Hot Giant

**Hot Giants** are a form of giant planet which orbit very close to their parent stars (between 0.015 and 0.5 AU) and thus have temperatures in excess of 1000 K. They have a slightly lower density than normal giant planets of a similar composition. Their orbits are very circular (low eccentricity between 0.001 and 0.09), due to tidal forces from the star, which also force the planet to be tidally locked to the star. High wind speeds distribute heat throughout the planet, however, and thus the night side has little difference in temperature from the day side. Hot Giants are unlikely to have moons larger than small asteroids, due to the star’s tidal forces. Many Hot Giants orbit in retrograde (opposite in direction to the rotation of the star).

Giant planets cannot form within the frost line of a star, and thus are thought to have migrated inwards, which has great effects on the system’s inner planets. For instance, a computer simulation has shown that the inward migration of a gas giant scatters the inner protoplanetary disk outwards, but the disk can reform and planets up to 2 M⊕ can form within the habitable zone of a star even after such an event. The inward migration of a giant planet can introduce materials from beyond the frost line into the inner regions of the system, thus these inner planets that formed after the migration would be water rich.

They are often found around F and G stars, and less commonly found around K stars. They are very rarely found around M stars. It is even possible for a regular giant planet, orbiting beyond the frost line, to become a hot giant once the main sequence star it orbits becomes a giant star, due to the size and intensity of radiation of a such a giant.

**Gas giants** which are also hot giants are known as **Hot Jupiters** (or Pegasids, after 51 Pegasi b), while hot ice giants are known as **Hot Neptunes**.

### Puffy Planet

**Puffy Planets**, also known as **Hot Saturns** due to their densities being comparable to Saturn’s, are hot jupiters which have incredibly large radii for their mass. Puffy planets have an upper mass limit of 2 MJ, as a planet with more than 2 MJ would have sufficient gravity to counteract the expansion.

One theory explaining the unusual nature of these puffy planets is that the interaction of stellar winds and the planet’s magnetosphere would create an electrical current through the planet that heats it up, causing it to expand.

## Worldbuilding in Practice

> Taggart is a typical hot neptune planet orbiting at 0.06 AU from the K star Ascott. At 0.10 MJ and 0.36 RJ, it is easily visible with the naked eye from Dagny, even during the day. Taggart has no moons and is incredibly hot on both day and night sides. As a result, it is largely ignored by the system's inhabitants in favour of the gas dwarf Galt, despite its easy access from Dagny.
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> Astronomers observing the uncolonised Clytemnestra system noticed a large planet transiting the star every 47 days. The planet has an astounding radius of 1.21 RJ despite weighing only 0.79 MJ. This incredibly low density (0.549 g/cm3), combined with an orbital semi-major axis of 0.34 AU, strongly suggests that this planet, dubbed Clytemnestra-b, is a puffy planet.
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## References

1. ↑ Wikipedia: Hot Jupiter
2. ↑ Martyn J. Fogg, Richard P. Nelson: On the formation of terrestrial planets in hot-Jupiter systems
3. ↑ Wikipedia: Hot Neptune
4. ↑ Adrián Brunini, Rodolfo G. Cionco: The Origin and Nature of Neptune-like Planets Orbiting Close to Solar Type Stars
5. ↑ Kenneth Chang: Puzzling Puffy Planet, Less Dense Than Cork, Is Discovered