A Strange Lonely Planet


An artist’s impression of the planet-sized L dwarf PSO J318.5-22. (MPIA/V. Ch. Quetz)

Earlier this week, a press release hit my inbox that made me say, “Ooooooh!” out loud. Its headline was: “A Strange Lonely Planet Found without a Star” and it came with an image.

“Oooooh!” I said again. An image of a planet without a star? Free-floating through the lifeless void? My imagination rumbled to life and started to jump to conclusions.

You see, I’ve wanted for a while now to write a very sad science fiction novel about such a starless scenario. This dream of mine has been motivated by a real science problem: planetary migration.

In the early days of exoplanet studies (way back in the mid-1990s!), the very first planets to be discovered were known as hot Jupiters—giant gaseous planets closer to their host stars than Mercury is to our Sun with temperatures in the thousands of degrees (Centigrade, Fahrenheit, or Kelvin—take your pick). Their existence continues to be a puzzle, because they could not have formed where they are. Both a star and its surrounding planets form when gravity pulls a cloud of gas together into clumps. But in these cases, the energy radiating from the young star should have blown the gas away before it could coalesce into a planet. Only small, rocky planets should be able to form there. The logical theory is that these hot Jupiters had to form much further out where it was colder—like where our own Jupiter is in our solar system—but then somehow migrate in.


The “planet” imaged by the Pan-STARRS1 telescope. It was discovered by a US-German team, with follow-up observations from Mauna Kea. (N. Metcalfe & Pan-STARRS 1 Science Consortium)

Multiple theories have been proposed in which the laws of physics conspire to do just that. In one, the newly formed planet slowly spirals in, losing momentum as it plows through the disk of gas surrounding the young star; in another, the gravitational presence of a nearby companion star perturbs the planet, triggering a wild, eccentric orbit that ends next to the star.

But here’s the rub: Either way, if a Jupiter comes barreling in through its solar system, its gravity will likely throw the planets out! Its enormous mass would scatter the planets like a bowling ball, slingshotting them into the dark, vast coldness of space faster than a tumbling Sandra Bullock. In fact, based on the number of these planets astronomers have already detected through gravitational microlensing, we expect upwards of billions of planets to be lost in space, away from their stars.

What a great end-of-the-world sci-fi story that would be! A helpless population, doomed by the inexorable dance of physics! The Earth becomes both our interstellar starship and our coffin! And people look for strength and hope in a world where every day is just a little bit darker and colder than the one before, without end. An art-house apocalypse—not with a bang, but the saddest, coldest whimper.

At first, I thought this discovery was a direct image of a planet so ejected! But then I read the paper, and the word “planet” isn’t how the authors described it in their title: “A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets.”

In other words, it very well might not a planet—at least, not based on how we use the word planet in everyday life—but it’s like a planet. The team seem to think it probably formed like a star based on the fact that it’s moving in the same direction as other nearby stars, as opposed to an ejected planet that could be going any which way. It just happened to be so small it could be mistaken for a planet! What a bummer.

Actually, outside of my imagination, it’s not really a bummer—it’s a neat opportunity. Actual exoplanets are very difficult to study directly; they’re so close to their stars they get lost in the glare. To the extent that these planet-sized objects actually resemble planets, they give us a chance to nail down their physics unimpeded by their pesky host star. Judging by attributes like its mass, color, and brightness, this “planet” does a fair impersonation, but we still don’t know if objects like these form in exactly the same way as planets.

Regardless, I’ll keep dreaming about writing my novel…


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