An X-ray binary is formed when a neutron star or black hole is orbited by a much larger, less dense, massive star. The material accretes onto the dense stellar remnant, heats up and ionizes, and emits X-rays. A recent dip in the X-ray flux from a region in the galaxy M51 is suggestive of a transiting exoplanet, but the evidence isn’t sufficient to draw such a dramatic conclusion. (Credit: NASA/CXC/M. Weiss)

No, we probably didn’t detect our first planet in another galaxy

Named M51-ULS-1b, it’s certainly a curious astronomical event. But the evidence is far too weak to conclude “planet.”

Over the past 30 years, one of the biggest revolutions in astronomy has been the discovery of enormous numbers of planets beyond our own solar system. We assumed, based on what we observed in our own backyard, that planets were common around stars beyond our own, but we didn’t know anything about them. Were all solar systems like our own, with inner, rocky planets and outer, giant ones? Did stars of different masses house different types of planets? Were there planets out there with masses smaller than Mercury, larger than Jupiter, or in between the rocky and gas planets we have here at home?

Since that time, our understanding of what’s out there has morphed from speculative and theoretical into one with enormous amounts of observational evidence pointing the way towards answers. Of the nearly 5,000 planets that have been detected and confirmed, however, almost all of them are relatively close by: just a few hundred or thousand light-years away. While it’s always the case that the easiest planets to find are the ones we find most abundantly at first, we’ve seen some rarities as well. In a new study just announced in…