It's astounding to think we can detect stars and planets hundreds of light-years away with some confidence. Unfortunately, at those distances, the science can get a little imprecise, as is the case with HD131399Ab — a supposed exoplanet 320 light-years from Earth. Discovered last year, the planet had some unusual properties that weren't easily explainable. Well, they can now: HD131399Ab isn't a planet — it's a star, one not even in the HD131399 system.
When Gizmodo interviewed Kevin Wagner, the lead author on the paper that revealed the existence of HD131399Ab, back in July 2016, here's how he described it:
"I'd venture to say this is the weirdest orbit of any exoplanet we've ever found ... We know of no other planet in a configuration like this."
Unsurprisingly, HD131399Ab's interesting nature demanded additional study, the results of which were published in November last year. Turns out the planet isn't one at all, with a fresh analysis suggesting it's a background star.
Two things gave it away. First of all, using better data, researcher were able to narrow the object's spectral type to a "K or M dwarf". Secondly, if the object was orbiting HD131399A, it "would exceed escape velocity given the mass and distance" from the star.
We shouldn't be so hard on the original discoverers, however. The paper goes on to state that "a combination of multiple unlikely factors led to the conclusion" of a planet, rather than a star:
A speckle in the third SPHERE dataset, combined with the SPHERE YJH spectral coverage ending at ∼ 1.64 µm, mimicked a Tlike spectrum in the H band. The apparent magnitude of the background star was consistent with a planet of the inferred spectral type and age of the system ... Further, the background object was one third of the way between HD 131399 Ab and HD 131399 BC, at the outer limit of predictions for stable orbits of planets in multiple systems.
Basically, the object was "almost tailor-made to pass the standard tests performed in the analysis of direct imaging data".