# Hot Jupiters

## What Are Hot Jupiters?

Hot Jupiters are a class of extrasolar planets whose mass is equal to or greater than the mass of Jupiter and whose orbit is about 0.015 to 0.5 AU away from their star14.

Artist's impression shows a gas-giant exoplanet transiting across the face of its star
Credit: ESA/C. Carreau15

## Discovery

In 1995, Michel Mayor and Didier Queloz discovered the first planet and hot Jupiter around a sun-like star using the radial-velocity technique. They used the spectrum of the star 51 Pegasi to detect periodic Doppler shifts caused by the planet's gravitational tug on the star16 . This technique is biased towards finding hot Jupiters around less massive stars. Hot Jupiters have an orbital period of a few days and are much easier to detect than Earth-size planets very far from their stars8 .

In 2006 the European Space Agency launched the COROT spacecraft, which was the first satellite used to search for extrasolar planets. COROT has discovered 24 extrasolar planets. NASA also launched a spacecraft called Kepler in 2009. So far, Kepler has discovered 25 confirmed planets and 1,235 candidates17. Both of these missions use the transit method. The transit method relays on the planet passing in front of its star, blocking a very small proportion of the starlight, which can be detected8 .

As of November 12, 2011, 697 extrasolar planets have been confirmed, about 415 of which are hot Jupiters18.

## Formation of Hot Jupiters

According to the theory of solar system formation, massive Jupiter like planets can only form in the cold outer regions of the star system and have nearly circular orbits. Hot Jupiters on the other hand are massive jovian planets that are close-in and have highly elliptical orbits8.

One possible explanation is planetary migration; that is hot Jupiters are formed in the outer regions of their solar system and then migrate inward. This migration is caused by the propagation of waves through the gaseous disk around the young planet. Young jovian planets get massive enough that their gravity is strong enough to clear its orbital path of the gas disk. As a result, the planet creates waves that propagate through the disk, which causes the material to bunch up. Then this bunched up material gains enough gravitational force that it reduces the planet's orbital energy causing the planet to migrate inward.8

Image from a numerical simulation of the interaction of a massive planet with a gaseous protoplanetary disk19

Another theory to explain the highly eccentric orbits of hot Jupiters is gravitational perturbation. Gravitational perturbation is when two young jovian planets have a close gravitational encounter. This counter can send one planet out of the star system while the other planet is sent inward towards the star into a highly elliptical orbit.8

## Characteristics Due to Location

 Hot Jupiters are very close to their stars, so they are receiving very intense levels of sunlight causing their cloud-top temperature to be much warmer then Jupiter's8. The flux of sunlight a planet is receiving is inversely proportionally to the square of distance separation. The closer the planet is the greater the flux is and the greater the intensity level of sunlight. Intensity is proportional to the fourth power of temperature, therefore the greater the intensity of sunlight the greater the temperature will be10. Jupiter has a cloud-top temperature of 130K while hot Jupiters' cloud-top temperatures can be up to 1300K8. Intensity of starlight is inversely proportionally to the square of the distance20

These high temperatures of hot Jupiters can affect the composition of the clouds. Jupiter's clouds are made up of ice flakes of ammonia and water because the cloud-top temperature of Jupiter is low enough for this compounds to condense. Hot Jupiters, however, are too hot for these gases to condense. Rocks can only condense at these high temperatures, therefore the clouds of hot Jupiters are made up of rock dust.

Also, the high intensity of solar heat makes Hot Jupiters puff up. The intense heat will cause the planet's atmosphere to inflate, resulting in a larger radius and lower density8. Hot Jupiters' upper atmospheres can extend beyond three times the radius of the planet. The expansion of the atmosphere is due to the hydrodynamic state, where the gas has a velocity upwards, which is caused by the intense heating of the upper atmosphere21,22. These high temperatures also give gas particles very high kinetic energy, which is given by

Kavg=(3/2)kT
where T is the temperature of the gas and k is Boltzmann constant10. As temperature increases, the kinetic energy increases. Moreover the velocity of the particles increases, which is given by
$V_{avg}=\sqrt{\frac{8kT}{\pi M}}$
Also, as the altitude increases, the temperature increases, meaning that the upper atmosphere may not be hydrostatically stable and the velocity of the particles is larger then the escape velocity, which is given by
$V_{escape}=\sqrt{\frac{2GM}{\ R}}$
where R is the radius, G is the gravitational constant and M is the mass of the particle10,22. Therefore the hydrogen gas in the atmosphere flows upward and escapes causing a blow-off situation, where the light gases drags the heavier gases to escape with it. This blow-off effect is maintained by constant energy input from the star. The closeness to the star also alters the shape of the gravitational field as the altitude increases from spherical to an elongated shape. This elongated shape allows more gas to escape making the escaping components resemble a comet tail that is moving away from the planet and the star23.

Computer simulation of the evaporating extra-solar planet HD209458b. The cometary planet orbits around the parent star. The absorption of the stellar emission during transit is seen at the bottom-right panel.
Credit: Alain Lecavelier des Etangs, IAP, France23

It is expected that hot Jupiters are affected by strong tidal forces because they are so close to their stars. Tidal forces occur when the gravitational force on a side of a large object is stronger than the other side causing the object to stretch. As hot Jupiters orbit around their stars, the strength and direction of tidal forces change due to the elliptical orbit. This variation causes the planet to be flexed in different directions generating much friction inside it. This friction tends to warm up the planet. Tidal forces have also locked hot Jupiters and their stars into synchronous rotation. A synchronous rotation is when an planets's rotation period and orbital period are equal, therefore the same side of the planet is always facing the sun. This means that hot Jupiters are rotating rapidly because their rotational period is only a few days long 8.

The action of the Earth-moon tidal force24

It is also predicted that tidal forces are so strong that they are destroying the planets. Tidal forces on hot Jupiters are so strong due to their very close distance from their stars, causing the drag to reduce the orbital energy of the planet and then the planet moves closer to the star. As a result, the tidal forces get stronger causing more orbital energy of the planet to be lost until the planet crashes into the star or is torn apart by the star's gravity.25

Artrist's impression of a star devouring a planet
Credit: NASA, ESA, and G. Bacon 26
Last Updated: November 14,2011