Tag Archives: irradiation

Hot Jupiter irradiation and the efficiency of heat recirculation

Here’s an interesting plot from a new paper by Michael Zhang et al.

The x-axis is the irradiation temperature for a sample of hot-Jupiter exoplanets; that is, how blasted the day-side of their atmosphere is by irradiation from the host star. This depends on the temperature of the star, its size, and the closeness of the orbit.

The heat of the day side of the planet is then transported to the night side by winds (the planets are phase-locked, so the same side always faces the star). The efficiency of this re-circulation of heat then determines whether the hottest regions of the planet are directly facing the star, or whether they are offset by some angle. This angle can by measured by looking at the “phase curve” radiation in the infra-red.

The y-axis then shows the observed offset angle as a function of the irradiation. The plot shows that the offset angle appears to be highest for cooler planets, and then decreases as irradiation increases, but then perhaps increases again for the very hottest planets such as WASP-33b.

There is, however, also a lot of scatter in the plot. The authors speculate that this might result from differing metallicities of the planets, which affects how well they form clouds, which can then determine the albedo of the planet, and thus how much irradiation is simply reflected.

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What happens to short-period hot-Jupiter planets?

Hot-Jupiter planets close to their host star will arouse tides in the host star, and the gravitational pull from tidal bulges will cause the planet to gradually spiral inwards. What happens to them? One possibility is that they end up spiralling into the star and are engulfed.

Another possibility is that strong irradiation from the star blasts off the planet’s atmosphere. Over time, all that would be left might be the small, rocky core of the original Jupiter-size gas giant. Maybe, then, the small, rocky, Earth-size planets seen by Kepler at ultra-short orbital periods are the remnants of hot Jupiters?

The figure shows the planetary radius versus the orbital period for a sample of Kepler planets. The Earth-size, ultra-short-period planets are in red, the hot Jupiters are in orange.

Planet radius versus orbital period for Kepler planets

Now, a team led by Joshua Winn of Princeton has tested this idea. The looked at the host stars of both the small, rocky ultra-short-period planets and of the hot Jupiters, and measured their metallicity (the fraction of elements heavier than hydrogen and helium, for which the iron abundance, denoted [Fe/H], is a good proxy).

They ended up with the following figure, which shows the metallicity distribution for the small, rocky planets (red histogram), for medium, sub-Neptune-size planets (blue histogram) and for hosts of hot Jupiters (orange histogram).

Metallicity distribution for hot Jupiter hosts versus hosts of small, rocky, ultra-short-period planets

The distribution for the rocky-planet hosts is significantly different from that for the hot-Jupiter hosts, showing that they cannot be part of the same population. This means that it is unlikely that hot Jupiters turn into rocky, ultra-short-period planets. Such planets might, however, be descended from hot Neptune-sized planets, for which the host-star metallicity does have the right distribution.