Despite what you may think NASA does not just point a high-powered telescope into the night sky and identify thousands of planets. NASA uses 4 distinct methods to discover (Exo)planets — and most do not involve actually viewing the (Exo)planet.
- Gravitational Microlensing
- Radial Velocity
- Direct Imaging
Disclaimer: I have left out Astrometry — using different metrics to identify a(n) (Exo)planet — even though NASA considers it a viable method. I simply do not know enough about the subject.
Gravitational Microlensing Method:
We can thank Albert Einstein’s wonderful brain for this technique of identifying planets. Einstein claimed that gravity causes a warp in the plane of space. This effect causes the planet to bend light from its host star.
How do we know that the bent light indicates a planet?
Well, typically only a large enough mass is able to create a gravitational field capable of warping the ray of light. So knowing the light is bent is a good indication that there is a planet in that stellar system.
Based on various attributes of the light observed from the lensing event, scientists can more accurately determine the actual characteristics of the planet.
Summary For Dummies: We observe light bending → only big masses can bend light → there must be a(n) (Exo)planet.
This method is tangentially similar to the Gravitational Microlensing Method because it relies on observing the dimming of a star’s light. When there is a significant dip in the light for a certain amount of time, scientists can predict there is an (Exo)planet passing in front of its host star.
Imagine staring at a projector screen. When nothing is blocking the light you expect to see a bright yellowish-white background, however, when you see a shadow on the screen you realize something is blocking the projector. No one has to tell me there is something blocking the projector it is easily inferred.
From the amount of time a planet covers the host star’s light to the amount of light blocked a scientist can calculate many of the attributes of the (Exo)planet.
Summary For Dummies: We observe a star’s light is dimmed → only large masses can block the light of radiant star → there must be a(n) (Exo)planet.
Radial Velocity Method:
Newton’s 3rd law states “every reaction has an equal and opposite reaction”. The Sun pulling the Earth is not a one-way relationship. Obviously our Sun has more pull on the Earth, but the Earth also pulls back a little on the Sun. This causes our Sun to wobble (or move in its own mini-orbit).
Now if some aliens were watching us from their galaxy and saw our much larger Sun wobble they could infer something else must be pulling on the Sun. Using this information they deduce some other planets are present in our solar system.
How do the aliens know our Sun is wobbling?
Well somehow our world-famous “Doppler Effect” has reached their galaxy and they know that the change in wavelength of visible light identifies the orbit characteristics of this star.
Reddish light waves indicate waves with a larger period (yup that term from 10th-grade math class)
Blue light waves indicate waves with a shorter period (all bunched together).
Summary for Dummies: We observe per the Doppler Shift the wobble of the star → wobble tells us some large mass must be tugging at the star → there must be a(n) (Exo)planet.
Direct Imaging Method:
This method is probably how most people think planets are observed; scientists use a large powerful telescope to take pictures of other stellar systems and after some image processing they sometimes identify some (Exo)planets.
Now, Direct Imaging is in the early stages of development and there are numerous methods coming out about how to identify (Exo)planets. Currently, in the 4000 some odd number (Exo)planet database only 50 have been identified through Direct Imaging.