Thirty years after the discovery of the first exoplanets, there are still many loose ends

Although exoplanet research has exploded in recent decades, thirty years after the discovery of the first exoplanets, astronomers have to conclude that there is still a lot we don’t know.

Exoplanet Discovered: We’re Not Surprised Anymore. It was very different thirty years ago, when astronomers announced on January 22, 1992 that they had discovered some planets outside our solar system for the first time. “It was revolutionary,” says Michel Meen, an astronomer and exoplanet researcher at the Netherlands Institute for Space Research (SRON). Until that time, we knew only the planets in our solar system. And there were astronomers who suspected that there were planets also orbiting other stars, but we didn’t know for sure.”

Planets around a dead star
Until 1992. Because then the Polish astronomers Alexander Wolsztzan and Maciej Konaki discovered two exoplanets: PSR B1257 + 12 c (Poltergeist) and PSR B1257 + 12 d (Phobetor). “And those were immediately very strange samples, because these two planets orbit a pulsar, which is actually a dead star.” And with that, those first planets were quite alien; The system they were a part of—and of which a third planet was discovered in 1994—was nothing like our familiar ancient solar system.

Hetty Jupiter
The discovery of 51 Pegasi b in 1995 was a little more familiar. “This was the first exoplanet found around a Sun-like star.” But the similarity with the planets in our solar system ended immediately, because we’ve never seen anything like 51 Pegasi b. “The planet takes only four days to orbit the star and has a mass roughly half that of Jupiter,” Min said. This makes it one of the hot Jupiters: gas giants – unlike the gas giants we know from our solar system – are very close to their star. “No one expected it to exist, because we know nothing like it in our solar system.”

A wide range of planets
These early nascent discoveries indicate that there is more than we thought possible based on what we see in our solar system. In the following years, this assumption was proved at length. Thousands of (candidate) exoplanets will be discovered within a few decades. And there are a lot of exotic birds among them like hot Jupiters for example. But also the so-called super-Earth or mini-Neptune. “Planets large and small were expected to exist outside our solar system – just as there are inside it,” Main says. “But now it turns out that the giant Earth is the most common planet. It is remarkable that we do not know this scale in our solar system.” In addition to alien planets, there are also alien systems. “For example, planetary systems that are very small and where the star and the parent planets are very close to each other.” By far the most famous example of this is TRAPPIST-1, which contains no fewer than 7 planets and fits perfectly into the orbit of Mercury. “It indicates that nature is doing everything it can,” says Main, when talking about the wide range of exoplanets and planetary systems discovered so far.

TRAPPIST-1 has seven Earth-like planets. The entire system fits into the orbit of Mercury (the innermost planet in our solar system) and is therefore very compact. Images: NASA/JPL-Caltech/R. Hurt (IPAC).

Lots of planets
At least as startling as the discovery that planets come in more shapes and sizes than we thought possible, is the fact that exoplanets are being discovered just about everywhere. “It was really surprising,” Main says. “And we now know that there are more planets in our Milky Way than there are stars. That’s cool and exciting. That means when you look at a star, you can be almost certain that one or more planets orbit it as well.”

Explodes
It is really wonderful. And astronomers since 1992 can’t get enough of it either. “Exoplanet research has really exploded. It has become a full-fledged field that involves a huge amount of money.” Earlier this century, NASA built a nearly $600 million Kepler planet hunter, which discovered thousands of (nominated) planets between 2009 and 2018 and was followed by a $200 million TESS, which now also contains several exoplanets around nearby stars. . Whereas the focus was on discovering exoplanets, it is now gradually turning to investigating them more closely. For example, the European Space Agency and NASA are working on space telescopes specifically developed to study the size, composition and atmosphere of already discovered exoplanets. “It flourish Discipline.” And our understanding of exoplanets is expected to expand dramatically in the coming decades. Maine predicts: “If the twentieth century is the century of cosmology, this will be the century of an exoplanet.”

Here you see the light curve of a star orbiting a planet. The moment the planet moves in front of the parent star, a dip in the star’s light occurs. Photo: NASA Ames.

Obviously, there is still much to discover. Because we know very little about the thousands of (candidate) exoplanets that have been discovered so far. Its existence is usually inferred from the regular dips in the light of its parent star, which are caused by planets moving in front of that star as we have seen. In addition to being present, these dips in the light curve usually also reveal the planet’s orbital time, size, and thus also its type (rocky or not). And in the case of hot Jupiters, it may be possible to detect the atmosphere during such a transition through which light leaks from the parent star. And sometimes, thanks to the light filtered through the atmosphere, we can tell more about the composition of the atmosphere of such a gas giant. But in most cases, the regression remains somewhat ambiguous in a steady light curve. This makes for endless curiosity. Because what are we going to face if we take a closer look at those exoplanets? Just as in our solar system, where Mars is very different from Venus and no match for Mercury on Earth, can we conclude that one rocky planet is not like the other? Will we see ring systems around some planets? or exomoons? There are still many questions.

Extraterrestrial life
This means that thirty years after the discovery of the first exoplanets, there is still a lot to look forward to. Are there any other discoveries coming up that are at least as amazing as those of the first exoplanets? Undoubtedly, according to Maine. He is thinking, for example, of discovering the first biomarkers. “Particles in the Atmosphere of Exoplanets That May Indicate Biological Activity.” In other words: an indication of the existence of extraterrestrial life. For a long time, such research was a distant proposition, but that is changing. The James Webb Space Telescope, which launched in December, should already be able to explore the atmospheres of large planets. “And its successor – LUVOIR – should be able to do the same for Earth-like planets.” This space telescope is expected to specifically look for a group of particles that you wouldn’t normally expect to find together. “If we look at life here on Earth, it actually constantly disrupts the chemical balance,” Maine explains. After all, living organisms remove gases from the atmosphere (such as carbon dioxide) and introduce other gases (such as oxygen) into the atmosphere. Thus our chemically unbalanced atmosphere bears witness to our existence. It does not seem inconceivable that the atmospheres of exoplanets would reveal the existence of living beings in this way. “So astrobiologists are looking for a good indicator of chemical imbalance and this research is very promising.” If aliens are somewhere chemically disrupting the atmosphere, Min is very optimistic that we will find their traces somewhere in the coming decades.

“We had to fix our theories by discovering exoplanets and we still don’t understand everything”

The solar system is like
That would be revolutionary. But when it comes to exoplanet research, even more exciting discoveries are conceivable, Maine stresses. “For example, finding a version of the solar system – with small planets in the innermost part and large planets on the outer edge – would also be a find that really warms me up. Also with the aim of looking for life. Because it is believed that Jupiter was also very important for the balance on our planet.” And if that is the case, the emergence of life elsewhere may require not only an Earth-like planet, but even larger planets.” In addition, Maine also hopes that the huge gaps that have appeared in our theories about planet formation since the discovery of the first exoplanets can be gradually filled. “We had to fix our theories by discovering exoplanets and we still don’t understand everything.”

Many of the exoplanets discovered so far are located at great distances from Earth. But every now and then an exoplanet is discovered close to home. The closest is Proxima Centauri b, about 4 light-years away and featured in this artist’s impression. Photo: ESO/M. Kornmesser.

What doesn’t help, of course, is that so far we’ve detected only a fraction of the exoplanets in our Milky Way. In addition, due to the most common detection technology – the transit method – we also have to deal with a huge blind spot, which means that we can now easily detect large planets with a short orbital period. “Using the transit method, you’re looking for dips in the star’s light, due to a planet moving in front of the star.” To make sure that the dip in brightness occurs regularly and is caused by a planet, astronomers want to see that dip at least three times. With hot Jupiters orbiting their star in just a few days, this isn’t a problem; You only need to look at them briefly to see them move three times in front of the star. “But imagine a planet, like Earth, that takes a year to orbit its parent star, to prove its existence, you have to observe the light of its parent star for three years!” In addition, the transit method requires that the observer (for example, a space telescope), the planet to be detected and the parent star are roughly connected, otherwise the planet will not pass in front of the parent star as seen by the observer. “In this regard, the closer a planet is to its parent star, the more likely it is to see it moving in front of the star. The odds of seeing an Earth-like planet the size of our planet and further away from its parent star when our planet passes in front of its parent star is only 0.5 percent. This means that for every Earth-like planet we discover Right now, we’re losing about 200 planets.” The hope is that many of the planets we don’t see now will be discovered in the near future with the help of better, more patient satellites that stare longer. But even if we had these tools, it could be decades before we get a representative picture of the estimated 100 to 400 billion (!) exoplanets that live in our Milky Way.

At the moment, the counter stands at 4,905 officially discovered exoplanets. It leads Maine 30 years after the discovery of the first exoplanets to a conclusion that is both realistic and exciting. “There’s still a lot we haven’t seen.”