In any solar system, the star or stars that occupy its center dominate absolutely everything. In the particular case of our solar system, our star accumulates 99.8% of all its mass . The same thing happens around the rest of the stars in our galaxy and in the universe. This will imply an almost total control over the orbits of the rest of the objects that orbit around said star, be they planets, asteroids or comets. In addition, the star will strip the atmosphere of the inner planets that are not able to retain it, it will evaporate any presence of liquid water on the planets that are too close, etc. Seen from a distance, the central star will also outshine all other objects with its brightness . In our particular case, the Sun is about a billion times brighter than any light that the planets can reflect.
As you may be imagining, all this makes it extremely difficult to detect exoplanets. Especially considering that not only is it enough for us to detect them , to know that they are there, but we also seek to find out how far they orbit their star, their mass and how big they are . Even with all this, nearly 5,000 different exoplanets have been detected, ranging from giants larger than Jupiter to Kepler 37b, a rocky exoplanet barely larger than our moon. This feat has been possible thanks to the ingenuity of those who dedicate their research careers to this task. In recent decades , different techniques have been devised that allow us to find the needle in the astronomical haystack . Let’s see the ones that have given the best results.
Radial velocity and astrometry
Above I commented that the star will have almost total control over the orbits of the rest of the objects that orbit it. That was almost not accidental or an attempt to be overly cautious, but, indeed, the domain is not absolute. Any object that exerts a gravitational force on a different object will receive an equal but opposite force from that other object. That is, the Sun attracts the planets, but the planets also attract the Sun. That force, which will be the same in both directions, will affect the Sun to a lesser extent due to its enormous mass, but it will affect it.
In fact, the correct way to talk about planetary orbits is not to say that these bodies orbit around the central star, but rather that all of them (planets and stars) orbit around the center of mass of the system. We usually ignore this detail because the center of mass of the solar system is practically in the center of the Sun. However, this supposed movement of the Sun, this orbit around the center of mass, can be measured. The Sun is moving at about 13 m/s due to Jupiter’s pull, but only about 9 cm/s due to Earth’s pull.
This speed can be easily measured by taking advantage of the Doppler effect , which is nothing more than the change in frequency of light due to the relative speed between emitter and receiver. That is, if the star (or any other source of light) moves away from Earth, its light will drop in frequency, becoming more reddish; if it gets closer, the opposite will happen, becoming more bluish. This was the most effective method during the 1990s and 2000s, allowing us to detect the first few hundred exoplanets. This technique gives us information about the mass of the planet, but not about its size.
Also, and for the closest stars or those with truly massive planets around them, this wobble can help us measure the star’s own motion. By measuring its position over time with incredible precision, we can see how it oscillates over the days. This technique is called astrometry.
Transit
This method takes advantage of the decrease in brightness that we observe when a planet comes between our telescopes and the star . When it is the Moon that does it, we call it an eclipse, managing to block all the brightness of the Sun. In the case of exoplanets, this decrease is really small, but enough for us to measure it. This technique is, to date, the most effective when it comes to finding exoplanets, having given us more than half of the total number of planets discovered . Of course, this technique has the drawback that we will only be able to detect planets whose orbital plane points more or less towards the Earth. In addition, this technique allows us to obtain information about the size of an object, although not about its mass.
Direct observation
This technique is the most intuitive to understand, although one of the most complex at a practical level. It consists of direct observation of the planets. The problem with detecting exoplanets directly, by detecting the light they reflect from their star, is not their low brightness per se, but their low brightness compared to the star they orbit . To reduce the brightness of the star, coronagraphs are used, which block the star’s light, allowing us to detect any less bright objects around it.
This technique works best with planets that are large and relatively separate from their star. If they are too close they will be difficult to distinguish from the star itself, but if they are too far away they will not reflect enough light to be detected. The direct observation of exoplanets has not produced many results, although during the last decade there have been great advances that seem to promise a good future for it.
These are just some of the ways we have of identifying planets orbiting distant stars. Other techniques, such as those that take advantage of disturbances in transit or astrometry techniques produced by the presence of more than one planet, or gravitational microlenses that take advantage of the temporal alignment of two distant stars . Event that despite being very unlikely, has resulted in the detection of several dozen exoplanets.
Today some space missions are being planned whose main objective will be to detect new planets around stars in our galaxy and, above all, increase our knowledge of the planets already discovered. This will be done, in part, by the James Webb Telescope , which will study the atmosphere of known exoplanets . Their study will take advantage of the light reflected or transmitted by the atmosphere of different extrasolar planets to study their chemical composition and physical characteristics. There are certainly some exciting decades ahead of us.
