We have exactly one world, in all the Universe, that we know for a fact to be hospitable to life: ours.
So when we're looking for habitable planets in other planetary systems, beyond our own corner of the galaxy, we often use Earth as the perfect template.
But a new study has revealed Earth isn't as habitable as it could be. In fact, it could be even more livable, if Jupiter's orbit shifted slightly.
It's an important study because there are many moving parts and ingredients in the Solar System, and figuring out which ones contribute to Earth's habitability is extremely tricky.
It could also help us better understand what makes a habitable world habitable.
"If Jupiter's position remained the same, but the shape of its orbit changed, it could actually increase this planet's habitability," says planetary scientist Pam Vervoort of the University of California, Riverside.
"Many are convinced that Earth is the epitome of a habitable planet and that any change in Jupiter's orbit, being the massive planet it is, could only be bad for Earth. We show that both assumptions are wrong."
The results also have implications for the search for habitable worlds outside the Solar System, by providing a new set of parameters by which potential habitability can be assessed.
Although we don't currently have any tools that can conclusively gauge the habitability of an exoplanet – planets that orbit stars outside our Solar System – scientists have been collecting a population of worlds at which we ought to take a closer look, based on several characteristics.
The first is where the exoplanet is in relation to its host star – it needs to be at a distance not so close that any surface liquid water would evaporate, nor so distant that the water would freeze.
Increasingly, it has seemed that a Jupiter-like gas giant in the same system might be a good indicator for habitability. But there seem to be some caveats.
In 2019, the international team of researchers published a study in which they showed, based on simulations, that altering the orbit of Jupiter could very quickly make the entire Solar System unstable.
Now more simulations have shown that the opposite can be true, which will help narrow down the range of gas giant orbits that help or hinder habitability.
The study was based on the eccentricity of Jupiter's orbit – the degree to which that orbit is elongated and elliptical.
Currently, Jupiter has only a very slightly elliptical orbit; it's almost circular.
However, if that orbit gets stretched, it has a very noticeable effect on the rest of the Solar System. That's because Jupiter is massive, 2.5 times the mass of all the rest of the Solar System's planets combined.
So, tweak Jupiter's eccentricity, and the gravitational effect it will have on the other planets is real.
For Earth, that also means an increase in eccentricity. That means, the researchers found, that some parts of the planet would get closer to the Sun, warming up into a temperate and habitable range.
But if you move Jupiter closer to the Sun, Earth's habitability suffers. That's because it will cause our home planet to tilt more sharply on its rotational axis than it does currently, a feature that gives us seasonal variations.
A sharper tilt, however, would cause large sections of our planet to freeze, with more extreme seasons. Winter sea ice would extend to an area four times greater than it currently does.
These results can be applied to any multi-planet systems we find, to assess them for potential habitability, the researchers said.
But they also highlight just how many factors may have influenced our presence here on our pale blue dot – how very nearly we may have never existed, perhaps. And what could happen to the Solar System if it ever destabilizes.
"Having water on its surface [is] a very simple first metric, and it doesn't account for the shape of a planet's orbit, or seasonal variations a planet might experience," says astrophysicist Stephen Kane of the University of California, Riverside.
"It's important to understand the impact that Jupiter has had on Earth's climate through time, how its effect on our orbit has changed us in the past, and how it might change us once again in the future."
The research has been published in The Astronomical Journal.