Radio observations have turned up evidence of a cloud of hot plasma near — or even in — the solar system. But its distance estimate is still up for debate.
Radio astronomers have detected a cloud of hot plasma at the edge of the solar system, and no one has a clue about its nature or origin.
Researchers didn’t observe the mystery cloud directly. Instead, they saw its effect on the radio waves of a background quasar known as J1402+5347. Its presence caused the quasar’s light to flicker on a timescale of minutes. “It’s a spectacular twinkler, and a very nice surprise,” says Mark Walker (Manly Astrophysics, Australia), who studies intra-hour variability of quasars but wasn’t involved in the new work.
Until recently, astronomers knew of three rapidly twinkling quasars, whose scintillations suggested the presence of plasma clouds at distances of a few tens of light-years. But the new cloud is much closer, says study lead Tom Oosterloo (ASTRON Netherlands Institute for Radio Astronomy).
Interstellar radio scintillation produces a speckle pattern, comparable to the speckle pattern in the optical image of a star that results from atmospheric turbulence. As Earth moves through that pattern, astronomers observe rapid brightness variations. “From the observed twinkle frequency of J1402+5347,” Oosterloo says, “we deduce a distance of just 0.8 light-years for the plasma screen, which is within the solar system’s Oort cloud.”
The discovery is the first published scientific result obtained with the new wide-angle Apertif receivers (Aperture Tile In Focus) on ASTRON’s venerable 14-dish Westerbork Synthesis Radio Telescope. Apertif increased the instrument’s field of view by a factor of 40, making it much easier to find rare sources.
The plasma cloud, thought to be at least a few billion kilometers (several astronomical units) across, could be an interloper from interstellar space. Or it could be some kind of remnant from the formation of the solar system, says Oosterloo. “That would be really exciting.”
However, Walker isn’t so sure about the cloud’s estimated proximity, which is based on a simple, idealized model. Each of the previously known twinkling quasars appeared to be associated with a nearby hot star (Vega, Spica, and Alhakim or Iota Centauri, respectively), suggesting that the intervening plasma was somehow expelled by the star.
As Oosterloo and his colleagues discuss in their Astronomy & Astrophysics paper, the new quasar’s sky position is close to Alkaid (Eta Ursae Majoris), which is just over 100 light-years from Earth. “It’s very unlikely that we’d get a random coincidence between [a twinkler] and a hot star like Alkaid,” says Walker. “The chances are less than one in 1,000 for a match as close as this — so my guess is that they are indeed physically related.”
But according to Oosterloo, that would imply an improbably compact and extremely elongated plasma cloud. Moreover, since the team serendipitously found the new intra-hour variable quasar in April 2019, they’ve discovered 10 more of these rare sources that do not seem to be associated with nearby stars, though the intervening plasma appears to be many light-years away in these newer cases.
Radio astronomer Hayley Bignall (CSIRO, Australia), who was also not involved in the new study, says she would “naively agree with the arguments the authors make for a very nearby plasma screen. On its own, I would say the case for the very small distance is compelling, but not 100% convincing.” Follow-up observations at other wavelengths and the study of additional twinkling quasars would help strengthen or refute the claim, Bignall says.