By Emily Levesque, University of Washington
In our own solar system, a planet like Jupiter, or even a dwarf planet like Pluto, can be spotted by observing the sunlight that they reflect, but exoplanets are much too small and dim and far away for such a method to be effective. So, how can we know that there’s a planet around another star and search for them?
A Solar Eclipse
When it comes to observing tricks and techniques that the astronomers might employ, in order to search for exoplanets, there are a few possibilities. To begin with, in order to capture an image of an exoplanet, we need to remember that we normally can’t see the faint little stars, next to the sun, in the blinding light of day. And yet, Arthur Eddington managed it just fine in 1919, during a solar eclipse. What if we could do the same thing for another star, one with small faint planets around it? What if we could simulate an eclipse, blocking out the light of a star and taking a good close look at its surroundings with a powerful telescope to see if there are exoplanets there?
Astronomers do this all the time, using an instrument called a coronagraph that places a circular or bar-shaped mask in the center of a telescope’s field a view. If the name coronagraph sounds a bit familiar, there’s a reason: the first coronagraphs were used at solar observatories to study our own sun’s corona.
Potentially, we can equip our largest and most powerful telescopes with coronagraphs and use them to study other stars. Blocking the light from these stars acts much like blocking our eyes from a bright light, making it easier to see the dimmer surroundings.
The downside is that this method of trying to capture a direct image of an exoplanet is incredibly difficult. It assumes that the planets are big enough and close enough to their host star to reflect enough light to detect them, but not so close that they’re covered under the mask of the coronagraph.
Coronagraphic observations are also time-consuming and can only be carried out by focusing on one star at a time, making it an inefficient method for carrying out any kind of large-scale exoplanet search. Ideally, we’d like a method that can search many stars at once.
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Light Curve to Detect Exoplanets
One might also consider using something like a light curve to detect exoplanets. What if a star is being orbited by an exoplanet that passes directly in front of it, blocking some of its light? Just as in the case of Pluto, we can monitor the light curves of stars very carefully. There should be a nice clear drop when a planet passes in front of them, and that drop should repeat regularly every time the planet completes another orbit!
However, this idea depends on the orientation of the exoplanets orbit; it only works if the planet is passing in front of the star. What if, from our perspective, the system is tilted, with the planet appearing to move around the periphery of the star rather than passing directly in front of it? Or what if the planet is simply too small to block enough light for us to notice?
The Tug of a Planet’s Gravity
Another idea worth exploring would be gravity. We could look for the tell-tale tug of an exoplanets’ gravity on its host star. At first it might sound like an impossibly small effect, but it can be measured by closely monitoring the motion of the star.
To imagine how this works, let’s picture two objects orbiting one another. One of the key principles of physics tells us that since both of these objects have mass—and, as a result, gravity—one isn’t really going around the other. Instead, both of these objects are orbiting around some point between the two of them known as their center of mass. This means that both objects are, in fact, moving! The center of mass will be much closer to the more massive object, so it may only be moving a very little bit, but the motion is still there.
Observing the Redshift
This would also involve observing the redshift as an object moving away will have the wavelength of its light stretched, making it appear redder. Conversely, an object moving towards us will have the wavelength of its light shortened, making it appear bluer. We can observe this clearly if we collect a spectrum of the object; in these data, the tell-tale stripes of atoms like hydrogen or helium will appear noticeably redder or bluer.
Now imagine applying these concepts to a star with an exoplanet around it. If that star has a planet, the two objects will orbit around a common center of mass, and the star will periodically move toward or away from us. If we repeatedly take a spectrum of a star, we’ll see the lines in its spectrum shift back and forth as it moves toward and away from us, tugged by the planet and orbiting that center of mass.
Another option could be gravitational lensing, that consequence of general relativity where we see massive objects warping spacetime and bending the path that light takes as a result. Shouldn’t the mass of an exoplanet have a lensing effect—even just a tiny one—on its surroundings? It turns out that the answer is yes, though maybe not in the way we might imagine!
Planets are far too small to measurably lens stars by themselves. However, their host stars could lens other background stars, and we know that how the light of these background stars is lensed depends strongly on the mass of the object—or system—doing the lensing. If the lensing effect is off, even a little, from what we’d expect, based on the mass of the star, it tells us that there’s more mass present and that this mass could be in the form of a planet.
This method is difficult but could potentially work really well, capable of picking up extremely small amounts of excess mass from exoplanets around a star.
Common Questions about Exoplanets
Coronagraphic observations are time-consuming and can only be carried out by focusing on one star at a time, making it an inefficient method.
Gravitational lensing is that consequence of general relativity where we see massive objects warping spacetime and bending the path that light takes as a result.
If the lensing effect is off, even a little, from what we’d expect, based on the mass of the star, it tells us that there’s more mass present and that this mass could be in the form of a planet.