A new paper by Shreyas Vissapragada and colleagues reports a new technique for detecting material boiling off hot-Jupiter exoplanets. The idea is that helium atoms in escaping material should be strong absorbers of light at the wavelength of 1083.3 nm, one of the transitions of neutral helium. Thus, if one records a transit in an ultra-narrow-band filter around that wavelength, the planet should look bigger and so the transit should be deeper.
Vissapragada et al pointed the 200-inch Hale Telescope at a transit of WASP-69b. Here’s the result:
The blue line is the usual transit depth expected in continuum light. The data and fitted red line are the transit observed in the 1083.3-nm helium line. The authors compute that the extra depth of the transit implies that 30 million kilos of material is evaporating off the planet each second, as a result of stellar irradiation. This sounds a lot, but adds up to only a few percent of the planet’s mass over the host star’s lifetime.
Earlier this year helium was found in the outer atmosphere of WASP-107b, the first detection of helium in an exoplanet. Several teams have now used similar techniques to find helium in WASP-69b, HAT-P-11b and HD 189733b, leading to a slew of papers and accompanying press releases from the Instituto de Astrofísica de Andalucía, the University of Exeter and others (see , ,  and ).
Artist’s impression of an escaping envelope of helium surrounding WASP-69b. (Credit: Gabriel Perez Diaz, IAC)
Lisa Nortmann, lead author of the WASP-69b paper, explains that the helium is escaping from the atmosphere, forming a comet-like tail: “We observed a stronger and longer-lasting dimming of the starlight in a region of the spectrum where helium gas absorbs light. The longer duration of this absorption allows us to infer the presence of a tail.”
The press releases have led to extensive coverage including by CNN, the Daily Mail and Tech Times.
The IAA press release includes a video illustration of WASP-69b, created by Gabriel Perez Diaz of the IAC: