combined
observations, the planet could then be confirmed. The first planet TESS
discovered, Pi Mensae c, orbits a star previously observed with the
radial-velocity method on the ground. Scientists compared the TESS data
and the radial-velocity data from that star to confirm the presence of
planet “c.”

Scientists using the radial-velocity detection method see a star’s
wobble caused by a planet’s gravity, and can rule out other kinds of
objects such as companion stars. Radial-velocity detection also allows
scientists to calculate the mass of the planet.


6. …or at least another telescope.

Other space telescopes may also be used to help confirm exoplanets,
characterize them and even discover additional planets around the same
stars. If the planet is detected by the same method, but by two
different telescopes, and has received enough scrutiny that the
scientists are more than 99 percent sure it’s a planet, it is said to be
“validated” instead of “confirmed.”


7. Write a paper.

After thoroughly analyzing the data, and running tests to make sure
that their result still looks like the signature of a planet, scientists
write a formal paper describing their findings. Using the transit
method, they can also report the size of the planet. The planet’s radius
is related to how much light it blocks from the star, as well as the
size of the star itself. The scientists then submit the study to a


8. Wait for peer review.

Scientific journals have a rigorous peer review process. This means
scientific experts not involved in the study review it and make sure the
findings look sound. The peer-reviewers may have questions or
suggestions for the scientists. When everyone agrees on a version of the
study, it gets published.

9. Publish the study.

When the study is published, scientists can officially say they have
found a new planet. This may still not be the end of the story, however.
For example, the TRAPPIST telescope in Chile first thought they had
discovered three Earth-size planets in the TRAPPIST-1 system.
When our Spitzer Space Telescope and other ground-based telescopes
followed up, they found that one of the original reported planets (the
original TRAPPIST-1d) did not exist, but they discovered five others
–bringing the total up to seven wondrous rocky worlds.


10. Catalog and celebrate – and look closer if you can!

Confirmed planets get added to our official catalog. So far,
Kepler has sent back the biggest bounty of confirmed exoplanets of any
telescope – more than 2,600 to date. TESS, which just began its planet
search, is expected to discover many thousands more. Ground-based
follow-up will help determine if these planets are gaseous or rocky, and
possibly more about their atmospheres. The forthcoming James Webb Space
Telescope will be able to take a deeper look at the atmospheres of the
most interesting TESS discoveries.

Scientists sometimes even uncover planets with the help of people like you: exoplanet K2-138 was discovered through citizen scientists
in Kepler’s K2 mission data. Based on surveys so far, scientists
calculate that almost every star in the Milky Way should have at least
one planet. That makes billions more, waiting to be found! Stay up to
date with our latest discoveries using this exoplanet counter.

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