For decades, astronomers have endeavored to forecast with confidence the fate of planetary systems, including our own, throughout the cosmos. And these experts’ predictions have one central principle: to confidently guess what will eventually befall a planet, you have to know the size of its star.
Tiny stars don’t really burn out but rather fade away as they shine dimly for hundreds of billions or even trillions of years, likely keeping their planetary companions in tow. Massive stars go out with a bang, expiring as a supernova that leaves behind a neutron star or black hole. Such events tend to be cataclysmic for planetary systems. And stars of middling mass, like our own, expand into a red giant, engulfing or scorching their planets and then dissipating to become a slow-cooling stellar ember called a white dwarf.
This dismal fate is expected to befall our sun in some five billion years, setting what has been considered the last-gasp expiration date for life on Earth and perhaps throughout the solar system.
But insights from fresh studies of dying stars and doomed worlds elsewhere in the Milky Way challenge this consensus. Increasingly, it seems that the eventual fates of planetary systems, ours included, are not wholly written in the stars.
Specifically, two new findings—the discovery of a giant planet closely orbiting around a red giant star and the identification and estimation of the number of so-called rogue planets adrift in our galaxy—have highlighted that there are many more nuanced scenarios to consider. Planets can survive the ruin of their star, and the vast majority of planetary systems shed numerous worlds throughout their history.
The Planet That Shouldn’t Exist
When our sun eventually enters its red giant phase, its radius will likely extend well beyond Earth’s present-day orbit. Even if our planet and the solar system’s other inner rocky worlds escape engulfment, the sun’s swelling will probably still spell their end because of the scorching temperatures they will experience. For the former scenario, astronomers have been seeing signs of this demise in the atmospheres of white dwarfs: researchers have found such stars littered with the remnants of dead planets they likely swallowed.
In fact, astronomers believed the fate of any planet orbiting a star within its red giant radius was likely sealed. That was until the discovery of the planet 8 Ursae Minoris b (8 UMi b), also known as Halla (after the South Korean mountain Hallasan and in honor of the South Korean astronomers who initially identified it in 2015).
“We used to think that planets just couldn't survive around stars that become red giants—but this system provides a loophole,” explains Malena Rice, an assistant professor of astrophysics at Yale University, who co-authored new research on Halla postulating how it improbably survived.
Halla was discovered by the wobbling its orbital tugging induced on its red giant home star, 8 Ursae Minoris (8 UMi). Track the period of that wobble over time, and you can discern the length of a planet’s year and its distance from its star. Such scrutiny showed that Halla orbits a mere 75 million kilometers from 8 UMi—that is, just half the distance between Earth and the sun. But standard modeling of 8 UMi’s red giant phase suggested that the star’s puffy, hot stellar atmosphere should have expanded about 30 million km farther out than that at its swollen peak. That is, Halla appeared to be a planet that shouldn’t exist. It should’ve been consumed and obliterated. Instead it had somehow escaped.
“This planet was very lucky,” Rice says. “In its past, we think that it may have orbited two stars rather than one, and this helped it to survive what could have been a fiery fate.”
Binary stars can exchange material back and forth, and they can even merge to become a single star, allowing a rich diversity of novel possibilities for any orbiting worlds. Such major redistributions of mass can alter planetary orbits while also profoundly influencing how a star shines, adding or siphoning away gas to change the nature and timing of its subsequent stellar evolution. According to the careful modeling work of Rice and her colleagues, the most likely explanation for Halla’s survival is that 8 UMi was once accompanied by a smaller close-in companion star, with which it eventually merged. Among other effects, the merger would’ve stifled 8 UMi’s red giant expansion, sparing Halla.
Although this mechanism clarifies how some fortunate worlds might survive their star’s antics, it offers scant hope for our own solar system because our sun lacks a stellar companion to tamp down its eventual evolutionary swelling.
“It will be tough for our rocky planets to make it through that process if the sun swells beyond their orbits,” Rice says. “But perhaps finding more systems like these might teach us about interesting natural ‘loopholes’ that occur in at least some types of planetary systems.”
Rogue Worlds by the Trillions
Bountiful discoveries of newfound worlds—and with them, perhaps, the revelation of more “loopholes”—could come relatively soon via NASA’s Nancy Grace Roman Space Telescope, which is due to launch by May 2027. Much of Roman’s potential comes from its planned exoplanet survey, which will rely on a relatively underused technique known as microlensing. In this method, Roman will stare at many stars simultaneously, looking for instances where, by chance, a planet-bearing star will be perfectly aligned to cross in front of another “background” star much farther away. In such cases, some of the foreground star’s planets can act as gravitational lenses and magnify the background star’s light in a way that allows astronomers to reconstruct a lensing world’s mass and orbit. The technique is especially sensitive to planets orbiting far from their stars—a circumstellar region that remains scarcely probed by other planet-hunting methods.
And in fact, it’s also capable of finding worlds that have left their stars behind entirely—something Roman could leverage to discover hundreds of rogue planets in interstellar space. Already preexisting microlensing surveys have found a handful of these free-floating worlds, and the statistics of this largely hidden population suggest most planetary systems have a surprisingly turbulent history.
The latest example comes from the MOA (Microlensing Observations in Astrophysics) survey, a project conducted at the University of Canterbury Mt. John Observatory on New Zealand’s South Island by an international team, including scientists at NASA and Japan’s Osaka University. Running for almost a decade, MOA has gathered enough data to weigh in on the galactic abundance of rogue planets down to and even below Earth mass.
“This number turns out to be somewhat larger than we would have guessed,” says David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center and co-author of two new papers reporting on these findings that were posted on the preprint server arXiv.org. These papers are set to be published in a future issue of the Astronomical Journal.
So far MOA has only detected six microlensing events that are consistent with magnification by a low-mass rogue planet, says MOA collaborator Takahiro Sumi, a professor at Osaka University, who co-authored both preprint studies. “Taking into account the low detection efficiency and our detections, we estimated that there are many such low-mass objects in the galaxy,” he adds.
“We found that there are about 20 free-floating planets per star in the galaxy, and the number is dominated by low-mass planets with a mass similar to or smaller than that of Earth,” Bennett says. Those numbers, in turn, suggest an astounding two trillion rogue worlds in the Milky Way alone—six times more than the planets that are estimated to be bound to stars.
If this estimate is correct, it means most planetary systems are essentially dissolving across cosmic time, jettisoning many of their members via dynamical interactions between planets or their host stars that can slingshot unlucky worlds out into the interstellar abyss. It’s possible that when we look out into the solar system and other multiplanetary systems, the remaining planets we see are rare vestiges of once-bustling neighborhoods.
Bennett explains that most rogue worlds likely get ejected during the early stages of planetary formation, after which planetary systems settle into more stable configurations. The probability of ejections should generally decrease throughout a sunlike star’s life, he says. But when it swells into a red giant and begins shedding its outer layers of gas, the resulting shifts in planetary orbits can spark new rounds of world-ejecting instabilities.
Stars that are much heavier than the sun and end their life as a supernova, Bennett suggests, could also provide a rich source of rogue worlds and help to explain MOA’s outsize estimates.
Scott Gaudi, an astronomer and microlensing expert at the Ohio State University, thinks MOA’s surprising results are the best currently available but cautions that they remain very uncertain, so they “should be taken with a grain of salt.” Roman, he says, should beef up the statistical certitude, thanks to the unprecedented sensitivity of its prospective microlensing survey.
The Question of Life
If MOA’s estimates are accurate, however, the sheer number of rogue worlds raises an interesting question: Could any of them provide conditions favorable to life? Ravi Kopparapu, a planetary habitability expert at NASA’s Goddard Space Flight Center, says life on a rogue planet would be problematic—but not impossible.
“Without a star, life on a cold rogue world would likely need to get its energy from internal sources,” Kopparapu says. “That could be in the form of tidal/frictional heat like in some of Jupiter’s moons where there are subsurface oceans, from residual energy when the planet formed or from the radioactive decay of heavy elements in the planet’s core.” Such worlds might resemble the large moons of our outer solar system and harbor potentially clement conditions beneath an icy crust.
For surface habitability, Kopparapu says a thick hydrogen atmosphere could possibly insulate a rogue planet and keep its surface temperature warm enough for living things to endure. Such atmospheres are easily blown away by stellar radiation, but because rogue planets do not orbit stars, they might be able to cling to an insulating atmosphere of hydrogen far longer than any sunbathed world could.
Amid so much uncertainty, life’s prospects in such alien environments can seem either dizzying or dim. Might biospheres someday be found eking out existence around post-red giant stars or on worlds without a star at all? The thought is staggering, to say the least—and the fact that we could soon have real data to better answer such grand questions is all the more so.