Edge-On: Good for Planets, Bad for Galaxies

Every time we see amazing photos of galaxies or planetary disks, we can see most of the detail since we see them face on.  But since the orientation of spiral galaxies in the universe is random, there are a plethora of galaxies ignored by image processors since we just can’t see much of the detail.  We can still learn from edge-on spiral galaxies, just not as much as we can from those that are face on.

Edge on spiral galaxy NGC 5866 Image Credit: NASA, ESA, Hubble Legacy Archive; Processed & Copyright: Hunter Wilson

We can see some fascinating dust lanes in the image above, and a ton of detail considering the view, but we don’t know what the spiral arms of this galaxy actually look like.  And we never will.  Galaxies that are face on simply look better and offer more scientific value.

With planets it’s the opposite.  And it’s all because of the way we find exoplanets, the planets that orbit stars other than the Sun.  The easiest and most prolific method is to look at the light from a star, and then watch for it to dim as a planet passes in front of it.  Even if the star is too far away to see the planet physically pass in front, we can measure the minute dip in what we call the light curve, or brightness as a factor of time.  If it happens several times, with the same dip at regular intervals, we can prove there is a planet orbiting that star, and infer properties such as it’s size, mass, and orbital period.

Credit: News.mit.edu

But this method completely falls apart if the planet doesn’t pass in front of it’s parent star.  And since the orientation of planetary disks is also random, there are likely a large number of interesting nearby exoplanets that we are just missing.  However there are a few ways to combat this, though they are all much less effective.

For one, you can directly image a planet orbiting a star.  Because planets reflect starlight, ones closer to Earth can be seen directly in photographs.  This only works for a few hundred nearby stars, but as our technology improves we can extend that number.  Along these lines you can also look at the polarization of light, since reflection by a planetary atmosphere polarizes starlight.  If you look at the light from a star and see a fraction of it polarized, you can confirm there is a planet.

Another method is through gravitational microlensing.  Just like gravitational lensing with galaxies, the gravity from stars and even planets can bend the light from background stars.  Planets have been found by observing their distortion of background starlight.  This method does require exceptional precision, and lots of background stars, meaning it works best when searching for planets near the galactic plane.  There are several other methods that work for face-on exoplanet hunting, but it is far more difficult than finding the planets that orbit edge-on from Earth.

The job of astronomers is to find ways to overcome these difficulties, to obtain knowledge from the edge-on galaxies, and develop methods to characterize face-on solar systems.  As our technology and methods improve, we will reach this goal, it will just take a bit longer.


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