Most people know planets are objects that go around a star, just like Earth goes around the Sun.
But what about a planet without a star?
Rogue planets are objects that have mass similar to planets but are not orbiting stars.
They’re free-floating planets, and there are lots of them.
The Euclid spacecraft has been tracking them down, finding previously unknown examples and confirming the existence of previously discovered ones.
Christopher Conselice is Legacy Science Lead for ESA’s Euclid telescope and professor of extragalactic astronomy at the University of Manchester.
We spoke to him to find out more about rogue planets and how Euclid is tracking them.
See Euclid's first images and Euclid's first science images.
Why use Euclid to detect rogue exoplanets?
ESA’s Euclid is a space telescope like James Webb Space Telescope or Hubble, but it’s unique because it covers a huge fraction of the sky – one third – and it’s doing so at very great depths.
Euclid can image an area of the sky 9,000 times larger than is possible with Hubble at the same resolution.
With Euclid’s deeper measurements, we hope to better understand the population of these rogue planets throughout our Galaxy.
It’s a bit harder to do this in other galaxies, because rogue planets are so faint.
How do you detect rogue planets?
We had a good idea that many were in the Orion Nebula, where you can detect faint points of light.
If you have an idea of the distance to an object and you know how bright it appears, you can work out how bright it intrinsically is, which tells you its mass.
We measured the mass of these faint objects and found that they had to be planets. They aren’t massive or bright enough to be stars.
Stars emit more light because they’re undergoing nuclear fusion in their cores.
However, young planets will emit some radiation, just like Earth did when it was very young and hot.
Do rogue planets typically look a certain way?
One of the questions we’re trying to answer is, what kind of distribution of masses do rogue planets have?
For planets and stars, the more massive you go, the fewer objects exist. At lower masses, there should be more objects.
You can see this in our Solar System: you only have one massive Jupiter, but many more less massive planets like Venus, Earth and Mars.
One of the things that our studies show is that rogue planets follow the same trend, down to planets with masses around four times that of Jupiter.
What are rogue planets like?
The rogue planets we can see are all very massive, much more massive than Jupiter, because of how deeply we can observe.
Maybe we would see lower-mass rogue planets if we could observe more deeply, as these planets would be much fainter than those we have currently detected.
We don’t know yet if you need a star to form certain types of planets.
So rogue planets are an interesting test for understanding planet formation and whether you need, for example, an asteroid belt to form rockier planets.
We see some hints that there are Earth-like planets in between different stars, but more work needs to go into detecting them.
Could life exist on rogue planets?
It’s unlikely because you don’t have a star to heat up the planets and make them hospitable; we think that places that have induced life need warmth.
You certainly wouldn’t have liquid water on these planets, because they’re so cold.
At the same time, you don’t have the threat of destruction of a star, or supernova, that could destroy life.
If there were life, it would have to be quite simple and would have to be able to survive in a very cold environment.
We don’t know of anything that can do that very effectively at these temperatures.
What’s next for your research?
The obvious thing to do is to look deeper for rogue planets.
Past a certain depth of observation, you start getting contamination from background stars, maybe even unresolved galaxies, so we have to be careful with how these observations are made.
Euclid’s large viewing area helps us get better statistics, which will help us with this problem.
This interview appeared in the August 2024 issue of BBC Sky at Night Magazine.
