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I got to thinking about all the planets in the galaxy that we know about (or are about to discover). Are there weird ones that aren’t quite in the same category as the Earth-types, hot Jupiters, super Neptunes, etc?

Well, sure. There are rogue planets. They’re little worlds (usually smaller than Earth) that, end up wandering the galaxy instead of marching around stars.

For one reason or another, these “rogue planets” escaped their systems at some point during the formation process. It happens as worlds are jostled around in the disk of material from which they form. Most of the time, the jostling results in a reshuffling of worlds. This is likely what happened with our gas giant planets early in the solar system’s birth process. Gravitational interactions basically push things around. A planet born in one area moves to a more distant orbit. Or, in the case of a rogue planet, it is ejected from the system altogether. In some cases, they have never been formally gravitationally bound to a star. No matter their origins, these lonely rogues are doomed to wander the galaxy forever.

A free-floating rogue planet transits across a distant star, and the light from the star is microlensed by the planet. Many of these rogue planets exist in the galaxy. — An artist’s impression of a gravitational microlensing event by a free-floating planet.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw

Finding Rogue Planets

How do these planets get detected? About 4,000 know rogues exist in the galaxy. But, they’re not easy to see. Most are thought to be smaller than Earth. Since they’re not around stars, they don’t reflect light. But, their presence is detectable when their paths take them between us and a star. If the alignment is just right, they block a little light from the star. That short-term dimming during such a transit can be detected. (Measuring these short-term brightness flickers is, in fact, what the Kepler Space Telescope did, before it went out of commission.)

A long-term survey of stars, called OGLE (which the Optical Gravitational Lensing Experiment), spots such transits for lensing events. Lensing happens when a massive object, such as a star passes between Earth and a more distant object. The light from the more distant one gets ‘lensed’ by the gravitational influence of the larger one. That changes the brightness of the more distant object a bit.

The lensing objects don’t have to be stars. A planetary system can be involved. So can a rogue planet. And OGLE has been studying thousands of stars to find these microlensing events. In some cases, rogue planets are involved.

In the future, other telescopes, such as the upcoming Nancy Grace Roman Space Telescope will be able to measure these things from space. On Earth, a few ground-based observatories can also do broad surveys to look for microlensing, and thus find more rogue planets.

Could These “Free-floating” Planets Support Life?

The hunt for any planets in the galaxy (and beyond) has, at its heart, the search for other places where life could exist. Most astrobiologists agree that the places with the BEST chances for life are on planets orbiting stars. In particular, they should be in or very near the habitable zone around the star. That’s where liquid water could exist on a planetary surface. You also need light and warmth (which the star provides), and sources of food.

I really doubt that a rogue planet is a very friendly habitat for life. For one thing, there’s no warmth, and may not be any atmosphere or liquid water on its surface. Plus, since it got flung out from its star early in its evolution, it probably didn’t have time to entertain the formation of life.

Astronomers estimate there could be billions of these lonely worlds, floating between stars. So, with such a large number, there could be variations on the rogue planet theme. That’s why I’d never say “never” to any life. It’s possible, I suppose, that some form of microbial life could have gotten started before the planet was kicked out. So, there might be evidence of that.

Other Uses for Rogue Planets

I could certainly think of plenty of sci-fi scenarios where rogue planets could be useful in a galactic empire sense. I mean,what a great place (if you had the technology to do so) to stash a rebel base. Or a secret hideout. The possibilities are endless. But, any techno-savvy group is going to have to provide power, technology, and livable spaces for these places. That’s going to cost a lot. And, I would have to wonder about the return on investment. Still, it’s an intriguing idea, and until we get more data on these worlds, it’s fun to imagine such alternate timeline stories about them.

A curious little story about a minor planet becoming something else landed in my email box yesterday. I’ve been working on a project about exoplanets for the past few weeks. Part of that involves looking into the formation of such systems around distant stars. Obviously, planets form, but other objects get formed when larger worlds do.

As part of the project, I read up on the kinds of worlds we have in our own solar system. Of course, we have planets. We also have dwarf planets (same as planets, but with a few qualifiers). There are minor planets, asteroids, comets, moons, and rings (made up of particles of broken-up moons). Oh, and we have a star, the Sun.

And Now We Have the Naming of Parts: Solar System Edition

Just because solar system objects get “binned” into various categories doesn’t mean they always STAY in those categories. And, those categories sometimes say more about our understanding of solar system objects than about the worlds themselves.

We all know about the IAU’s silliness of “demoting” Pluto (based on a vote by a remnant of astronomers at the end of a meeting). They didn’t even bother to consult planetary scientists, who have the best knowledge of planetary system objects. It’s still a planet, albeit termed a “dwarf planet”. But, the name we call it didn’t change Pluto. It’s still a fascinating world out in the Kuiper Belt and it still orbits the Sun and has activity under its surface. Nothing has changed there.

But, what if an object appears to have changed? Say, like a comet. It dives in toward the Sun, and as it gets close, its ices start to sublimate. That creates a cloud of material around the nucleus, called a coma. A pair of tails sprouts out from the comet, and they (and the coma) last until the nucleus gets too far away from the Sun to sublimate. In that sense, the comet DID change, from being just a frozen chunk of ice to being one that acts when heated to form more characteristics. But it’s still a comet, whether it’s in its active state or a quiescent phase. What also changes, as we learn more about how comets work, is our understanding of them.

Minor Planet Discovery

Now, what if a minor planet did the same thing as a comet? Does that change the minor planet designation? In the case of at least one object, it has done exactly that. It’s called Centaur 2014 OG392. This little worldlet orbits out between the orbits of Jupiter and Neptune. Centaurs sometimes show features that comets usually exhibit: outgassing and coma-building. That’s pretty amazing since the region of space they orbit in pretty cold. That makes it harder for water to make the transition from a solid chunk of ice to a gas cloud.

A Minor Planet Becomes Something Else

This new image of C/2014 OG392 (PANSTARRS) and its extensive coma combines many digital images into a single 7,700 second exposure. The dashed lines are star trails caused by the long exposure. Images captured October 14, 2020 using the Large Monolithic Imager on the 4.3-meter Lowell Discovery Telescope

Centaur 2014 OG392 caught the attention of a team of astronomers at Northern Arizona University, led by doctoral student and Presidential Fellow Colin Chandler. They studied it and found that this Centaur is pretty active. It’s outgassing and forming a coma. The team suspects that the culprit activity causing that outgassing is the sublimation of carbon dioxide or ammonia (or both). The team measured a coma that stretches out about 400,000 kilometers from the body of the minor planet.

Well, so does that coma formation make Centaur 2014 OG392 an active minor planet or a comet? Apparently, there are other active minor planets, so that’s an interesting question. The Minor Planet Center, which has the task of cataloging such objects, has responded to the discovery of the coma around Centaur 2014 OG392 by designating it as a comet. So, it’s new name is now C/2014 OG392 (PANSTARRS). And, that’s perfectly appropriate. It now reflects a better understanding of what this object really is.

Change is Good, Even for Minor Planets

I think this is all very interesting, and really reflects very nicely on our growing awareness of objects and activities in the solar system. It also shows how, as time goes by and new equipment and observing methods become available, our understanding of solar system objects is what changes. Comets and planets and asteroids and other objects will continue to do what they do, regardless of what we call them. But, the names we call them need to accurately reflect our understanding of them — not rely on outdated naming methods and misguided votes.

The Spacewriter's Ramblings

Life on a Rogue Planet…. Or, Not