X-ray mystery shrouds Pluto

X-rays appear to be trickling away from Pluto, even though the dwarf planet has no obvious way of making the high-energy photons, a new study reports.

Given what researchers have learned about Pluto since the New Horizons spacecraft flew by in 2015 (SN: 8/8/15, p. 6), the discovery is surprising. For many planets and comets, X-rays are generated when the solar wind, a stream of charged particles from the sun, runs into neutral gas atoms or magnetic fields from these bodies. But the environment around Pluto isn’t conducive to producing X-rays: the dwarf planet has no measurable magnetic field, its atmosphere is very thin, and it’s losing that atmosphere at rates much lower than expected.

“We naively thought Pluto might be losing its atmosphere at the same rate as [some] comets,” says Carey Lisse, a planetary astronomer at the Applied Physics Laboratory in Laurel, Md. “We knew comets make X-rays, so we hoped that Pluto did, too.” Instead, interactions between the solar wind and a tenuous tail of methane gas hundreds of times longer than Pluto’s diameter might be the culprit, Lisse and colleagues suggest online October 25 on arXiv.org.

Lisse’s team used the Chandra X-ray telescope, once in 2014 and three more times in 2015, to look for Pluto X-rays. Chandra detected just seven photons streaming from Pluto in a total of about two days’ worth of observing time. Though the signal isn’t strong, that’s about six or seven more photons than expected based on New Horizons’ measurements of Pluto’s atmosphere and the solar wind.

“It’s a very puzzling finding,” says Konrad Dennerl, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. “I’m not fully convinced,” he adds. “It’s a very low signal.”

Lisse and collaborators note that the signal appears to follow Pluto across the sky. They detected X-ray photons on four separate occasions. The energy of the photons doesn’t appear to match that of the spurious X-ray noise that peppers the telescope, so the signal appears genuine. Still, Lisse and Dennerl are teaming up to get some Pluto time with another X-ray observatory, the European Space Agency’s XMM-Newton satellite.

“We understand that there’s a bit of skepticism,” Lisse says. “We’re going to do some follow-up with a totally different instrument to verify this.”

X-rays from Pluto aren’t just a quirky detail about this specific dwarf planet. If other bodies in the Kuiper belt, the ring of icy debris just past Neptune’s orbit, have atmospheres, then X-ray observations could help detect them.