Thirty years ago, we knew of no planets orbiting other stars. Not a single confirmed one. Today the tally has passed 6,000, and it is still climbing week by week.

That is a striking jump on its own, but the number is not the most interesting part of the story. The surprise is what all these worlds turned out to be like, because the most common kind of planet in the galaxy is one our own solar system does not contain.

I am a writer with a long interest in this beat, not an astronomer, so treat this as a careful reading of the field rather than expert testimony.

From zero to thousands

The modern era began in 1995, when Michel Mayor and Didier Queloz confirmed a planet orbiting the Sun-like star 51 Pegasi, work that won them a share of the 2019 Nobel Prize in Physics. The planet itself was a shock: a gas giant whipping around its star in four days, far hotter and closer in than anything our own system would have led us to expect.

Then came the flood. NASA’s Kepler space telescope, staring at one patch of sky, found thousands of candidates, and the confirmed count has since grown to more than 6,000 worlds. The great majority were found by the transit method, watching for the tiny, repeated dip in a star’s brightness as a planet crosses in front of it.

In three decades, planets went from a thing we assumed were out there to a thing we can count.

The planet type we do not have

Here is the finding that genuinely reordered expectations. Among the most common planets in the galaxy are super-Earths and sub-Neptunes, worlds larger than Earth but smaller than Neptune. They turn up around star after star.

Our solar system has nothing in that range at all. There is a large gap between Earth, at one Earth-radius, and Neptune, at nearly four, and nothing of ours sits in it. The single most abundant category of planet out there is one we have no local example of, which means for the commonest planets in the galaxy we are working entirely from other stars’ systems.

There is even structure inside that population. Astronomers have found a relative scarcity of planets at around 1.5 to 2 times Earth’s radius, a feature nicknamed the radius valley, which seems to divide rocky super-Earths from gas-wrapped sub-Neptunes. We did not predict any of this. We inferred it only once the data forced us to.

Finding the ones like Earth is the hard part

It would be easy to read 6,000 planets as 6,000 possible Earths, and that is where care is needed.

The methods we use are biased towards the planets that are easiest to see. A big planet close to its star blocks more light and tugs on it harder, so the early catalogue filled up with hot giants and short orbits, not because those dominate the galaxy but because they shout the loudest. Small, temperate, rocky planets on wide orbits, the ones most like home, sit right at the edge of what we can currently detect.

Even the idea of a habitable zone is more slippery than it sounds. I have written about how icy moons complicate the neat picture of a habitable ring around a star, since worlds far outside it may hold liquid water underneath their ice. And the recent excitement about possible chemical signs of life in exoplanet atmospheres, such as the debated detections around the planet K2-18b, remains contested and unconfirmed. A candidate signal is not a discovery.

What the count actually changed

Step back from any single planet and the real shift is statistical. Before 1995 we had exactly one planetary system to reason from, our own, and no way to know if it was typical. Now we know it is not the only pattern, and in some respects not even a common one.

The firmest conclusion from all this is simply that planets are the rule rather than the exception. The data from Kepler and its successors indicates that most stars host planets of some kind, which means our galaxy contains planets in the hundreds of billions. That is a genuine change in how full the universe looks, quite apart from whether any of those worlds hold life.

It also means our own solar system, once the only template we had, now looks like one arrangement among many, and a slightly unusual one at that.

What to watch

The frontier has moved from counting planets to reading them. The James Webb Space Telescope is beginning to probe the atmospheres of some of the more accessible worlds, and NASA’s Roman telescope, due to launch shortly, will run a microlensing survey expected to add large numbers more.

The count will keep rising, but the harder prize is a small, rocky planet at the right distance from a quiet star, with an atmosphere we can actually study. Three decades in, we have proven the galaxy is full of planets. Whether any of them resemble ours, in the ways that matter, is still an open question.