WORLDS WITHOUT SUNS
Astronomers are still on the hunt for rogue planets, but what can they tell us about the layout of our cosmos?
When we think about planets, a star usually comes as part of the package. After all, where would Earth be without the Sun? Imagine Luke Skywalker’s home planet of Tatooine without its twin suns, or Superman without Rao, the red supergiant around which Krypton spun. Yet you’d be wrong to imagine that planets are never starless. Astronomers have already found swarms of rogue planets – worlds wandering the open chasm of space alone.
As far as we know, planets always start off with stars. They are the leftover fragments from star formation, the offcuts that you’d throw away in a skip if you were building a house. A cloud of gas and dust that was otherwise minding its own business may begin to contract if it’s hit by a shock wave from an exploding star. As the cloud compresses, gravity takes over and bundles it into an ever-smaller space until fledgling stars ignite inside.
The shrinking cloud begins to spin faster and faster, flinging leftover gas and dust into discs around new stars. Gravity continues its work here, too. Dust grains merge into lumps the size of golf balls, which continue to snowball until they reach a kilometre or so across.
Astronomers call these planetary building blocks planetesimals. Close to the star they are made of metals – the only materials with high enough melting points to stay solid in the face of the inferno. Further out – beyond an imaginary boundary called the ice line – temperatures plummet below freezing and the planetesimals take the form of frigid lumps of water, ammonia and methane ices. That’s why we have rocky planets close to the Sun and ice giants the furthest out.
Looking at the Solar System today, it seems a somewhat serene place. Yet this relative calmness belies the cataclysmic pinball machine that it used to be. Jupiter formed further from the Sun before migrating inwards, scattering many of the asteroids like a flock of pigeons. Uranus and Neptune moved outwards, and may
have even swapped order. At least that’s according to the Nice model, which is named after the city in France where it was devised. It says that the Solar System started off in a different configuration before evolving into the set-up we see today.
Crucially, when planetary scientists run computer simulations of the Solar System’s evolution, they recreate the modern planetary set-up more often when they include a fifth giant planet from the outset. There’s more evidence for extra planets, too. We already think, for example, that a Mars-sized planet called Theia careened into the Earth just after our planet formed. The resulting surge of debris into space eventually coalesced to form the Moon.
But if there really was a fifth giant planet patrolling the Solar System where did it go?
After all, there are clearly only four giant planets now. Astronomers conveniently say that it was discarded in a gravitational tussle with its siblings, or perhaps picked off by the gravitational pull of a star that buzzed close by. Maybe it ended up marooned in the darkest depths of the Solar System. That would certainly fit with recent ideas about a so-called Planet Nine lurking far beyond Pluto, the dwarf planet that we used to regard as the ninth planet.
Yet it could have been ejected from the Solar System entirely. It’s a notion that once may have seemed far-fetched, but is gaining significant ground. After all, we’ve recently seen jettisoned visitors from other solar systems in the form of ‘Oumuamua and Comet Borisov.
While it would take considerably more force to orphan an entire planet, the fact that we seem to have lost one from our own Solar System bolsters the argument. Stars could also lose their gravitational grip on outer planets as they die.
The Sun will one day become a red giant and lose mass as it sheds its outer layers. This could cut the invisible tether to Neptune, irretrievably sending it out into the universe like a child accidentally letting go of a helium balloon.
It’s therefore no surprise that there are vagrant planets trekking through the void. Though given their relatively small size, considerable distance from us and the fact they are not illuminated by a star, it’s a wonder we’ve been able to spot them at all. It’s only possible thanks to a clever technique called gravitational microlensing.
We often think of gravity as a pull – that the
Sun pulls on the Earth, and that’s why we orbit around it. It’s certainly the way Isaac Newton thought about gravity. Yet over a century ago Albert Einstein replaced this simple notion with the idea that gravity is the result of curved space. A heavy object like the Sun warps the fabric of space around it, creating a divot in the universe that astronomers call a gravitational well. Earth is caught in this cosmic trap, which is why we’re stuck in orbit around the Sun.
Any light travelling across the universe will be forced to alter its path if it encounters the curved space around a local heavy object. Rays of light are bent around it, just as a lens in a telescope bends light. The result is the same: the light gets magnified. If an otherwise-invisible rogue planet passes in front of a distant light source, we can tell it’s there by the way it temporarily amplifies that background light.
The technique does have its drawbacks, however. The alignment between the rogue planet – the lens – and the background light source is always temporary and fleeting. It typically lasts between a few hours and a few days. After that we’re never going to see it again, and so we only have that single measurement to go on. Fortunately, the duration of the event does tell us something important: mass. The lighter the lens, the shorter the magnification spike lasts.
It’s been over ten years since Japanese scientists used the Optical Gravitational Lensing Experiment (OGLE) to find rogue planets. They observed 50 million stars in the Milky Way and found just 474 microlensing events. Only ten turned out to be consistent with objects with planetary masses.
But just because an object has the same mass as a planet doesn’t make it a planet – it could be a miniature black hole with the same mass. More plausibly it could be a failed star called a brown dwarf. They start off with a mass considerably higher than Jupiter’s, but could cool and shrink.
Once you get down to objects with masses more similar to Earth’s, it’s harder to argue that they aren’t rogue planets. In September 2020, for example, astronomers announced the discovery of the smallest ever rogue planet candidate – one with a mass roughly equal to Earth.
We may even have found rogue planets in other galaxies. In 2018 astronomers at the University of Oklahoma looked at a galaxy that was lensing light from the distant quasar RX J1131-1231. Irregularities suggested the presence of an enormous number of objects with masses between the Moon’s and Jupiter’s. The total number of such objects in that galaxy could top 1 trillion. While some could be black holes or brown dwarfs, it’s likely that a significant proportion are rogue planets.
Even in our own galaxy there could be as many as 50 billion starless planets, according to a 2019 study that looked into how frequently worlds get kicked out of their home solar systems. That’s why finding rogue planets is about more than just collecting cosmic curiosities. Working out the
ratio of vagabond worlds to stars in the Milky Way is an important part of understanding how solar systems form and evolve.
Soon there will be a step change in our ability to complete this celestial survey. In 2025, NASA plans to launch the Nancy Grace Roman Space Telescope. When Roman reaches space, it will carry with it a mirror 2.4 metres (7.9 feet) in diameter, equal in size to the Hubble Space Telescope’s. However, it will have a field of view much greater than its cousin, allowing it to observe an area one hundred times wider than Hubble can.
Roman is designed for many purposes, including helping to decipher the twin puzzles of dark matter and dark energy – the mysterious entities that bind a galaxy together and push galaxies apart respectively. Yet it will also be able to use microlensing to search for rogue planets – or freefloating planetary-mass objects, to give them their full name. Roman is sensitive enough to discover rogue planets that range in mass from Mars, at ten per cent of Earth’s mass, all the way up to
100 Earth masses. In doing so it will provide an estimate of the number of rogue planets in the Milky Way that is ten times more accurate than the data we currently have.
Only then will we have the tools to understand how our Solar System came to look the way it does, telling us something important about what kinds of planetary systems to search for if we want to find other life-supporting planets in this vast and complicated universe.
“Working out the ratio of vagabond worlds to stars in the Milky Way is an important part of understanding how solar systems form”