Tag Archives: metallicity

Water in exoplanet atmospheres

The Cambridge Institute of Astronomy have put out a press release based on a new paper analysing the water abundance in the atmospheres of 19 exoplanets, 11 of them being WASP planets.

The plot shows the measured water abundance versus the planet’s mass. Welbanks et al state that: “We find a mass–metallicity trend of increasing H2O abundances with decreasing mass”, and also that: “The H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms”.

The press release explains that: “The researchers found that while water vapour is common in the atmospheres of many exoplanets, the amounts were surprisingly lower than expected, while the amounts of other elements found in some planets were consistent with expectations”.

The press release has led to coverage in the Daily Express, Astronomy Now, and Science News, among other sites, accompanied by this graphic:

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.