Expert Voices

Extremely Hot and Incredibly Close: How Hot Jupiters Defy Theory

Inside Exoplanets: Motley Crew of Worlds Share Common Thread
An artist's impression showing a close-up of the extrasolar planet XO-1b passing in front of a Sun-like star 600 light-years from Earth. The Jupiter-sized planet is in a tight four-day orbit around the star. (Image credit: NASA, ESA and G. Bacon (STScI))

Paul Sutter is a visiting scholar at The Ohio State University's Center for Cosmology and Astro-Particle Physics (CCAPP). Sutter is also host of the podcasts "Ask a Spaceman" and "RealSpace," and the YouTube series "Space in Your Face."

As usual, we thought we had it all figured out. See that gas giant over there in the outer solar system? It was born there. It will spend its whole life there, and it will die there. Sure it might wiggle around a bit every few hundred million years — who doesn't? — but, by and large, planets don't move.

Surprise: Planets move. And not just a little. They move a lot. All over the place. In fact, in the early days of a solar system's formation, planets are a little rambunctious: squirrely little toddlers jostling about underfoot. But it wasn't until we started observing planets in other solar systems ("extrasolar planets" or "exoplanets" for the astronomer on the move) that we really noticed this fact. 

Hot Jupiters whipping around

And it wasn't just any type of exoplanet that kicked off this re-think; it was the hot Jupiters. Imagine: a planet more massive than the largest one in our solar system and 10 times warmer, a monstrous beast of hydrogen and other elements, complete with swirling bands of gas and a rich, dynamic atmosphere, orbiting closer its star than Mercury orbits the sun. In some solar systems, such a planet orbits so quickly that its year is shorter than the Earth's day. That means these worlds can whip around their parent stars in hours. The physics involved can reduce the most hardened scientist to tears.

When astronomers spotted the first hot Jupiter (51 Pegasi b, the first exoplanet to be found around a sunlike star, no less), the reaction was mostly, "Ha ha, mother nature, that's cute. You got us this time, but no more funny business, OK?"

But then another hot Jupiter was found. And another. Then half a dozen more. They went from goofy oddballs to … normalcy. For a while, it started to look like our own solar system was the weird one. Maybe they should just be called "regular Jupiters," and ours re-named a "cold Jupiter?"

Astronomers searching for another Earth are getting closer, thanks to recent discoveries by the Kepler space telescope. [See our full infographic on the types of alien planets here. (Image credit: Karl Tate, Contributor)

Seeing stars — and planets

In retrospect, it's not surprising that astronomers spotted these massive planets living so close to their parent stars. After all, our detection methods are most sensitive to exactly these scenarios.

One method is based on the motion of the parent star itself. Have you ever taunted a dog on a leash, running back and forth? The dog, frantically trying to chase you, runs until the leash stops it. You go the opposite direction, and so does the dog, until "thunk!" and the leash again reaches its limit. 

In this really bad analogy, each planet is taunting its parent sun through gravity. During one part of the planet's year, the world sits at a certain position in the system. Gently, week by week, the planet tries to pull the star over to it, because that's how gravity works. But some time later, the planet finds itself on the opposite side of the system. "No, star, I meant come over here, not over there!" 

Back and forth the star goes, sloshing around — just a tiny bit — it is huge compared to its planets, after all. But with precise-enough measurements, we can detect that wobble by a telltale red- and blue-shifting of the star's emitted light. [Direct Imaging: The Next Big Step in the Hunt for Exoplanets ]

A second powerful method — and nowadays, the method most commonly used to find new planets — is to simply look for distant eclipses. If we get the alignment just right, and stare at enough stars, every once in a while, a planet will cross the face of its parent, ever so slightly dimming the star. Bingo: a transit detection!

Both of those methods will more easily find a planet if it is big, producing a stronger pull from wiggling or a more significant dip in the brightness. 

So these methods will first pick out the massive, close planets, because those will make the strongest, clearest, least-ambiguous signals. And with planets that have fast orbital speeds, you can get more signal bang for your observational buck. 

That led to the initial worry: For a while, it seemed like every exoplanet was a hot Jupiter. Fortunately, as our detection methods improved and we could spot smaller exoplanets, we've learned the galaxy is a mellower place. There are plenty of hot Jupiters, but also plenty of regular Jupiters, and every other kind of planet you can imagine

Almost a sun, but not quite

Still, how did the hot Jupiters get so hot? To seed a gas giant, you need more than rocks for a core, simply because there aren't enough rocks in a solar system to make a decent Jupiter-size planet core. You also need to glue together a bunch of ices, and last time I checked, there aren't exactly a lot of ices near the surface of a star. So obviously the hot Jupiters didn't form in the Mercurial positions where we now find them. What gives?

The best guess we have so far — and it really is a guess at this point — is that a Jupiter-like planet forms in an appropriately Jupiter-like orbit in an early gaseous, nebulous not-quite-a-solar-system. The big world clears a gap in the gaseous disk, because that's what giant planets do. It's stuck to the middle of the gap like a car on a racetrack. If it moves too close in, the bands of gasses around the star are rotating faster than the planet is orbiting, and so nudge the giant young planet back out. If the planet scoots out too far, the slower-moving gas bands located there nudge it back into its proper place. 

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But since the system is so young, it's not done contracting and compressing. The gas continually brushes against the planet, playing a fantastically huge game of curling to keep the planet within the gap. And as the entire disk of gas continues to squeeze inward to its final, compact size, it carries the gap — and the newly formed planet — with it. Voilà: a Jupiter-size planet in the inner solar system!

But if it's so easy, why does it happen only sometimes? How come our solar system's Jupiter is where it "belongs"? And what stops a hot Jupiter from becoming a very hot Jupiter and just crashing into its star? And, honestly, the whole mechanism seems a little dodgy, if you ask me.

There are certainly many things we don't understand, and hot Jupiters offer us yet another tantalizing clue about the larger puzzle of how solar systems form, both here and abroad. To solve this riddle, we have to do what scientists do best: think about it some more. And more data wouldn't hurt, either.

Learn more by listening to the episode "What's Up with Exoplanets?" on the "Ask a Spaceman" podcast, available on iTunes and on the Web at Thanks to Jon Ziegler, Dan Cataldo, @infirmus, @MarkRiepe and Kieran Price for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and at

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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.