In the spring of 2022, NASA announced the achievement of an important symbolic mark: its astronomers confirmed the existence of the 5,000th exoplanet. This is, of course, only the beginning, as reports on the discovery of new extrasolar worlds appear almost every day. But what are exoplanets, and how do scientists find them?
Let’s start by defining our terms. The International Astronomical Union defines an exoplanet as a body whose mass is insufficient to trigger fusion reactions within them (that is, less than 13 Jupiter masses) and which revolves around stars, brown dwarfs, or stellar remnants.
It is easy to see that this formulation hides a number of nuances. One of them is that the term “exoplanet” refers only to a body revolving around another star or stellar object. When it comes to planetary bodies that are not part of stellar systems, scientists apply other labels, such as “free-floating planets,” “orphan planets,” or “rogue planets.”
Sometimes, astronomers use broader terms like “planetary mass objects,” or planemos. These are bodies with enough mass to qualify planets (that is, more than an asteroid, but less than a brown dwarf), but which may have properties that do not correspond to typical planets.
History of the search for exoplanets
It is unlikely that we will ever know who the first person was to suggest that the bright little dots that light up the sky were actually distant suns that could have their own planets. It is known that Democritus, Epicurus, and several medieval Arab cosmologists at least considered this idea. The 1584 treatise of the Italian monk Giordano Bruno titled On the Infinite, Universe, and Worlds proved to be a milestone. In it, Bruno suggested that planetary systems like the solar system must exist everywhere in space.
After the invention of the telescope and the accumulation of scientific knowledge, astronomers gradually accepted the concept that the stars in the sky were the same type of bodies as our Sun, but located very far away. And if this was the case, why shouldn’t they really have their own planets?
The problem was that the astronomers of the past were not in a position to test this hypothesis. Due to the extreme dimness of exoplanets, it was simply impossible to see them even with the most powerful telescopes. They could only be shown to exist through indirect methods, such as searching for anomalies in the movement of stars. The idea was that since the gravity of an exoplanet should act upon its star, then the star’s movement in the sky would be different from what it would otherwise be projected to be without exoplanets.
In 1855, the head of the Madras Observatory, William Stephen Jacob, published an article in which he spoke about the anomalies he had identified in the binary system 70 Ophiuchi. The astronomer came to the rather bold conclusion that these anomalies were caused by the influence of the gravity of an invisible third body in the system, which he reasoned must be an exoplanet with an orbital period of 26 years.
As we now know, Jacob was wrong: if there are any exoplanets in 70 Ophiuchi, then they were not the body he predicted at all. There is nothing surprising about this mistake, given the large margin of error in the measurements he carried out via telescope and the hand sketches he did based on his observations. But in any case, Jacob inscribed his name into history as the first astronomer to try to find an exoplanet.
It is curious that if things had turned out a little differently, scientists could have received very solid evidence of the existence of exoplanets a little over a hundred years ago. In 1917, astronomer Adrian van Maanen discovered a previously unknown star that now bears his name. Studying its light spectrum, he noticed unusual absorption lines which indicated the presence of many heavy elements in the body he was observing, such as calcium, magnesium, and iron. As a result, van Maanen erroneously classified the body he found as a star of spectral type F. Only a few years later would it turn out that he had actually found a white dwarf.
However, the fact is that all elements heavier than helium located on the surface of white dwarfs “sink” rather quickly, descending towards the star’s core. So why did van Maanen’s star have such a strange spectrum? We now know that the answer is that white dwarfs can be “enriched” with matter from the disks surrounding them, which consists of fragments of destroyed exoplanets. That is, at least in the past, van Maanen’s star had its own planetary system. Alas, scientists of that time did not know this, so no exoplanets were discovered.
In the 1960s, astronomer Pieter van de Kamp caused a sensation when he reported the deviations he had discovered in the motion of Barnard’s star. In his opinion, they were caused by the gravity of a Jupiter-like exoplanet. Even though van de Kamp’s conclusions were not universally accepted, and subsequent observations did not reveal the deviations he claimed, his claim to have found exoplanets around Barnard’s star sparked a great public stir. For a very long time, this red dwarf was seen by many as the only star that was certain to have its own planetary system. The creators of the Daedalus interstellar spacecraft, developed in the 1970s, even proposed sending it to Barnard’s star rather than to the closer Alpha Centauri. Oddly enough, astronomers cannot say for sure even now whether Barnard’s star has exoplanets.
The turning point in the search for extrasolar worlds began in the 1980s with the digital revolution in astronomy. The transition from photographic plates to digital images, the use of computers for data processing, the introduction of more powerful telescopes, the emergence of new technologies such as adaptive optics – all this laid the groundwork for the discovery of exoplanets. The first signs were obtained in 1984 pictures of the star Beta Painter, which demonstrated the surrounding planetary disk.
However, the first exoplanets to be discovered were ironically not found with optical observations, but with the help of radio telescopes. At the same time, they revolved around … a neutron star. Here is how it happened: in 1990, the Arecibo radio telescope discovered a new pulsar, which received the designation PSR 1257+12. In the course of further study, astronomers discovered a strange anomaly. From time to time, the pulsar’s signal was interrupted, and this would always occur at a consistent time interval. After considering all options, the scientists found the only possible explanation: the “breaks” in pulsation were caused by a body orbiting PSR 1257 + 12 (or rather, bodies, as we now know about three exoplanets in this system).
An article about the find was published in 1992, making a splash in the scientific community. The discovery gave rise to a number of questions, the main one being how a pulsar could even have exoplanets. The fact is that these objects were formed as a result of supernova explosions, which should completely “clean up” their surroundings and destroy all nearby bodies. The celestial companions around PSR 1257+12 are now thought to be “secondary” exoplanets, most likely formed from the debris of the original exoplanets destroyed by a supernova.
The very first exoplanet to ever be found near a “full-fledged” star was found in 1995 by Swiss astronomers Michel Mayor and Didier Quelo. It was a gas giant orbiting the sun-like star 51 Pegasi. For this achievement, Mayor and Kelo were awarded the Nobel Prize in Physics in 2019.
Exoplanet Search Methods
How exactly are modern astronomers looking for extrasolar worlds? The exoplanet in the 51 Pegasus system was found using what is called the radial velocity method. It is based on the search for changes in the radial velocity of a star caused by the impact of a massive object in its vicinity. These manifest as periodic shifts of the spectral lines – very small fluctuations, which can be less than a meter per second. But modern technology is already able to find these deviations.
This radial velocity method was the main tool in astronomers’ arsenal in the early years of hunting for exoplanets. It made it possible to determine the exoplanets’ masses, as well as periods of circulation. In total, thanks to this method, it was possible to find a little more than a thousand extrasolar worlds (about 20% of the total number known today). However, over time, the radial velocity method was replaced with a new one, called the transit method.
The transit method is based on the idea that if a star has a companion, the brightness of the star will decrease as the companion passes through the star’s disk. By plotting the light curve, astronomers can determine an exoplanet’s diameter and orbital period, and even detect whether it has rings or its own satellites (exomoons). Of course, it remains quite rare to find exomoons right now, but as technology improves and the next generation telescopes are put into operation, their number should increase markedly.
Another advantage of the transit method is that if an exoplanet has an atmosphere, astronomers can learn a great deal while studying the spectrum of light that passes through it, including its chemical composition, its structure, and even its cloud cover. If, say, astronomers can find a large amount of oxygen, as well as certain biomarker compounds in the atmosphere of an extrasolar world, this may indicate its potential habitability.
Unfortunately, despite all its obvious advantages, the transit method has one very significant drawback. The probability that an arbitrarily any given exoplanet will be transiting at a given time is negligible. For a celestial body with the same size and orbit as our planet, the chances of the conditions being right for detection at any given point is only half a percent. Fortunately, there are just so many opportunities to detect them, as our Milky Way contains about 200 billion stars. So even with such a small probability of an individual exoplanet being detected in a star system, there are billions and billions of them out there to be found.
It is precisely because there are so many star systems out there that the transit method is a key means of searching for exoplanets. The main contribution so far has been made by the Kepler telescope, which operated from 2009 to 2018. Analysis of its data revealed more than 2,700 confirmed exoplanets.
In 2018, NASA launched Kepler’s successor, the TESS space telescope. So far, it has already confirmed the existence of about 300 exoplanets, with another 6,000 still listed as candidates. In total, as of December 1, 2022, out of more than 5,200 known exoplanets, almost 4,000 were found by the transit method.
The third most popular way to detect exoplanets so far is the gravitational microlensing method (145 confirmed exoplanets). Microlensing occurs when the gravitational field of a star closer to us magnifies the light from a distant background star like a lens. If a nearby star has an exoplanet, then its gravity will also affect the light, and this effect can be calculated. The main advantage of the method is that it makes it possible to detect both bodies at great distances from the Earth (even in other galaxies) and objects that are not part of stellar systems and which cannot be found using other methods. The main disadvantage of microlensing is that it relies on a very rare event. If you take any arbitrary star and wait for it to microlense, it could take hundreds of years.
Curiously, astronomers have been able to find several dozen exoplanets by taking direct images of them. But here there is an important nuance: so far, almost all exoplanets discovered in this way are very massive, newly formed bodies. Their atmospheres produce a very large amount of infrared radiation, which allows them to be identified.
Of course, there are other ways to search for exoplanets (astrometry, the transit variation method, etc.). But due to a number of difficulties, their application is very limited. In total, all the other invented methods for searching for exoplanets have so far made it possible to discover less than a hundred extrasolar worlds.
The future of the search for exoplanets
The study of exoplanets is one of the most rapidly developing branches of modern astronomy. Every year, new telescopes and instruments are put into operation to learn more and more about the extrasolar worlds. Astronomers thus have high hopes for the recently launched James Webb telescope. Its technical capabilities will make it possible to conduct a more detailed analysis of exoplanetary atmospheres and obtain direct images of large objects whose orbits pass at a considerable distance from stars.
Over the course of the next decade, then a number of new observatories will be put into operation, which will have to make a significant contribution to the search for exoplanets. Astronomers are especially interested in Earth-like bodies in so-called habitable zones: the area of distance from a star where a planet receives enough energy so that, with suitable atmospheric conditions, liquid water can exist on its surface.
The European Space Agency (ESA) plans to send the PLATO observatory into space in 2026. Its priority will be to search for exoplanets in the habitable zone around yellow dwarfs. Another European space telescope called ARIEL is set to join PLATO in 2029, and will study worlds already known to astronomers. It will focus on determining the chemical composition of their atmospheres and better understanding their orbital characteristics.
NASA is also hard at work. A coronagraph will be installed aboard the new Roman infrared telescope (scheduled to launch in 2027). This device is capable of filtering out a star’s light to directly study its orbiting satellites. The Roman coronagraph will use new light blocking and wavefront distortion compensation technology. This will allow the telescope to obtain the clearest images of protoplanetary and circumstellar disks, as well as to find several thousand new exoplanets and get direct images of the largest one.
There are also sure to be many discoveries made by the autonomous Chinese Xuntian telescope module, which is planned to be sent into orbit in 2023-2024. It will be equipped with a two-meter mirror, which will allow it to take pictures with a resolution close to that of the Hubble telescope. However, the Chinese telescope will have a 300 times larger field of view than its American “colleague.”
There are also ground-based telescopes to consider. Under construction now in Chile, the Extremely Large Telescope (ELT) will become the largest ground-based observatory conducting research in the optical range. Thanks to its gigantic 39-meter mirror, it will receive images with a resolution that is an order of magnitude higher than the famous Hubble. The ELT is expected to be able to directly photograph exoplanets, study their atmospheres and observe the birth of new planetary systems. The Giant Magellan Telescope, which is also being built in Chile, will also have to make a big contribution to the study of exoplanets. At the moment, both telescopes are scheduled to go into operation at the end of this decade.
Talking about longer-term prospects, one of the most interesting projects for the study of extrasolar worlds involves the construction of a telescope that will use our very own Sun as a magnifying glass. The idea is to position the telescope in line with an exoplanet and our star in the middle. This will allow the Sun’s gravity to act as a gravitational lens. According to calculations by engineers, a telescope placed in the right place, equipped with the same mirror as Hubble, will be able to get an image of an exoplanet at a distance of 100 light years, which will be as clear as a photograph of the Earth taken from the moon. To get a similar picture using a traditional telescope, one would need a mirror with a diameter 20 times larger than that of our planet.
The main problem of this project concerns the huge distances involved. To take advantage of the solar lens effect, the telescope will have to be placed at a distance of 550 astronomical units (82 billion km) from the Earth. For comparison, the legendary Voyager 1 has traveled a mere 130 AU from us so far, 45 years after leaving Earth. Another disadvantage of the project is that a solar-gravity telescope would be able to study only one particular system. For greater coverage, scientists will have to use a whole swarm of these devices.
However, with the further development of technology, such a project may well become a reality, especially if astronomers can find an Earth-like exoplanet with clear signs that indicate its habitability. And who knows? Perhaps right now some other civilization is studying our planet through a similar telescope.
In the second part of our series, we will talk about the most unusual exoplanets astronomers have found.