Thousands of new worlds have been added to the list since the discovery of two planets outside of the Solar System in 1992.

We've learned a lot from this large collection of alien worlds. There is a small detail that stands out. Our own Solar System is the only one like it out there.

The home star and its brood could be outliers in some way, according to some.

It's possible that the conditions that formed Earth and its veneer of self-replicating chemistry are difficult to replicate.

The outlook is bad if you just look at the numbers. By a large margin, the most numerous exoplanets we've identified to date are giants and subgiants of the gas and perhaps ice.

Most exoplanets are so close to their stars that their sizzling temperatures are much higher than the known habitability range.

Artist's impression of an ultra-hot exoplanet about to transit in front of its host star
Artist's impression of an ultra-hot Jupiter transiting its star. (ESO/M. Kornmesser)

We might see more places that remind us of our own backyard as the statistics balance out. The issue is more complicated than just looking at numbers. Our technology limits the scope of exoplanet science. Our imagination can limit our view of the true variety of alien worlds.

It's possible that what's out there in the universe is not what we see.

Expectations, and how to thwart them

From the very beginning, exoplanet science has subverted expectations.

When Jonti was a child, we only knew of one planetary system.

An implicit assumption was made that all planetary systems would be like this. You would have rocky planets near the star that were small, and gas giants a long way from the star that was large. That would be the way planetary systems would be.

It took scientists a while to find an exoplanet around a main sequence star. If other solar systems were like ours, the tell-tale signs of planets tugging on their stars would take a long time to see.

It didn't make sense to sift through a long history of observations for many stars to find out what happened to a fellow main-sequence solar system.

They found a gas giant half the mass of Jupiter that was so close to its host star that it took 4.2 days for it to warm up.

These 'Hot Jupiter' type planets are not strange at all. It seems like they are relatively common.

We know that there's a lot more variety in the universe than we know. It's important that we don't assume that what we can see is all that the Milky Way has to offer. Our own Solar System may be beyond our detection capabilities.

It's difficult to find things like the Solar System at the moment.

None of the surveys we have done so far have picked up the planets on the ground. Mercury, Venus, Earth and Mars are not likely to be found around the Sun.

How to find a planet

The methods used to detect exoplanets are very clever. The transit method is one of the workhorses of the exoplanet detection toolkit.

You need a telescope with sensitivity to the light of a star. Each is looking for a different signal.

If you want to use the transit method, you need a telescope that can keep a star fixed in its view for a long time. That's why instruments such as NASA's space-based Transiting Exoplanet Survey Satellite (TESS) is such a powerhouse, able to lock onto a segment of the sky for over 27 days without being interrupted by Earth's rotation.

Invading astronomy one exoplanet gif at a time! This time with a gif showing the transit method for detecting exoplanets ๐Ÿ˜Š

โ€” Alysa Obertas (parody) (@AstroAlysa) September 1, 2021

The aim of these kinds of telescopes is to spot the signal of a transit, when an exoplanet passes between us and its host star. You can imagine how small these dips are. There are many things that can dim a star's light, many of which are one-offs. The gold standard is multiple transits that show regular periodicity.

Smaller planets that are closer to their stars than Mercury is to the Sun are favored in the data.

In case you missed it, my gif showing how exoplanets are detected via the radial velocity method is now available in dark mode!

โ€” Alysa Obertas (parody) (@AstroAlysa) August 15, 2022

The wobble of a star is detected by the radial velocity method. A planetary system doesn't really circle a star, so much as dance around it. The barycenter is where the stars and planets are located. Due to the influence of Jupiter, that is a point very close to the surface of the Sun, or just outside it.

The shift in the star's position is an ongoing change that doesn't require constant monitoring. The motion of distant stars affects the light they emit.

As the star moves towards us, the waves of light coming in our direction are slightly smaller, towards the blue end of the spectrum. The star's light indicates the presence of a companion.

The data favors larger planets that exert a stronger influence on their star.

It is possible to image an exoplanet as it travels through the stars. It's not easy to do, but it may become more common in the JWST era.

This approach would uncover a different class of exoplanet than the short-orbit variety. To see an exoplanet without it being swamped by the glare of its parent star, the two bodies have to have a very wide separation. The direct approach favors planets that are long-lived.

Larger exoplanets would be spotted more easily through this method.

The discovery methods have their own biases.

He says it's difficult to find planets with a one year circle around the Sun because it sits between the extremes of different detection techniques.

What's out there?

The class that isn't represented in the solar system is the most numerous group of exoplanets. There are gas-enveloped exoplanets that are smaller than Neptune and bigger than Earth.

Rocky planet surrounded by purple haze and a star in the distance on the left
Illustration of the mini-Neptune TOI 560.01, orbiting its solitary star. (W. M. Keck Observatory/Adam Makarenko)

More than half of the confirmed exoplanets have a less than 20 day lifespan.

Our Sun is similar to most of the exoplanets we've discovered. Only a small amount of people are in multi-star systems. The stars in the Milky Way are part of a multi-star system with estimates as high as 80 percent.

For a second, think about that. Is it possible that exoplanets are more common around single stars than around multiple stars? The presence of more than one source of light can distort or obscure the signals we're trying to detect from exoplanets.

This brings us back to our solar system. Home seems strange in the context of everything we've found.

Bayliss says that there are some very common types of planets that are missing from the solar system.

We don't have anything like that because it looks a little bit like Earth. These mini-Neptunes are not available to us. I think it's fair to say that there are some very common planets that we don't see in our own solar system.

I don't think I'd go that far if that made our Solar System rare. There could be a lot of other stars that have planets that we don't know about.

An artist's illustration of many planets and stars in the Milky Way.
This artist's illustration gives an impression of how common planets are around the stars in the Milky Way. (ESO/M. Kornmesser)

On the brink of discovery

Thirty years ago, the first exoplanets were discovered around a star completely different to our own. The technology is no longer seen. Scientists can use better and better ways to find them now that they know what to look for.

We will be able to find smaller and smaller worlds as the technology improves.

It's possible that exoplanet science could discover thousands of worlds hidden from view. There are a lot of small things in astronomy.

There are red dwarf stars. They're up to half the mass of the Sun and are the most common type of star. They're so small and dim that we can't see them with the naked eye, yet they make up 75% of the stars in the universe.

We're operating with incomplete information because we can't see all of the worlds.

It is going to change.

If you look back at statements from the early 1990s, you'll see that mini-Neptunes are the most common kind of planet with a lot of skepticism.

I could be proven wrong. Science works this way. There are more things that are Earth-sized and smaller than there are things that are Neptune-sized.

Maybe we will find that our weird planetary system is not alone in the universe.