How 'Ploonets' Tell Us About Planet Formation

An exomoon circles a gas-giant planet in this artist's impression. Exomoons may detach when their parent planets migrate inward, becoming "ploonets" that circle stars on their own.
An exomoon circles a gas-giant planet in this artist's impression. Exomoons may detach when their parent planets migrate inward, becoming "ploonets" that circle stars on their own. (Image credit: NASA/JPL-Caltech)

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to's Expert Voices: Op-Ed & Insights

One of the most surprising discoveries in the past couple decades is the existence of so-called "hot Jupiters," which are giant exoplanets that orbit way too close for comfort to their parent stars. In short, they shouldn't exist. Gas giant planets need a lot of gas to become giant (hence the name), but there isn't a lot of gas near stars. So they must have formed farther out and migrated in like a gaseous moth to a flame. In the process, any icy moons may have become detached and started to disintegrate in the blaze, leading to something that's not quite a moon and not quite a planet — a "ploonet." Detection of these hybrid creatures may tell us how hot Jupiters came to be so hot.

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The making of a giant 

We might be tempted to think that the majority of solar systems out there in the galaxy look a lot like our own — a small collection of rocky worlds orbiting close to a star, with gassy and icy giants, surrounded by retinues of moons big and small, dominating the outer orbits.

Boy is that wrong.

Astronomers were in for a nasty surprise once they first started detecting planets outside the solar system. The earliest methods depended on detecting the "wobble" of the parent star as one of its children gravitationally tugged it back and forth in the course of its usual orbit. The biggest wobbles will come from bigger planets orbiting at smaller distances.

And guess what: that's exactly what we found. 

Big planets — in some cases even bigger than our own reigning heavyweight champion, Jupiter — orbiting in frankly ridiculously close orbits. I'm talking closer than the orbit of Mercury. 

In the decades since those first surprising discoveries, we've come to know that these so-called hot Jupiters are actually pretty common. However the heck nature is able to fashion these systems, it's able to do it pretty efficiently.

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There and not back again

Of course, we're not exactly sure how hot Jupiters get so hot. The main problem is that stars are kind of warm objects, emitting a lot of radiation. In a young planetary system, an infant star is surrounded by a swirling cloud of gas and dust that will eventually coalesce into planets.

Near the star, pulses and outbursts blow away any lingering lighter elements like hydrogen and helium, and the heat turns frozen ice into more pliable water. Thus the inner worlds are dominated by heavier molecules and usually acquire a healthy dose of liquid.

In the colder outer reaches, dirt and rocks and glue together with ices, forming much more massive bodies than can be found inwards. With that enhanced mass, the newly formed planet can really bulk up by slurping on as much gas as it can possibly get its greedy gravitational hands on, building up to truly impressive sizes.

There's usually a bit of shuffling and rearranging as a planetary system starts sorting itself out, with planets moving inward or outward, crashing together, or even getting an unfortunate ejection from the system altogether. Through a mechanism we don't fully understand — possibly involving a complex dance between a newly forming planet and its surrounding gas that flows toward it to continue its feeding — sometimes giant planets barrel inward toward the star, crowding out anybody else.

So one mysterious mechanism pulls the big planets in (sometimes), but another mysterious process has to make the planet stop its inward migration; otherwise it would just go all the way and slam into the star itself. But once in place, the planet can last for at least a few million years. With the intense heat, the hot Jupiter's gaseous atmosphere slowly evaporates, but with so much raw bulk the planet can withstand the assault for some time.

Wayward moons

But giant planets don't come alone. Thy have lots of little friends — their moons, usually numbering in the dozens. While most are small and barely qualify as anything meaningful, some can be quite large, as big as Earth's own moon, swathed in layers of rock-hard water ice. 

Despite decades of searching, astronomers have not yet confirmed the existence of any exomoons — moons orbiting a planet outside the solar system. If any hot Jupiters have a moon, we haven't seen one. 

Did they lose all their moons in the chaotic processes that led to their inward trek? Did the moons detach and fall into the star? Did they instead get ejected from the system altogether?

The likely answer is "yes, usually", but recently a team of astronomers detailed a scenario where the moons not only survive the migration to the hot inside regions of the solar system, but manage to detach themselves from the gravitational grips of their parent planets and orbit the star on their very own. 

The team called these objects "ploonets," because why not.

These ploonets, if they survive the travails of their giant parent plants in the transformation from regular Jupiters to hot Jupiters, would be similarly affected by the uncomfortable proximity to the star. All those layers of rock-hard ices become much less rock-hard at those extreme temperatures, and begin evaporating off in a thick haze surrounding the ploonet.

This makes for intriguing observational consequences, as the spectrum of starlight passing through the haze would show an unmistakable signature. A potential close-in small planet may not be a planet at all, but the leftover remnants of an icy moon. Should such a ploonet be discovered, it would tell us that this scenario is not only possible, but viable, giving astronomers a vital clue in the formation of hot Jupiters, and solar systems in general.  

Read more: "Ploonets: formation, evolution, and detectability of tidally detached exomoons."

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Paul Sutter Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.