Have you ever looked up at the night sky and wondered… is there another world out there like ours? A planet with oceans, skies, maybe even something breathing under alien clouds?
That question sits right at the heart of ESA’s PLAnetary Transits and Oscillations of stars mission, or Plato for short. Its goal is to explore a surprisingly simple question to ask, yet incredibly difficult to answer: does a second Earth exist in the Universe?
What makes an “Earth-like” planet?
Before we can hunt for another Earth, we need to know what we’re looking for. There are lots of planets beyond our Solar System, but most are places you wouldn’t want to visit. Boiling hot gas giants, frozen rocks drifting far from their stars – not good holiday destinations! Earth is different, and one of the reasons for this is where its orbit is located around the Sun.
Astronomers call the region where Earth orbits as being within the Sun’s habitable zone, sometimes nicknamed the “Goldilocks zone” because everything has to be just right. Not too hot, not too cold, but just enough warmth for liquid water to exist on the surface. Most planets out there in the depths of space aren’t in their stars’ Goldilocks zones.
But location alone isn’t enough. To be truly Earth-like, a planet also needs a rocky surface, a similar size and mass to Earth, and a stable star to orbit, among other criteria. Plato is designed to find planets that tick all of these boxes, especially those orbiting stars similar to our Sun.
Meet Plato: the multi-eyed space watcher
Plato isn’t a typical telescope. For a start it’s in space, meaning it can achieve crisp views, unblurred by Earth’s atmosphere.
And then there are the cameras: instead of one, Plato carries 26, all working in unison. This means Plato can observe hundreds of thousands of stars over at least two years’ time, maximising chances of success.
But how does Plato actually find planets? There are a few planet-hunting techniques, but Plato’s main method is searching for transits.
Imagine watching a distant star. It shines steadily, like a cosmic lighthouse. Now imagine a planet passing in front of it. For a brief moment the planet blocks a tiny bit of light, and the star dims ever so slightly.
Plato watches for these tiny dips in star brightness. If the dimming happens regularly, it’s a strong clue that a planet is orbiting the star. This is a huge challenge though, because the change in brightness can be much less than 1%, and must be observed several times to confirm it’s caused by a planet.
Beyond discovery: weighing and understanding worlds
Finding a planet is only the first step. Plato is designed to go further and help scientists understand what these worlds are like. Plato finds planets by measuring how much a star’s light becomes slightly dimmer when a planet passes in front of it. From this, scientists can learn the planet’s size. By seeing how often the dimming happens, they can also find out how long the planet takes to go around its star.
With help from other instruments, scientists can also determine a planet’s mass, radius, and density. Plato focuses on bright stars, because this makes it possible to measure the masses of small planets — something that was difficult for earlier space missions studying exoplanets.
Plato uses its 26 cameras to observe many bright stars at the same time and can see a very large part of the sky — about as large as 10,000 full moons. This wide view allows it to collect a huge amount of information at once. With help from other telescopes on Earth, scientists can then calculate a planet’s mass, size, and density, which tells them what the planet is made of.
Low-density planets are probably made of gas, like Jupiter in our own Solar System. Finding a high-density planet is particularly exciting, because that’s likely made of rock, which is one of the conditions for being Earth-like.
So, from what seems like a little bit of information, Plato can follow the clues like a cosmic detective. Eventually it will build the first large catalogue of planets with known properties like composition, structure, and even age, which will help scientists understand how planetary systems evolve over time.
The secret music of stars
Not content to just study planets, Plato can also analyse the stars themselves! Using a technique called asteroseismology, scientists can detect tiny variations in a star’s brightness caused by internal vibrations. In a way, stars have their own subtle “music”.
By analysing these oscillations, that are movements that keep going back and forth, scientists can determine a star’s mass, size, and age with remarkable precision. This is crucial, because a planet’s properties are closely tied to its star. Without understanding the star, it’s impossible to fully understand the planet.
A story of origins and evolution
Planets do not appear fully formed. They grow from dust and gas swirling around young stars, slowly building into worlds over millions of years. Understanding this process helps us understand how rare, or common, Earth-like planets might be.
While Plato’s main focus is searching for Earth-like planets, as it surveys the sky it is expected to find new planetary systems with thousands of exoplanets of all sizes, exomoons (moons orbiting other planets) and even exorings (like Saturn’s rings, but around distant worlds). Each discovery adds another piece to the puzzle of how planetary systems form, change over time, and why some become habitable.
The big question
After all the cameras, calculations, and careful observations, everything circles back to that original question: does a second Earth exist in the Universe? Or is our planet truly unique?
Plato won’t answer that question overnight, but it will set us on the path by giving us something we’ve never had before: a detailed, reliable map of exoplanets and their properties.
Somewhere in that map there may be a small, rocky world, orbiting quietly in the right place, just waiting to be noticed.
