Sciency Words: Hot Jupiter

Today’s post is part of a special series that first appeared on Planet Pailly. Every week, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s scientific term is:

Hot Jupiter

Hot Jupiters are defined as large gas giants, roughly Jupiter-sized, in orbits less than 0.5 AU from their host stars (half the distance between Earth and the Sun). Many hot Jupiters orbit much closer than that.


Since the 1990’s, astronomers have catalogued hundreds of hot Jupiters. Current models of planet formation indicate that gas giants cannot form so close to stars, so hot Jupiters must begin life father away and migrate inwards.

These planetary migrations can have dramatic effects on the rest of a star system.

The Creator of Worlds

As star systems coalesce, gas giants like Jupiter are among the first objects to appear. In some cases, a young gas giant might migrate inward while the other planets are still forming. The denser the protoplanetary disk, the more likely it is that a gas giant will migrate.

In computer simulations, researchers found that an inward migrating giant is actually good for a developing star system. Its passage stirs things up, encouraging planet formation.

Terrestrial planets that form in this way would benefit from the mixing of material from different regions of the protoplanetary disk. In the simulations, some ended up with way more water than Earth could ever dream of!

The Destroyer of Worlds

Of course if a giant planet migrates inward after the inner planets form, all bets are off. These smaller planets would either be gobbled up by the giant or hurled out of orbit by the giant’s gravity.

This scenario could happen if a Jupiter-sized planet were nudged by gravitational interactions with other large planets or by interactions with nearby stars. Gas giants in binary star systems would be at especially high risk.

The Destabilizer of Stars

Hot Jupiters are often found in high inclination (tilted) or retrograde (backward) orbits when compared to the orbits of their host stars. For a long time, astronomers wondered what happened to the orbits of these planets. A better question might be what happened to the rotations of these stars?

The presence of such a large object so close to a star could have a destabilizing effect on the star. New research suggests that hot Jupiters cause their stars to tilt sideways or tip upside down. This would explain the highly inclined and retrograde orbits we’ve observed.

Is This Normal?

Astronomers have discovered a whole lot of hot Jupiters, but that doesn’t mean they’re common. It’s just that with our current detection techniques, hot Jupiters are among the easiest planets to spot.

Rare or not, hot Jupiters would be worth closer inspection by futuristic space explorers. What sorts of adventures might these explorers have? Please share in the comments below.


Why Doesn’t Our Solar System Have a Hot Jupiter? from Space Answers.

Build Your Own Orbit (Hot Jupiters) from Artifexian.

“Hot Jupiter” Systems May Harbor Earth-like Planets from

Mystery of “Hot Jupiter” Planets’ Crazy Orbits May Be Solved from

Today’s post is part of Jupiter month for the 2015 Mission to the Solar System. Click here for more about this series.

Article by James Pailly. Check out James’ blog for more great science articles.

  • Leonardo Faria

    “What sorts of adventures might these explorers have?”

    The same adventures as watching paint dry, I suppose. The quickest detectable events take place in thousands of years.

    • Paulo R. Mendes

      Unless of course, that this explorers have access to FTL flight. 😉

      • Leonardo Faria

        Speed is overrated in this crazy world of ours. We should all learn to slow down. It’s time for a Slowpunk Manifesto.

        • Kirov

          Space colonization without FTL is actually pretty interesting. I don’t know why it doesn’t show up more often in sci-fi.

        • we are making the crawl to snailspeed captain!

        • what about a windpunk manifesto?

    • Kirov

      You could probably do some pretty interesting stuff involving tidal forces without having to wait so long. Something similar was done in Sheffield’s Summertide, which I thought was a great read. But speaking of hot Jupiters, what do you guys think the chances are of an Earth-like orbiting a hot Jupiter in the habitable zone? I thought it would make for an interesting setting, so I threw one into my universe, but I never really researched the plausibility of such an occurrence, other than understanding that migration would probably mean fewer bodies in the system.

      • Leonardo Faria

        To begin the design of a story with something different from the characters is, I think, the basic formula of scifi thriller. Make something unexpected and bad happen, throw in it characters with different personalities and backstories, and see what happens. Another story of stranded explorers?

      • Christmas Snow

        Interesting point, Kirov. A hot jupiter may be outside the habitable zone on the warm side. A earth-sized moon orbiting the hot jupiter closely enough will be eclipsed for a few hours every local day. Will that be enough to cool the moon and push the habitable zone further inward?

        The moon will be tidally locked to the hot jupiter, so the side facing or nearing the planet (the “near” side) will have long solar eclipses at noon and lots of planet-shine at night. Will that side be warmer or colder than average? The “far” side away from the planet will experience a mundane day-and-night cycle and the Hot-Jupiter will never be seen from that side.

        • Kirov

          The day/night cycle would certainly be interesting, and so is the idea of the eclipse cooling off the planet. I don’t really know enough about that, so someone else will have to chime in there. But I had been under the assumption that any gas giant within a star’s frost line would be a hot Jupiter, which could theoretically put it well within the habitable zone we’re familiar with. If this isn’t the case, how close does a gas giant have to be to actually be considered a hot Jupiter?

          • Christmas Snow

            I guess Bellerophon’s orbit (51 Pegasi b) is just too close to harbor a habitable moon. Being so close to its sun, the moon must orbit tightly to stay within the gravity well of the planet and still remain too hot. I guess the term “Hot” is not a clear-cut, and is blurred as much as the difference of size between a pebble and a stone, or a stone and a rock. There could be “warm” jupiters out there as well.

  • John H Reiher Jr

    This makes me think that a large Jovian getting nudged into spiraling in to it’s sun, in a “Mature” star system. Say we have another gas giant with a habitable moon and the two get ejected from that system. And said moon was home to an intelligent species. What we have is the cause for the novel “When Worlds Collide” Bronson Alpha and Bronson Beta are a gas giant and moon pair ejected from their home system millions of years ago and by chance pass through the Sol system. A lot of planets are going to get jostled in the process, as well… Well, I won’t spoil the novel if you haven’t read it or seen the movie version.

  • Christmas Snow

    I have been experimenting with orbital perturbations by simulations available online. One of them is available at

    The PHET simulator has two drawbacks, but is nevertheless good to begin with and understand how planets nearing each other exchange momentum, collide, or catapult each other out of orbit. It was much harder than I thought to make one planet fling another towards the sun, unless the system had not enough initial velocity to start with.

    The first drawback is 2D rather than 3D: I could not simulate inclined orbits, for instance.
    The second drawback was the use of arbitrary units: Is the mass in Kilograms? Earth masses? Jupiter masses? What are the speed units? At first it may seem impossible to apply the orbital equations. To get a circular orbit, I had to change the orbital velocity until I got it right. To simulate orbital resonance between two or more planets, I expressed the equations as ratios (of masses, semi-major axis and so on). That would reduce the equations into simpler expressions.