An x-ray observatory spots a possible planet in another galaxy

In the 30 years since astronomers discovered the first exoplanets, two methods of searching for them have been most common—looking for periodic redshifts and blueshifts in a star’s wobble that are caused by an exoplanet’s gravitational tugs, and looking for the dip in a star’s brightness when an exoplanet passes in front of it. Those methods account for more than 4800 exoplanets spotted in the Milky Way, all of them less than 3000 light-years from Earth.

Three years ago, Rosanne Di Stefano and Nia Imara, both at the Harvard–Smithsonian Center for Astrophysics, made a bold recommendation: Astronomers should turn their telescopes toward x-ray binary systems to efficiently hunt for planets in certain extreme environments. Each binary consists of a collapsed star—a black hole, neutron star, or white dwarf—that is gravitationally bound to a normal star. The collapsed star accretes plasma from its much larger companion. Spiraling inward through an accretion disk, the plasma reaches temperatures high enough to emit x rays. So long as the distance between the bound stars is less than the so-called Roche lobe, in which tidal forces stream the gas from one star to the other, the x-ray-emitting region is exceedingly compact, on the order of a few Jupiter-sized planets. The dip in the x-ray light curve from a transiting planet against so small a region can be huge, large enough to produce a total eclipse. Planets orbiting ordinary stars typically cast a far smaller shadow from the host star, which makes them more difficult to spot.

Di Stefano, Imara (now at the University of California, Santa Cruz), and their collaborators now report finding what may be a planet orbiting one of the brightest x-ray binaries in Messier 51, known as the Whirlpool galaxy. The binary, whose x-ray spectrum is shown (boxed) in figure 1, its visible spectrum enlarged, resides 23 million light-years from Earth. To find it, the astronomers mined the vast archives of the Chandra X-Ray Observatory. They spotted the features of an exoplanet transit—an abrupt dip and rise in brightness—in a 2012 light curve (figure 2). From their analysis, the accreting object is thought to be either a black hole or a neutron star gravitationally bound to a blue supergiant companion with the luminosity and spectrum of a 20- to 30-solar-mass star. The newly found exoplanet is comparable in size to Saturn and moves around the binary’s center of mass at 17 km/s. According to Kepler’s laws, the exoplanet orbits the center at a distance of about 45 AU with a period of roughly 70 years.

That period would be the longest ever found for an exoplanet. And it may put the exoplanet’s confirmation—either from a repeat transit observation, Doppler spectroscopy, or both—out of reach. Even so, much of the researchers’ analysis is devoted to analyzing the transiting object’s identity. Fortunately, there is much more Chandra data to mine. To move the field ahead, more exoplanet candidates need to be found. (R. Di Stefano et al., Nat. Astron., 2021, doi:10.1038/s41550-021-01495-w .)

Editor’s note, 7 December: The axes and scale bar of Figure 1 have been corrected. We had inadvertently introduced errors when adapting the original figure.