The gruesome fate of every planet when the Sun dies

Today, our Solar System is relatively stable.

The inner Solar System, including the planets, asteroids, gas giants, Kuiper belt, and more, is minuscule in scale when compared to the extent of the Oort Cloud. Sedna, the only large object with a very distant aphelion, may be part of the innermost portion of the inner Oort Cloud, but even that is disputed. On a linear scale, depicting the entire Solar System in a single image is incredibly limiting; to characterize the orbit of a faraway bound object requires years or even centuries of data.

Eight planets, an asteroid belt, Kuiper belt, and Oort cloud all orbit the Sun.

A logarithmic chart of distances, showing Voyager, our Solar System, the Oort Cloud, and our nearest star: Proxima Centauri. If we imagine building larger and larger particle accelerators, every factor of 10 in radius gets us another factor of 10 in energy, but typically at the expense of another factor of 100 in cost. On planetary and stellar scales, this can get prohibitively large very quickly.

However, the Sun is evolving, and won’t live forever.

After its formation some 4.6 billion years ago, the Sun has grown in radius by approximately 14%. It will continue to grow, doubling in size when it becomes a subgiant, but it will increase in size by more than ~100-fold when it becomes a true red giant in another ~7-8 billion years, total, all while growing in brightness by a factor of at least a few hundred.

Over the next 7 billion years, it will heat up and swell, becoming a red giant.

When stars fuse hydrogen to helium in their core, they live along the main sequence: the snaky line that runs from lower-right to upper-left. As their cores run out of hydrogen, they become subgiants: hotter, more luminous, cooler, and larger, before evolving into true red giants that fuse helium in their cores. The red giant phase for a Sun-like star results in a luminosity that’s hundreds or even 1000+ times as bright as the Sun is presently.

Mercury and Venus, the innermost worlds, will quickly be engulfed.

As the Sun becomes a true red giant, the Earth itself may eventually be swallowed or engulfed, but it will definitely be roasted as never before during the subgiant phase and the evolution into a red giant. It remains to be determined whether any of the effects of swallowing Mercury, Venus, or even possibly Earth will be noticeable by a distant alien civilization.

The Earth, although there is a chance it will survive, should be the final devoured planet.

The Sun, when it becomes a red giant, will become similar in size to Arcturus. Antares is more of a supergiant star and is much larger than our Sun (or any Sun-like stars) will ever become. Even though red giants put out far more energy than our Sun, they are cooler and radiate at a lower temperature at their surfaces. Inside their cores, where helium fusion occurs, temperatures can rise into the tens of millions of K.

In the giant phase, the Sun will shine thousands of times as bright as today.

During the main phase of a star’s life, planets can orbit at nearly any distance from it, including very close in. As the star evolves, it becomes a subgiant and eventually a true giant. As the star increases in size, the frictional drag force on the innermost planet increases; eventually, it will come into contact with and be devoured by the parent star, while the increased stellar brightness has severe consequences for planetary atmospheres and ice-rich objects.

The asteroids, Kuiper belt, and inner Oort cloud objects should sublimate away, leaving rock-and-metal cores.

Comet 67P/Churyumov-Gerasimenko was imaged many times by the ESA’s Rosetta mission, where its irregular shape, volatile and outgassing surface, and cometary activity were all observed. The comet’s nucleus itself would have to have been much larger and more massive to be pulled into a “round” shape by self-gravitation.

The atmospheres of even the largest giant planets will entirely evaporate.

Photoevaporation is the process by which, when a planet is too close to its parent star or when a star evolves to become very bright, the planetary atmosphere heats up, where stellar emissions can strip particles out of the atmosphere, leading to photoevaporation. For a gas giant planet that orbits a Sun-like star, the act of the star evolving into a red giant can be sufficient to strip even a Jupiter-like or Saturn-like atmosphere away entirely.

Then the Sun should lose mass, ejecting its layers in a planetary nebula.

The dying red giant star, R Sculptoris, exhibits a very unusual set of ejecta when viewed in millimeter and submillimeter wavelengths: revealing a spiral structure. This is thought to be due to the presence of a binary companion: something our own Sun lacks but that approximately half of the stars in the universe possess. Stars lose approximately half of their mass — some more, and some less — as they evolve through the red giant phase and into an eventual planetary nebula/white dwarf combination.

This mass loss should eject the remaining Oort cloud and Kuiper belt objects.

The Egg Nebula, as imaged here by Hubble, is a preplanetary nebula, as its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to become fully ionized. Many of the giant stars visible today will evolve into a nebula like this before shedding their outer layers completely and dying in a white dwarf/planetary nebula combination. As the central star loses mass, the outermost objects in that stellar system, such as the analogue of our Oort cloud and Kuiper belt, become ejected.

If ~50% or more of the Sun’s mass is lost, as expected, Neptune and Uranus should get ejected, too.

Although our best-ever views of the planets of Uranus and Neptune still come from the Voyager 2 encounters with these worlds from the late 1980s, the reality is that these two planets are extremely similar in color, composition, and size, with the famous “azure” image of Neptune not representative of its true color. Uranus is just 3% larger than Neptune, but substantially less massive.

The Sun’s core, meanwhile, ought to contract down to a white dwarf.

When lower-mass, Sun-like stars run out of fuel, they blow off their outer layers in a planetary nebula, but the center contracts down to form a white dwarf, which takes a very long time to fade to darkness. Some white dwarfs will shine for trillions of years; others are on their way to an inevitable supernova when they collide with another white dwarf or accumulate enough mass to detonate.

The remnants of Mars, the asteroids, plus Jupiter’s and Saturn’s stripped cores should persist.

When lower-mass, Sun-like stars run out of fuel, they blow off their outer layers in a planetary nebula, but the center contracts down to form a white dwarf, which takes a very long time to fade to darkness. The planetary nebula our Sun will generate should fade away completely, with only the white dwarf and the surviving planets and asteroid left, after approximately 9.5 billion years. It is suspected by some that only Mars, Jupiter, and Saturn, among the planets, will survive in any form.

Only three known white dwarf systems possess planetary remnants.

Only three white dwarf systems are known to house orbiting planets or planetesimals, with WD 1856, illustrated here, possessing the highest-mass planet known. All of the sub-stellar mass objects orbiting white dwarfs are in extremely tight orbits, and transit across the face of their white dwarf companion.

Our far future still contains tremendous unknowns.

For planets that continue to orbit a stellar remnant, such as the white dwarf our Sun will become after another 7-to-10 billion years, it is likely that only a thin atmosphere, or no atmosphere at all, will remain on even massive, gas giant worlds. In addition, any object not swallowed or ejected may be torn apart into a debris disk, particularly for a fragile surviving world such as Mars.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.