If we can tell a planet exists by the dimming of the brightness of a star when it passes in front of our view of the star does that mean we cannot see a planet if it does not cross in front of our view?

If planet of a distance star goes around the star rather than in front of it than we cannot see that planet. Correct?


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The answer by choeger provides a correct outlook, but I wanted to provide a bit more detail on some exoplanet detection methods that have actually been used to find planets we couldn't see. Now I interpret "see" in the manner posed by the question in that "see" references observing a transit of the star by the planet. The questioner is correct that we cannot "see" the planet in this manner if the alignment is not perfectly correct. So how else might we detect the planet?

  1. Radial Velocity Method - We can try to detect its gravitational effects on the star through a method known as the Radial Velocity method. Both the star and planet(s) will be orbiting their common center of mass. That means if we watch the star, we can see it orbit that center of mass as well. If we see the star "wobbling", we can infer what mass must exist in that system to cause the wobble. I should point out that we are only able to see radial wobbling (that is, wobbling of the star towards and away from us) which means that while we can still detect planets which are not directly aligned with the star as they would be for the transit method, they are much harder to detect. I also want to point out that the answer by choeger mentioned tidal forces, but that is actually not something we can use to detect planets or measure their effects on their host stars.
  2. Direct Observation - This is exactly what it sounds. With some clever techniques and the right conditions, we can take a picture of the planet itself. One pretty famous example was the direct imaging of Fomalhaut b. Shown below is an image over a few years as it travels through its orbit.

enter image description here

  1. Microlensing - This uses the concept of gravitational mass acting as a lens to background, luminous objects by bending spacetime, and consequently, the path of light through space. If a planet passes in front of a background object (not its host star), it will cause a slight magnification of that background object. I should say that this requires a chance alignment and so it is not a repeatable measurement, meaning there is often little chance that we can actually confirm that we saw a planet. Below is a light curve of a background object and you can see it grow brighter as the main star passes in front of the background object. Pay attention to the little blip on the right which indicates the planet also passes in front of the background object.

enter image description here


This is not true in general. There are other methods to detect a planet that do not rely on a direct line of sight.

You are not completely wrong: Direct observation (e.g. of a planet's atmosphere) requires a line of sight. We can however notice the presence of a planet by indirect means: If the planet causes an effect on an observable body (e.g. tidal forces on its star) we can deduce its existence from this (regular) effect.

In principle, we could also use the gravitational waves a planet should emit, although no practical method exists to do so (yet).

  • $\begingroup$ Also "astrometry", but i think that this method has only detected planets that were already known from radial velocity measurement. The Gaia mission may find thousands. Astrometry detects "wobble" in the plane of the sky whereas the RV method detects it perpendicular. Thus , they are complementary. $\endgroup$ – Jack R. Woods Jun 14 '16 at 0:36
  • $\begingroup$ One more that's "off the charts weird", but actually detected the first exoplanets ever is pulsar timing. You'll have to read up on that one! $\endgroup$ – Jack R. Woods Jun 14 '16 at 1:22

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