A Look at the Next Eight Billion Years in Our Solar System

Turn your attention to the nearby sky: to the 8 planets, 288 moons, and countless smaller rocky objects whose precarious existence began with, depends upon, and may end with the Sun. Last month, we covered the future of life on planet Earth. This month, we’ll take a look at what’s in store for our little corner of the Milky Way.

Astronomers researching protoplanetary disks around low-mass stars are piecing together an increasingly detailed portrait of how stellar systems like ours formed. Interstellar space is filled with clouds of gas and dust, and one of these clouds eventually collapsed into our solar system. The bulk of the matter formed the Sun. The rest, spread out into a disk as the young Sun rotated, eventually coalesced into the many objects that make up our solar system.

That’s old news. In the same way that looking at baby stellar systems tells us where we came from, looking at dying ones tells us where we’re going. So here are scientists’ best predictions for the future of our solar system.

Planum Boreum, Mars, 36,000 years from now

It’s 36,000 years in the future, and there’s something new in the sky. Human eyes can’t quite see it. It twinkles just beyond the boundary of naked-eye vision. But for any hand-held amateur telescopes set up at the Martian north pole, the little red star Ross 248 flickers like an ember.

Or at least it does between breaks in the clouds. In the morning, those clouds are mostly ice crystals, dissolving into a humid mist in the afternoon. For the last 10,000 years, the clouds on Mars have been thicker, and atmospheric humidity has been higher than in the 50,000 years before.

That’s because the temperature in Mars’ northern polar desert, Planum Boreum, is about 10˚C higher than in the 21st century. Climate change has come for Mars’ glaciers; it is not manmade but brought on by the wobble in the axis of Mars’ rotation. Planum Boreum gets more sunlight now, and its melting ice caps have supercharged the atmosphere with humidity.

But Ross 248’s changing brightness isn’t just because of the waves of mist wafting in front of it. The surface of the star itself writhes in storms, and sunspots often dim it. Now, anything in the Solar System has a front-row view of its stellar activity: Ross 248 has supplanted Proxima Centauri as the closest star in the sky. It now sits a mere three light-years away. If any planets orbited Ross 248, and anything on them could communicate, their radio transmissions would receive a response from us in only six years.

However, according to all available evidence, Ross 248 has no planets.

Cresdemona, Uranus orbital system, 1 million years from now

Cresdemona is a new moon. Irregular and patchy, it orbits Uranus at about 50,000km. Most of its surface is water ice, but there’s enough mixed-in rock that when its parent moons, Cressida and Desdemona, collided, they didn’t shatter but combined into the nascent moonlet Cresdemona, probably no more than tens of kilometers across, encircled by shards of ice.

Meanwhile, another one of Uranus’s 28 moons, Juliet, carbon-rich and petite, tries in vain to escape the new moonlet. Soon, the two moons’ paths will cross, slamming heavy carbonaceous rocks into the icy surface of Cresdemona.

The resulting moon, Cressida-Desdemona-Juliet, will hurtle off into the future.

Neptune, 3.6 billion years from now

Over on Neptune, the moon Triton has strayed into more dangerous territory than just the path of its siblings. Its orbit has decayed past the planet’s Roche limit. Beyond this point, Neptune’s gravity is so immense and Triton sufficiently large that the difference in gravitational force on the near and far parts of the moon literally tears it apart.

The tidal forces ripping into Triton also heat it, and its icy surface sends out trails of steam that demarcate its descent toward the ocean world below. Then there’s the core. Neptune wrenches chunks of rock off the surface of Triton piece by piece and flings them into orbit, where they eventually coalesce into a ring.

A few of them careen down toward Neptune. They pass through the strongest winds in the solar system, buffeted along at 2,000kph. When they finally make it through Neptune’s atmosphere, it’s possible they hit molten oceans of water and methane, kept liquid by the unimaginable pressure above. If so, the remnants of Triton sink without a trace.

Atacama Desert, the Earth, 5 billion years from now

The Milky Way is no more. The Andromeda galaxy has collided with the Milky Way. Where familiar stars once stood, alien worlds now throng.

Over the last billion years, the force of the colliding galaxies condensed interstellar gas and dust at breakneck speed. New stars formed, lighting up the sky with red flecks like burning coal. Now, 5 billion years after the 21st century, young stars are still visible in some parts of the sky. For the most part, however, a monotonous smog stretches across the horizon.

Smog, perhaps, implies gas. That’s not the case for Milkomeda. What looks like a fine morning mist is actually made up of an uncountable number of stars, so far-reaching and dense that only a rare few individuals poke out of the crowd. The space in between has very little gas and dust left. All of it was used to form the last generation of stars.

For anything living in our Solar System, the sky is no longer a place of infinite galaxies just like us. We cannot see anything past the dense gauze of our own galaxy’s stars.

Somewhere in the nearby galaxy, 5.4 billion years from now

An unremarkable little star has just become remarkable. For billions of years, Sol has burned steadily, visible with the naked human eye up to 56 light-years away. It was faint and yellow. Now, practically overnight in astronomical terms, it has lit up like a strobe. It glows about 100-1,000 times brighter than before and has a deep red color.

That’s because the Sun has finally left the Main Sequence, that part of a star’s lifetime where its structure is supported by fusing hydrogen into helium deep within its core. Now it’s run out of hydrogen. With nothing to push back against gravity, its core begins to collapse.

Suddenly, two new sources of energy become accessible. Within the core, helium reserves wait to be fused into carbon and oxygen. Outside the core, there’s still plenty of hydrogen.

Neither energy source used to be accessible for burning. Before, the Sun’s gravity was only strong enough to fuse hydrogen in the very core. As soon as it reached the burning point of hydrogen, it stopped collapsing. It didn’t reach temperatures high enough to burn either helium in the core or hydrogen in the outer layers.

Now, as the core collapses on itself, it keeps collapsing until temperatures rocket high enough to burn helium. They also reach a shell of hydrogen in the outer layers of the star. Buoyed by this new source of energy much closer to the surface, the Sun expands. Its luminosity rockets up, and suddenly it’s visible to the naked eye within 500 light-years.

Titan, 7 billion years from now

One billion years ago, the Sun’s dramatic increase in brightness raised temperatures on the surface of Mars to those of the 21st century Earth. Constantly buffeted by high-velocity winds from the dying Sun, it’s unlikely Mars could have retained any liquid water on its surface. Still, if any life did crawl from its ancient river valleys, it’s now long gone. The rapidly expanding Sun is scorching the inner planets, disrupting their orbits. Soon, it will engulf them.

But in the outer reaches of the solar system, on Saturn’s moon Titan, conditions are ripe for a few determined cells to spring into existence. Not to be confused with Neptune’s moon Triton, Titan has long been a candidate for extraterrestrial life. In the 21st century, Titan’s surface teemed with lakes and rivers of liquid methane. Instead of rock beneath, though, the crust was made of thick ice. Beneath that ice cover lurked sub-surface water oceans.

Now the expanding Sun has melted Titan’s ice, drenching its surface with oceans of water and ammonia. The astronomers who first predicted this called it a “primordial gazpacho” — choppy, slower-moving than the volatile chemistry of the early Earth, but still with the potential for forming life.

Any life that forms has a few hundred million years before Titan, too, is burned to a crisp.

Pluto, 8 billion years from now

Pluto, barren and distant, orbits a kaleidoscopic nest of gas. The inner planets that once sat comfortably within the Sun’s habitable zone are gone, burned up as the Sun expanded to 256 times its original radius. Then the Sun, too, disappeared.

It’s still there, nearly unrecognizable. At the center of the expanding ring of gas ejected from its outer layers sits a little star called a white dwarf. The only thing holding it up against gravity is the pressure of electrons being forced too close together. Imagine trying to condense an entire elephant into the size of a matchbox, and you have an idea of how dense matter has to be before electron degeneracy pressure kicks in.

In this case, most of the mass of the Sun now occupies an object about the size of the dead Earth. The rest, jettisoned into space, forms a ring-like structure called a planetary nebula.

Out at the edges of this structure sits Pluto. It’s a far cry from the ice-coated dwarf planet it used to be. The Sun stopped short of evaporating it entirely, but all the water and ammonia ice on its surface boiled away. All that’s left is a rocky core, more like Mercury than an ice world.

Pluto drifts on, orbiting a dead star. It isn’t going anywhere.