How to Find an Exoplanet

  • By Ari Daniel
  • Posted 06.28.17
  • NOVA

If you want to find another world beyond our solar system, here are the five ways you can do it.

Running Time: 03:42


Onscreen: How do you find another world? Last week the Kepler space telescope added another 219 exoplanets (planets beyond our solar system) to the roster of known alien worlds. Including 10 in the “Goldilocks zone” that might have the conditions to support life.

Mario Perez: Kepler has been the most productive spacecraft in detecting exoplanets.

Onscreen: But how exactly do you find an exoplanet? Here are five methods.

Method #1 — Radial Velocity

Obviously, planets orbit their stars. But stars are also influenced by their planets.

Suvrath Mahadevan: The star and the planet are pulling on each other due to gravity.

Natalie Batalha: The planet’s going to move a lot, but the star itself is only going to move just a tiny bit, and that’s why we call it a wobble.

Onscreen: The light looks bluer when the star is wobbling towards us, and redder when it’s wobbling away. So if you see a star’s light change like that over and over again, it probably has an exoplanet tugging on it.

Exoplanets bagged by radial velocity: 638

Method #2 — Astrometry

Batalha: You can look at stars on the sky with exquisite spatial resolution. If you stare long enough, and if there is a planet orbiting that star…

Mahadevan: You can actually see that small perturbation in the position of the star on the sky.

Exoplanets bagged by astrometry: 1

Onscreen: Method #3 — Direct Imaging

Batalha: Now this is where you take a powerful telescope, you point it in the sky, and you actually see a tiny blur of light that’s indicative of a planet. The problem is these planets are literally lost in the glare of their parent star.

Onscreen: Which means you need a super hi-res telescope and a mask that blocks the star’s light. That mask either attaches to the telescope or, in the future, may unfurl as a “starshade.”

Exoplanets bagged by direct imaging: 43

Method #4 — Gravitational Microlensing

According to Einstein, gravity can influence light.

Mahadevan: So you take a massive object and you put light behind it. That massive object will bend light towards you.

Onscreen: Here’s how you exploit that. You watch the light of a distant star. It starts to get brighter. Which means a closer star is passing in front of it, bending more light from the distant star towards you. The exoplanet’s extra gravity will bend even more light towards you, leading to an extra blip.

Exoplanets bagged by gravitational microlensing: 47

Method #5 — Transit

Mahadevan: You have a star, and you have a planet going in front of it. This is a dark object passing in front of a bright object.

Batalha: These planets are literally casting a shadow out into the galaxy, and we will infer the existence of that planet by measuring a momentary dimming of light that repeats once every orbit.

Onscreen: Transit is the technique used by Kepler. And it’s bagged a remarkable haul of exoplanets — 2,732. Next year, NASA plans to launch the Transiting Exoplanet Survey Satellite. It’ll scan the entire sky for even more exoplanets. And one day, we might just find life on one of these distant worlds. 



Digital Producer
Ari Daniel
Editorial Review
Tim De Chant
Anna Rothschild
Production Assistance
Ana Aceves
Erin Dahlstrom
Bianca Datta
Greg Kestin
Elena Renken
© WGBH Educational Foundation 2017


ESO/J. Girard
National Park Service


(main image: exoplanet triptych)

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