Eccentric Exoplanets: The Smallest and the Speediest

Article February 22, 2025

Exoplanets, man. They can be so weird! I can remember (not well, mind you, but a little) the heady days of the first exoplanet discoveries, when our idea that other solar systems would all resemble our own got immediately blown straight out of the water. After all, the very first exoplanet we ever found was orbiting a dead star. Then the first one we ever found around a live star was a puffy planet bigger than Jupiter orbiting extremely closely to its star.

A series of artistic renditions of confirmed exoplanets. Credit: NASA/JPL-Caltech/R. Hurt

You’ll note that neither of those scenarios sound like our solar system. And as the years have gone on and the exoplanet discoveries have racked up (as of this writing we are at over 5,000 confirmed exoplanets with a stupidly large number of unconfirmed candidates) so have the number of weird systems that we would have once thought couldn’t exist.

Dead star solar systems and hot Jupiters? Obviously. Gas giants and rocky worlds intermixed? Yup. Solar systems with a bunch of rocky worlds and no gas giants at all? Uh-huh. Planets orbiting more than one star at a time? Yeah, we call them Tatooines for fun.

While there’s (as yet) no end to the delightful variety of solar systems the universe has to throw at us, we’ve recently found a pair of candidates (so their actual existence is not yet confirmed) that would definitely earn their spots on the weird exoplanet list. Let’s meet the new, wonderfully weird potential worlds, one tiny and one speedy.

Dead Star? No Problem.

As I mentioned above, the first exoplanet we ever found, way back in 1992, was orbiting a dead star. A pulsar, to be exact. These are rapidly spinning neutron stars, remnants of big stars that supernovaed. We can measure their spin times very, very precisely with radio observatories like Arecibo (RIP). They’re the finest timepieces in the universe, nothing throws them off.

An artist’s rendition of a pulsar. When pulsars spin we see their bright magnetic beams (represented here as columns of light) as bright radio pulses that allow us to precisely time their spins. Credit: Kevin Gill

Or, you know, that’s what we once thought. Then in 1992 astronomer Alex Wolszczan, while using the Arecibo Radio Observatory to do a survey for previously undiscovered pulsars, found PSR B1257+12 (catchy, I know). This pulsar had…anomalies in its spin time. Periodic anomalies, in fact, as though something was gravitationally tugging on the pulsar at regular intervals. You know, like a planet.

Wolszczan got a second astronomer, Dale Frail, to double check and make sure he wasn’t seeing things, and in 1992 they famously announced the discovery of the first known world outside our solar system, orbiting the corpse of a star (and later were able to confirm a second planet orbiting this pulsar, with the possibility of a third).

These days we know of a few more pulsar planets, but they don’t seem to be super common. We think they’re probably formed from the ragged remnants left behind after the supernova, whether from planets that were there before, the shreds of a companion star, or even bits of the exploded star itself. They are zombie worlds orbiting stellar corpses.

Tiny Zombie World

A recent discovery involves what might be the smallest exoplanet candidate we’ve ever seen. And it one ups all previously known pulsar planets—it’s not orbiting one dead star but three!

This system sits about 4,200 light years away from us and was first discovered in 2014. It consists of one pulsar (so remnant of a very big star that went supernova), and two white dwarfs (remnants of two smaller stars, more like the Sun). Three stellar corpses for the price of one! It made history when it was discovered because we’d never found a pulsar in a trinary system before.

We don’t even know for certain if this system has a planet in it. What we do have are mathematical models of the three dead stars that suggest they alone cannot explain blips in the spin of the pulsar. It’s possible those models are off. It’s possible this pulsar has something else going on. It’s possible there’s something up with one of the white dwarfs we’re not aware of.

An artist’s rendition of the PSR J0337+1715 system, with the pulsar and two white dwarfs in the middle, with a model of a prospective exoplanet’s orbit going around all of them. Credit: Guillaume Voisin/Fabrice Mottez/Paris Observatory

But so far the most likely possibility is that there is a fourth body in this system, orbiting all three of these dead stars at once. And this fourth body, if it exists, would be a very small planet. If confirmed it would be the tiniest exoplanet yet.

The smallest confirmed exoplanet with a confirmed size (look, pinning down the radius or even the very existence of a wee planet light years away is no easy task) is a world called Kepler-37b. Its radius is a little less than a third of Earth’s. If there is a small world orbiting these three dead stars, it would be roughly the size of Pluto. That’s about 70% the size of the Moon and only 60% the size of Kepler-37b. Or, put another way, that’s tiny.

It will take some additional efforts to try and confirm its presence, but if we can then we’ll have a new record-holder for smallest exoplanet that will be hard to beat with our current tech.

Speed Demons

And artist’s illustration showing stars speeding around the center of the Milky Way. The trails are meant to indicate star movement, with longer trails indicating faster stars, while the colors indicate motion towards (blue) or away (red) from us. Credit: NASA/JPL-Caltech/R. Hurt

Back in 2011 astronomers discovered a pair of objects being gravitationally lensed by some foreground stars. Because this pair of things was discovered via lensing, we were able to figure out the mass ratio between the two things, but not their actual masses.

The mass ratio was pretty big though, with one of the objects being 2,300 times as massive as the other. There are only so many types of pairs that could have a mass ratio like that. It could be a small star and a big planet, or it could be a big planet and a small moon (for a point of reference, the Sun’s mass is 332,950 times that of Earth and 1,057 times that of Jupiter).

While this image does not show the star we’re talking about in this post, it does show a star speeding through the central region of the Milky Way, creating a bow shock similar to that created by the front of a speeding boat. Credit: NASA/STScI/Aura. Acknowledgment: C. R. O’Dell

The good news was that, if this was a star and planet combo, the star should be visible in previous observations from other telescopes (if it was a planet/moon combo, well, those don’t really give off much light and we’d be out of luck). A little detective work looking back through images from the Keck Observatory and Gaia telescope (RIP) found a candidate, a small red dwarf, that looks about right to be the larger of the two lensed objects. It sits in the Milky Way’s central galactic bulge, about 24,000 miles away. And, if this is the lensed star, it’s moving ludicrously fast!

By measuring the distance this star theoretically traveled between its discovery in 2011 and the position in the Keck and Gaia data in 2021, this star has to be moving at a minimum of 1.2 million miles an hour, and could possibly be going quite a bit faster. In fact, it’s possible that it’s topping 1.3 million mph, which is the escape velocity of the Milky Way. If it’s going as fast as that, this star is on its way out into intergalactic space.

That’s not necessarily unusual. Stars get sped up to escape velocity for any number of reasons. But not all of them are thought to be dragging something along with them.

Propelled Planet?

Remember, this star (which, as far as I can tell, doesn’t even have a catalog number yet?), was one of two masses discovered via gravitational lensing. If the star is the bigger object, then the smaller object is small enough that it has to be a planet.

Not a small planet, mind. It’s what we might refer to as a super-Neptune (or, well, I suppose we could call it a super-Uranus, but astronomers are habitually wary of using Uranus as a reference for anything. We’ve been burned too many times by middle school jokesters), between one and two Neptunes big.

An artist’s illustration of what the planet being dragged through space by a speeding red dwarf might look like. Credit: NASA/JPL-Caltech/R. Hurt

If this star is dashing along at or near galactic escape velocity (all of this, it should be noted, must still be confirmed), it’s dragging this planet along for the ride. And that would make this super-Neptune the fastest exoplanet ever discovered—so far anyway! Who knows how many of these hypervelocity stars might be lugging planets along, now that we know that can be a thing!

Interestingly, if you lived here (it’s a gas giant and it’s in the central galactic bulge, both of which mean we don’t even have to bother thinking about if this planet has life on it, but just for a thought experiment), you probably wouldn’t notice anything particularly weird. The stars in your night sky would certainly move around a lot faster than they appear to here on Earth, but the movement probably wouldn’t have a huge effect on your life. Of course your sky would get super weird once your solar system left the Milky Way, but that’s some ways down the road.

Whacky Worlds Everywhere

It’s fun to recall how naïve we were about exoplanets before we started actually finding them. How very wrong we were. Shakespeare said “There are more things in Heaven and Earth, Horatio, then are dreamt of in your philosophy” (Hamlet, if you’re wondering). That seems even truer of exoplanets than of anything else. Every time we think we have a handle on what solar systems are likely to look like, the universe throws us a curve ball. 51 Pegasi. TRAPPIST-1. WASP-121. Now maybe PSR J0337+1715 and this as-yet unnamed speed demon star.

And who knows what kind of weird and crazy worlds we’re going to find next! Space seems to get a kick out of knocking us for a loop, and no doubt the next one is lurking gleefully just around the corner. And that’s why space is awesome.