What is the current accepted theory for the fate of hot Jupiters?

Question

It is well established that one main feature of many hot jupiters is their close proximity to their parent star, usually the equivalent of being within the orbit of Mercury. So, these planets are gas giants and are very hot (hence their category).

However, a few discoveries have lead to questioning about what is the fate of these planets.

example 1: HD 209458b a.k.a. "Osiris"

According to the NASA page "Dying Planet Leaks Carbon-Oxygen", Osiris is doing more than 'evaporating', it is leaking carbon, oxygen alongside hydrogen in an envelope behind the planet that has been detected from Earth. The significance of carbon and oxygen is stated from the article:

Although carbon and oxygen have been observed on Jupiter and Saturn, it is always in combined form as methane and water deep in the atmosphere. In HD 209458b the chemicals are broken down into the basic elements. But on Jupiter or Saturn, even as elements, they would still remain invisible low in the atmosphere. The fact that they are visible in the upper atmosphere of HD 209458b confirms that atmospheric 'blow off' is occurring.

It is stated in the article that Osiris is likely to become a hypothesised class of exoplanet known as a Chthonian, which is defined in "Evaporation rate of hot Jupiters and formation of Chthonian planets" (Hebard et al. 2003) as

new class of planets made of the residual central core of former hot Jupiters

These would be similar in size to Earth, but considerably denser.

example 2: CoRoT-7b

According to the NASA article "Most Earthlike Exoplanet Started out as Gas Giant", CoRoT-7b is an Earth-sized planet where a hot Jupiter usually is found, they describe it as

is almost 60 times closer to its star than Earth, so the star appears almost 360 times larger than the sun does in our sky," Jackson said. As a consequence, the planet's surface experiences extreme heating that may reach 3,600 degrees Fahrenheit on the daylight side. CoRoT-7b's size (70 percent larger than Earth) and mass (4.8 times Earth's) indicate that the world is probably made of rocky materials.

The high day time temperature means that the star-facing side of the planet is likely to be molten, any tenuous atmosphere is also blasted away. Scientists estimate that many Earth-masses may have been boiled off. It also seems that the decreasing mass is causing the planet to be drawn closer to the star -cause more material to be boiled off, hence the mass to decrease.

To summarise on of the scientists in the article:

You could say that, one way or the other, this planet is disappearing before our eyes,"

The question

As these are only 2 examples of a possible process, the question is, what is the current accepted theory as to the fate of hot Jupiter exoplanets?

Could this also be the reason a hot Jupiter does not exist in our solar system?

Answer 1

This is a fairly loaded question in that it depends heavily on what a "hot Jupiter" actually is defined to be. What is "hot"? What is a "Jupiter"? In reality, there's a continuum of planetary masses and distances from their parent star, and in the literature you'll commonly see references to "hot Neptunes", "hot Saturns," etc.

The predominant theory as to how giant planets form is that they first coalesce from rock and ice beyond the ice line, the distance from the parent star at which water becomes solid. This distance is approximately where Mars lies today in our solar system. What's surprising about "hot gas planets" is that they are found within this ice line, significantly within. This implies that after they formed their cores, they migrated closer to their host stars via some currently undetermined process (for which there are several good candidates, but for now let's assume that the existence of hot planets shows that at least one of these processes operates quite regularly).

And what about the word "hot"? Well, for the planets that are closest to their parent stars, there is known to be a radius anomaly: The radii of these planets are significantly larger than models of giant planet structure irradiated by their host stars would predict. So I would define "hot" planets as gas giants whose radii are larger than what would be predicted by the standard models.

Now that we got some of the definitions out of the way, there's the question of survival. When giant planets are close to their parent stars, they become tidally locked. As a consequence, there is very little energy tidally dissipated on the surface of the giant planet, the shape of the planet is fixed and there are little internal motions. However, the giant planet also raises a tide on its host star as well, and because it takes a lot of angular momentum to change the spin of an object with 1,000 times more mass, the host stars are almost never going to be tidally locked to their closest planet.

The rate at which energy is dissipated within the star is highly uncertain, and this uncertainty is typically swept into a fudge parameter "Q," the quality factor, with lower quality factors reflecting more dissipation. "Q" is measured for certain bodies in our own solar system (i.e. Earth and Jupiter) and in some stellar binaries, but is highly variable from body to body, ranging from about 10 for the Earth to 10^8 for some stars.

Whether a planet survives to be observed today depends on how long the orbital decay time, which is determined by Q, compares to the age of the system. For some systems, such as WASP-12b and WASP-19b, which feature highly inflated hot Jupiters, Q is estimated to be small enough to cause them to fall into their host stars in a surprisingly short time (< 10^7 years).

One other possibility is that the gas surrounding the rock/ice core is blasted away by the tremendous amount of heat deposited into the planet. This leaves you with a relatively low-density planet that's somewhat devoid of iron, as the cores of giant planets form further from their host stars than the rocky planets. There are a few candidate close-in, Neptune-mass objects that may have been produced as a result of them losing the bulk of their atmospheres in this way (Example: GJ3470b).

As for our own solar system, the formation of a hot Jupiter would have likely destroyed the inner solar system as it migrated close to the Sun, owing to the fact that it would violently perturb the inner planets' orbits. Additionally, the Sun would likely be enhanced in metals owing to the accretion of metal-rich material from this giant planet. While it's potentially possible that there was a hot Jupiter in our solar system before the other planets formed, it currently seems unlikely.