Nothing to do, planet Earth is blue

Though of course, those are only the planets in our own solar system, meaning the eight planets that orbit around our Sun. But our Sun is just another somewhat run-of-the-mill star among billions of others in our Milky Way galaxy alone. And the Milky Way is just another somewhat run-of-the-mill galaxy among billions of others in our universe. So, while we may not have heard of any planet other than these eight, it might strike you as unlikely and implausible that the Sun is the only star in all those unimaginable billions that has planets around it.

If that’s true about you, you’re not alone. Many others have also thought it unlikely and implausible. In particular, plenty of astronomers. This is why the search for exoplanets—planets outside our solar system, orbiting some other star —has been a serious astronomical pursuit for decades. The result is that that answer is no longer “eight".

Before I tell you what the number is today, spare a thought for how we find exoplanets. There are different techniques, but perhaps the most common—meaning the method that has found us the most exoplanets so far—is known as “transit photometry".

In essence, this is a very simple idea. Think of a solar eclipse. It happens because to us on Earth, the moon appears to travel across the face of the Sun. As it does, it blocks some of the sun’s light—sometimes all—from reaching us. Analogously, any given star we choose to observe has a particular brightness—but what if that brightness dims? One explanation for that is that an object between us and the star has appeared to travel across—“transit"—the face of the star. And what if the star dims and returns to its usual brightness regularly? Well, that object might be a planet, orbiting the star, transiting across it regularly.

To understand this, imagine an astronomer in some other corner of the universe who trains her telescope on our Sun and finds that its light dims once a year. She has probably detected the existence of our own planet, Earth.

So, the search for exoplanets using this transit method is a search for stars whose brightness fluctuates regularly. Admittedly, there are possible planets which this method will miss. For example, if the plane of a planet’s orbit around the star is perpendicular to our line of sight, we will never see it transit the star. Or the planet might be so small in comparison to the star, or so far from it, that its transit doesn’t lower the star’s brightness quite enough for us to detect, here on Earth.

In any case, here’s the score. We now know of over 5,000 exoplanets. Of those, nearly 4,000 were found using transit photometry.

Those are pretty remarkable numbers. That’s because even though astronomers have long suspected the existence of exoplanets, and the sheer number of stars out there suggests such existence as well, the first confirmed exoplanet was discovered only as recently as 1992. Or put it this way: until 1992, we knew our universe to contain only the eight planets that sail around our Sun, forming our solar system. But in just over 30 years, we have confirmed the existence of 5,000+ exoplanets. Even that number will almost certainly increase. Especially with the new James Webb Space Telescope now operational, we can expect many more exoplanet confirmations.

The ones we have found so far fall in four broad categories. There are “gas giants" (1636 such), composed largely of helium and hydrogen like Jupiter and Saturn, but usually much larger than those giants. Many have very tight orbits, close to their home stars; in some cases, the orbit lasts—and thus their “year" is—only 18 hours. There are “Neptunians" (1,834), also made of helium and hydrogen with a rocky and metallic core. They are about the same size as our Neptune and Uranus. There are “super-Earths" (1,602), larger than Earth but smaller than Neptune. We don’t know enough about their composition yet—they might be largely water or snow, or gaseous, or a mixture of rock and gas. Finally, there are “terrestrials" (195). These are essentially rocky worlds, like Mercury, Venus, Earth and Mars are. They vary from about half to twice the Earth’s size.

Some terrestrials are in the so-called “habitable zone". That is, they are far enough, but not too far, from their home stars so that water might exist on them in liquid form. We can’t (yet) confirm the presence of water, but their position relative to their stars at least suggests that these planets are potentially habitable.

That last suggestion is, of course, what underpins our abiding interest in exoplanets: mankind’s yearning to answer the question “are we alone?" That is, is there life of some kind elsewhere in the universe? If so, it will almost certainly be on an exoplanet. Now that we have discovered so many of them—and that we know even those are just a tiny fraction of the billions of them that probably exist—many astronomers believe detecting life on an exoplanet somewhere out there is only a matter of time.

This is not to say that we will necessarily find life as we know it—giraffes and Venus fly-traps, Donald Trump and the coronavirus. It may be utterly primitive. It may even be just the chemical building blocks that we know went into creating life on Earth. You could say we are looking for “biosignatures"—signs in the chemical make-up of an exoplanet that suggest life is possible. In searching exoplanets for these biosignatures, though, we also seek to answer a fundamental question about ourselves. Was life on Earth inevitable, meaning it is the result of the way the universe has evolved? Or are all of us just the manifestation of one glorious, fantastic, never-again accident?

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.