Exoplanets

"This space we declare to be infinite; since neither reason, convenience, possibility, sense-perception, nor nature assign to it any limit. In it there are an infinity of worlds of the same kind as our own."

- Giordano Bruno, 1584
 

For many thousands of years, we only knew of six planets, our closest neighbors: Mars, Venus, and Mercury, the two gas giants Jupiter and Saturn, and of course Earth. These are the only planets visible in the night sky with the naked eye. After the invention of telescopes, we could also see Uranus and Neptune further out in the Solar System. As they lie so far away, they were difficult to spot. Astronomers noticed that Uranus was not moving quite as expected, and calculated that the gravitational pull of an unseen planet must be responsible. When they finally pointed their telescopes at the predicted location, Neptune was right where the math said it would be.  

For a time, we also counted a ninth planet: Pluto, discovered in 1930. But in 2006, the International Astronomical Union officially reclassified Pluto as a dwarf planet, after the discovery of several similar icy bodies in the outer Solar System made it clear that Pluto was not unique, but rather one of many objects in a region now known as the Kuiper Belt.

Researchers have long searched for planets around stars other than the Sun, but if it can be so difficult to see the planets within our own Solar System, considering just how hard it is to find planets outside it! In 1992, they finally succeeded, and the first planet around another star was found, a so-called exoplanet, and since then many exoplanets have been discovered.

In fact, researchers estimate that almost every star has its own system of planets, and if you have looked up at the sky on a clear night, you might think that means there are quite a lot of planets out there. So far we know of more than 6,000, but we estimate there are more than 100 billion planets in our own galaxy alone, the Milky Way.

Strange worlds

Even though we had expected to find planets around other stars, researchers were still surprised when they found the first ones: Poltergeist and Phobetor. More than their presence, scientists around the world were surprised by how different they were from the planets we already knew. Researchers had expected to find planets resembling those in the Solar System, around stars resembling the Sun. But Poltergeist and Phobetor both orbit a so-called pulsar, which is a neutron star rotating incredibly fast so that its light pulses. A neutron star is a "dead" star, which is extremely different from the Sun.

It is the remnant of one of the most powerful explosions in our Universe, a Supernova (you can read more about neutron stars in the section on Supernovae). 

Even the planets themselves are a type we simply do not have in the Solar System. They are so-called "super-Earths," a middle size between our largest rocky planet, Earth, and our smallest gas planet, Neptune. These planets have since turned out to possibly be the most common planet type in the Milky Way.

The more exoplanets we discover, the more different they turn out to be. We have found "hot Jupiter" gas planets that lie so close to their star that they are several thousand degrees hot. Planets where it rains glass and gemstones. Planets whose rotation is locked to their star, so one side is in eternal day and the other in eternal night. Planets that are being torn apart and consumed by their star. Water worlds and "Hycean planets" where the entire outer layer consists of enormous oceans. Even planets entirely made of lava.

We have found planets around almost every different kind of star, moving in almost every kind of orbit. As puzzling as it feels, we are still yet to find a planetary system resembling the Solar System, with similar planets, orbits, or a central star like the Sun. 

This has baffled and slightly concerned researchers, as a strong driving motivation for exploring other planets in our Universe is to ask a simple and yet fundamental question: are we alone? When we search for life in the universe, we often look for systems and planets resembling our own, since that is the only kind of place we know that can support life. So why have we not found systems like the Solar System? Perhaps the Solar System is in fact a very unusual system, and it may turn out to be difficult to find systems and planets like ours. Perhaps it is also because we simply have not found them yet, as even the planets in the Solar System are actually difficult to see from a distance, and our methods may not be entirely designed to find them. However, one of our most exciting recent leads suggests we may not need to find an exact copy of our Solar System to answer that fundamental question. 

The TRAPPIST-1 system, discovered in 2016 and located about 40 light-years away (see The Solar System to learn how astronomical distances are defined), contains seven Earth-sized rocky planets, several of which orbit within their star's habitable zone, the region where liquid water could exist on the surface. That alone makes it one of the most remarkable systems we have ever found. Using the James Webb Space Telescope (see Telescopes and Observatories), scientists are now examining the atmospheres of these planets one by one, searching for the conditions that might support life. We do not yet have the answer, but for the first time in human history, we have the tools to actually look. You can read more about the hint for life in the Universe in Life in space .  

Watching Shadows and Dancing Stars

Even though Earth may seem large, and gas planets like Jupiter and Saturn are even larger, they are all tiny compared to the Sun. If we place Earth and the Sun side by side, the Sun would be approximately 109 times wider than Earth, and you could fit about 1.3 million Earths inside it. On top of that, it shines so powerfully that its light completely drowns out the faint reflected light of any planet beside it.

Earth is therefore very difficult to see. Trying to spot it next to the Sun would be a bit like trying to find a tiny pinhead next to a large, powerful lamp. The same problem applies to the search for exoplanets: they are much smaller and fainter than their host stars, making it nearly impossible to spot the planets themselves. For this reason, it is quite rare to be able to observe the planets directly. Luckily we have found a way around this problem! Instead of trying to spot them, we look at how the planets affect their stars. 

One of the techniques used to find the first exoplanets is called the radial velocity method, which involves how the star and planet pull on each other. Gravity works both ways: just as the star pulls the planet into orbit around it, the planet pulls back on the star with its own gravitational force. This means that while the planet moves in its large orbit around the star, the star itself is also being pulled along, wobbling back and forth in a tiny orbit of its own. This dance can be detected through the star's light, so even though you cannot see the planet itself, you can work out that it must be there because the star is dancing. The bigger and closer the planet, the bigger the wobble, and the easier it is to spot. This makes the radial velocity method best suited for large planets orbiting close to their stars.

Most planets detected so far were found using a different approach: the transit method, where we watch for the shadow a planet casts as it passes in front of its star.

When a planet passes in front of its star as seen from us, it blocks a small part of the star's light. So even though you cannot see the planet itself, you can see that some of the light from the star is missing, which tells us that a planet has probably passed by and cast a slight shadow. This method is especially powerful because it not only tells us something about the planet, but can also give a  lot of information about the planet's atmosphere. As the planet passes in front of its star, some of the starlight filters through its atmosphere before reaching us, carrying chemical fingerprints of what that atmosphere is made of. This is actually one of the key ways we search for signs of life in space! You can read more about this in our section on Life in Space.

Much like the radial velocity method, the transit method is also best suited to large planets orbiting close to their stars. And so both of our most powerful tools share the same blind spot. This makes things quite challenging as in the Solar System none of our large planets lie close to the Sun! In fact, all the planets in our Solar System orbit much further from the Sun than most of the exoplanets we have discovered orbit around their host stars. 

This might explain why we have not found planets like ours, as they are quite difficult to find with the methods we currently have. But as our telescopes improve, we are observing more exoplanets that are smaller and orbit further from their stars. The TRAPPIST-1 system, for instance, was discovered using the transit method, and its seven Earth-sized worlds would have been nearly impossible to detect with earlier technology. Another exciting example is K2-18b, a planet 124 light-years away whose atmosphere has been studied in detail by the James Webb Space Telescope. Using the James Webb Space Telescope, scientists have detected water, methane, and carbon dioxide in its atmosphere. In 2025, researchers even reported a possible detection of molecules that on Earth are only associated with living organisms. The finding is still being debated, but the fact that we can even ask that question from 124 light-years away says everything about how far our technology has come.

Already it is beginning to look as though Solar System-like planets are fortunately not as rare as we had feared, even though we have still not found the same combination of planets as we have in our own system. There are still so many worlds out there waiting to be discovered, and perhaps somewhere among them we will find the Solar System's twin.