Exoplanets

“We declare this space to be infinite; for neither reason, convenience, possibility, sense perception, nor nature sets a limit to it.
Within it, there is an infinity of worlds of the same kind as our own.”
- Giordano Bruno, 1584

For thousands of years, we only knew of six planets: our closest neighbours; Mars, Venus, and Mercury, and the two gas giants, Jupiter and Saturn. And, of course, Earth. These were the only planets visible to the naked eye in the night sky. After the invention of the telescope, Uranus and Neptune became visible farther out in the Solar System. Even with telescopes, they were difficult to spot because of their great distance. Neptune, in fact, was only discovered because its gravitational pull affected Uranus, allowing astronomers to calculate that another planet must exist there - long before Neptune could be seen directly.

Scientists have long searched for planets orbiting stars other than the Sun, but if spotting planets within our own Solar System is so challenging, imagine how difficult it is to find planets beyond it! Finally, in 1992, the first planet orbiting another star, a so-called exoplanet, was discovered, and since then many more exoplanets have been found.

In fact, astronomers now estimate that almost every star has its own system of planets. If you have looked up at the night sky on a clear night, you might begin to appreciate just how many planets there must be. So far, we know of more than 6,000 exoplanets, but it is estimated that there are over 100 billion planets in our own galaxy alone, the Milky Way.

Strange worlds

Although scientists had expected to find planets around other stars, they were still surprised when the first ones were discovered: Poltergeist and Phobetor. Not because the planets existed, but because they were so different from the planets we knew.

Researchers had anticipated finding planets similar to those in the Solar System, orbiting stars like the Sun. But Poltergeist and Phobetor both orbit what is called a pulsar, a neutron star that rotates incredibly quickly, giving the appearance of pulsing. A neutron star is a “dead” star, the remnant of a supernova (you can read more about neutron stars in the Supernovae section). In short, it is a star that is very, very different from the Sun.

The planets themselves are a type not found in our Solar System. They are so-called “super-Earths,” which are intermediate in size between our largest rocky planet, Earth, and our smallest gas planet, Neptune. These super-Earths have since turned out to possibly be the most common type of planet in the Milky Way.

The more exoplanets we discover, the greater the variety we see.

We have found “hot Jupiter” gas planets orbiting so close to their star that they reach temperatures of several thousand degrees. Planets where it rains glass and gemstones. Planets whose gravity is locked to their star, so one side is in eternal day while the other is in eternal night. Planets being torn apart and devoured by their star. Water worlds and “hycean planets,” where the outer layer consists entirely of vast oceans. And planets made completely of lava. We have found planets around almost every type of star, moving in almost every kind of orbit.

It almost feels as if the only thing we haven’t found is a planetary system like our own Solar System. We have not found a single system with a composition of planets resembling ours, orbiting in similar paths, around a star like the Sun. We haven’t even come close. This has puzzled and worried scientists somewhat, because when searching for life in the universe, we often look for systems and planets that resemble our own, as it is the only place we know can support life.

So why haven’t we found systems like the Solar System? Perhaps our Solar System is actually a very unusual system, making it difficult to find other systems and planets like ours.

But it may also simply be because we haven’t found them yet. Planets in the Solar System are actually difficult to see from a distance, and our methods may not be fully suited to detecting them.

Looking at shadows and dancing stars

Although Earth may seem large, and gas giants like Jupiter and Saturn even larger, they are all tiny compared to the Sun. If we place Earth and the Sun side by side, the Sun would be about 12,000 times larger than Earth and shine roughly a billion times brighter.

So spotting Earth next to the Sun is a bit like trying to see the head of a pin next to a powerful lamp. In other words, Earth is extremely difficult to see.

It’s the same problem faced when searching for exoplanets. They are much smaller than their stars, and the star’s light is so bright that it can be nearly impossible to spot the planets directly. As a result, astronomers rarely observe the planets themselves; instead, they look at how the planets affect their stars.

The method used to find the first exoplanets is called the radial velocity method, which observes how the star and planet pull on each other. Planets orbit their star because the star’s gravity pulls on them, but the planet’s gravity also pulls on the star. This means that while the planet moves along its large orbit around the star, the star itself moves in a tiny orbit, rocking slightly back and forth. This “dance” can be detected from the star’s light, so even if the planet cannot be seen directly, its presence can be inferred because the star is moving. The larger the planet and the closer it is to the star, the more it pulls on the star, and the more pronounced the star’s motion. The radial velocity method is therefore particularly effective for detecting very large planets that orbit close to their stars.

The method that has discovered the most planets so far is the transit method, which observes the shadow cast when a planet passes in front of its star. When a planet moves in front of its star from our perspective, it blocks a small portion of the star’s light. Even if the planet itself cannot be seen, the drop in light reveals that something has passed in front of the star.

This method is particularly clever because it can tell us not only about the planet itself but also about its atmosphere. When the planet is between us and its star, some of the star’s light passes through the planet’s atmosphere before reaching us. By analysing this light, astronomers can learn a great deal about the atmosphere’s composition and structure. This is, in fact, one of the methods used to search for life in space, which you can read more about in our section Life in Space.

The larger the planet, the more light it blocks, and the closer it is to its star, the more frequently we can observe its shadow. For this reason, the transit method is most effective for detecting large planets that orbit close to their stars.

This means that the two most commonly used methods for studying exoplanets are both best suited to finding large planets that orbit very close to their stars. But in our Solar System, none of the large planets lie close to the Sun. In fact, all of our planets are relatively far from the Sun compared with many of the exoplanets we have discovered. So the reason we haven’t yet found planets like ours may simply be that they are difficult to detect with the methods we currently use.

As our telescopes improve, we are discovering more exoplanets that are smaller and orbit farther from their stars. Already, it seems that the planets in the Solar System are fortunately not as rare as we once feared, even though we still haven’t found a system with the same composition of planets as ours.

There are still so many planets left to discover out there, and perhaps among all these wondrous worlds, we will one day find a twin of the Solar System.