From my understanding we detect planets by measuring dips in light intensity from the star the habitable planet is orbiting when it passes by it. There are 2 things I don’t understand about this method:

  1. Planets in a solar system tend to orbit their star in one disc like plane. Can we only detect planets if this plane is in-alignment with our sensors here on earth/ space. I assume if someone looked at our sun from the ‘bottom’ they would never see a planet cross it. If so this would exclude a big portion of the stars we are looking at?

  2. Our orbit is 1 year. I assume the orbits of habitable plants can vary and having a short/ close to 1 year orbit is not a criteria for habitability. Even so we have to measure 1 dip every year or 10. How can we tell that this a planet with a regular yearly orbit or just anything passing between the star and us. So maybe a better question to ask for number 2 is what can we find out from these dips?

  • 1
    $\begingroup$ In addition to John Duffield's answer, planets closer to their star with shorter orbital periods are easier to detect due to more regular wobbles. More distant planets can still be detected, but it takes longer. Similarly (and somewhat obviously), larger/more massive planets are easier to spot than smaller ones. $\endgroup$
    – userLTK
    Apr 18, 2018 at 18:26
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    $\begingroup$ One of the things that surprised astronomers when they started discovering exoplanets is how many have incredibly short periods. $\endgroup$
    – Barmar
    Apr 18, 2018 at 23:50
  • $\begingroup$ I think that it's important to remember that we have just begun a "Golden Age" of exoplanet study. The answer to most questions should still be "Let's wait and see!". If mankind can be resourceful enough to detect and analyze gravitational waves, I think we should be able to catalog all of the "planets" orbiting stars within fifty light years in the next fifty years. It's all about money and politics. $\endgroup$ Apr 29, 2018 at 19:52

2 Answers 2


It isn't the only way, Aaron. See the Wikipedia Methods of detecting exoplanets article. The first method listed is radial velocity. That's where we measure the Doppler shift of the star's "wobble".

The second method is transit photography. And as you say it has a drawback. In fact there's more than one:

"This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets...

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems..."

  1. Yes and yes. Transit detection is only effective for that (small) fraction of planets that pass between the star and our line of sight. Most planets will go undetected by this method.

  2. Also correct. The detection of multiple transits is required to find planets. Even then the dips can be caused by other things (e.g. grazing eclipse binary star systems). This means to find Earthlike planets in Earthlike orbits would require years of observation. But habitable zones around less luminous stars are closer-in and have shorter orbital periods. It is these stars that planet-hunting missions like TESS are focused on.


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