by Ludwig Scheibe, February 2025
The most common and most succesfully methods of finding planets around other stars are the transit and radial velocity methods. Using those, humanity has discovered thousands of exoplanets and is still discovering more. But both of these methods rely on seeing the planet system “edge-on”, meaning the planet passes between us and its star. Planets orbiting their star such that we look at it “top-down” can’t be discovered with them.
Left: ideal orbit configuration of telescope and planetary orbit for transit and radial velocity method. Right: Configuration of telescope and planetary orbit, which makes it impossible to find the planet with the transit or radial velocity method. Note that distances and sizes are not to scale.
Looking for the wobble
Astrometry is a fascinating field of research in astronomy that involves measuring and tracing the position and movements of stars and other celestial bodies as precisely as possible. This method has many useful applications, such as measuring the distances to stars. Here, however, we will focus on what such measurements tell us about the possible existence of planets.
The basic effect we use is the same as in the radial velocity method: when a planet orbits around its star, the star does not remain motionless. It also performs an orbital motion around the common center of gravity, which is much smaller than that of the planet, but still present. This minimal movement causes the position of the star in the sky to change in relation to the neighboring stars.
The principle behind the astrometric detection of exoplanets. Credit: NASA
Tiny signal
The problem with this method lies in the extremely small change in position of the stars. Even for large and heavy planets in orbit around a nearby star, the change in position in the sky is only about one thousandth of an arc second – for comparison, an astronaut at the height of the ISS would only take up one arc second when viewed from Earth. For lighter planets or those that are more than a few light years away from us, this change can be smaller by a factor of a thousand.
At best, these changes can only just be measured with our current telescopes. This is why we have only discovered a handful of exoplanets using astrometry so far – as of early 2025. However, the great advantage of this method is that the signal is independent of the viewing angle of the planetary system. As a result, astrometry has already been successfully used to more accurately determine the mass of planets discovered using other methods.
The Gaia mission
Artist’s depiction of the Gaia Space Telescope. Credit: ©ESA
The Earth’s atmosphere slightly alters all the light that reaches us from space. This means that the best chance of measuring truly accurate astrometric data of the stars is with telescopes in space. One of these is the European Space Agency’s (ESA) Gaia telescope. Since its launch at the end of 2013, it has been measuring the position of over a billion stars with unprecedented accuracy. Individual planets have already been discovered with the data now available, for example Gaia 4b. This planet is one of the first exoplanets to be discovered using astrometric technology, and it is one of the heaviest planets known to orbit a small, low-mass star. However, processing and analyzing the entire data set takes a lot of time. It is therefore expected that many more planets will be discovered astrometrically with the Gaia data in the coming years.
Interesting questions about astrometry:
What can we learn about an exoplanet via astrometry?
The measured maximum displacement of the star depends on the planet’s mass and its orbital radius. We can learn the latter by measuring frequency of the star’s movement to give us the orbital period. Orbital period and orbital raidius are linked directly, so that we can use the star’s movement to leanr the planet’s mass.
Which planets are particularly suitable for the astrometry?
The heavier a planet is, the stronger is the movement it induces in its star. That means it is easier to detect high-mass planets than lower-mass ones.
Furthermore, the induced movement is stronger the larger the planet’s orbital radius. Thus, as opposed to transit and radial velocity method, particularly planets on wide orbits detectable.
Finally, the star’s displacement is more easily measurable if it is closer to us. Thus, astrometry can mostly find planets around very nearby stars.
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