A quick review of photographs of galaxies will reveal that not all galaxies are orientated in the same plane as the field of view from Earth. Some galaxies are parallel to our field of view and we see them side on. Others are perpendicular to our field of view and we see them as if we are looking either from above or below them.

Given this fact, it would be fair to assume that the orientation of planetary systems is similarly varied. Not all of them are parallel to our field of view.

All the exo-planetary systems I am aware of have been seen in a side view perspective and any exoplanets have been detected by either a wobble of the star at the center of the planetary system or via dimming of the star's light as the exoplanet transits the star.

Have any exo-planetary systems been discovered that are perpendicular to our field view, looking from above or below the system and if so, how have any exoplanets in these systems been detected? If no such systems have been observed, how would astronomers detect exoplanets in such systems?

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    $\begingroup$ While detection by transits does indeed require the system to be nearly edge-on, detection by radial velocity (“wobble”) doesn’t require that, and most of those systems are likely not exactly edge-on. For a given planet and star mass, the detected size of the velocity variation changes as sin(i), where i is the inclination of the orbit to our line of sight. (i=90 is edge-on, i=0 is face-on.) So at a 45-degree angle, the signal is reduced by less than 30% compared to edge-on. Even at 60 degrees from edge-on you still get half the radial velocity signal. $\endgroup$ Commented Aug 22, 2020 at 16:28

2 Answers 2


The multi-planet system HR 8799 is pretty close to face-on. Here's a video made by interpolating images taken over 7 years showing the orbital motion.

The planets of HR 8799 were detected by imaging. The majority of exoplanets have (at the time of writing) been detected by the radial velocity method, which requires radial motion and thus is most sensitive to planets close to edge-on, and the transit method which requires that the planet passes in front of the star as seen from Earth (thus will only spot planets extremely close to edge-on). The imaging method does not have this issue, so can find both face-on planets (like HR 8799) and edge-on ones, like Beta Pictoris b (video).

Astrometry is another exoplanet detection method that would allow detection of face-on systems. It has been plagued by a number of false starts (e.g. the infamous "detection" of planets around Barnard's Star by Peter van de Kamp) but hopefully once the full Gaia dataset is released there will be numerous astrometric exoplanet detections, which may include near-face-on systems.


For an overview of the many methods: Wikipedia - gravitational microlensing.

Each detection method has a different sensitivity to the inclination of the exo-planet's orbit:

  • Radial velocity is most sensitive to side-on planetary orbits as it looks for changes in velocity along the line of sight. Purely face-on planets are undetectable. You need the inclination to get the mass of the planet rather than lower limit on mass.
  • Transit can only detect planets nearly side-on.
  • Imaging does not depend much on inclination, but the system must be big and close.
  • Astrometry does not depend much on orbital inclination as it measures side-to-side motion of star against star.

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