Mysterious Fomalhaut b Might Not Be an Exoplanet After All

The famous exoplanet was the first to be directly imaged by Hubble in 2008 but many mysteries surround its identity — so astronomers are testing the possibility that it might actually be an exotic neutron star.

Located 25 light-years from Earth, the bright star Fomalhaut is quite the celebrity. As part of a triple star system (its distant, yet gravitationally bound siblings are orange dwarf TW Piscis Austrini and M-type red dwarf LP 876-10) Fomalhaut is filled with an impressive field of debris, sharing a likeness with the Lord Of The Rings’ “Eye of Sauron.” And, in 2008, the eerie star system shot to fame as the host of the first ever directly-imaged exoplanet.

At the time, the Hubble Space Telescope spotted a mere speck in Fomalhaut’s “eye,” but in the years that followed the exoplanet was confirmed — it was a massive exoplanet approximately the size of Jupiter orbiting the star at a distance of around 100 AU (astronomical units, where 1 AU is the average distance the Earth orbits the sun). It was designated Fomalhaut b.

This was a big deal. Not only was it the first direct observation of a world orbiting another star, Hubble was the aging space telescope that found it. Although the exoplanet was confirmed in 2013 and the International Astronomical Union (IAU) officially named the exoplanet “Dagon” after a public vote in 2015, controversy surrounding the exoplanet was never far away, however.

Astronomers continue to pick at Fomalhaut’s mysteries and, in new research to be published in the journal Monthly Notices of the Royal Astronomical Society, Fomalhaut b’s identity has been thrown into doubt yet again.

“It has been hypothesized to be a planet, however there are issues with the observed colors of the object that do not fit planetary models,” the researchers write. “An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut’s disk.” The research team is lead by Katja Poppenhaeger, of Queen’s University, Belfast, and a preprint of their paper (“A Test of the Neutron Star Hypothesis for Fomalhaut b”) can be found via arXiv.org.

Fomalhaut b was detected in visible and near-infrared wavelengths, but followup studies in other wavelengths revealed some peculiarities. For starters, the object is very bright in blue wavelengths, something that doesn’t quite fit with exoplanetary formation models. To account for this, theorists pointed to a possible planetary accretion disk like a system of rings. This may be the reason for the blue excess; the debris is reflecting more starlight than would be expected to be reflected by the planet alone. However, when other studies revealed the object is orbiting outside the star system’s orbital plane, this explanation wasn’t fully consistent with what astronomers were seeing.

Other explanations were put forward — could it be a small, warm world with lots of planetesimals surrounding it? Or is it just a clump of loosely-bound material and not a planet at all? — but none seem to quite fit the bill.

In this new research, Poppenhaeger’s team pondered the idea that Fomalhaut b might actually be a neutron star either in front or behind the Fomalhaut debris disk and, although their work hasn’t proven whether Fomalhaut b is an exoplanet or not, they’ve managed to put some limits on the neutron star hypothesis.

Neutron stars are the left-overs of massive stars that have run out of fuel and gone supernova. They are exotic objects that are extremely dense and small and, from our perspective, may produce emissions in visible and infrared wavelengths that resemble a planetary body. Cool and old neutron stars will even generate bluer light, which could explain the strange Fomalhaut b spectra.

Neutron stars also produce ultraviolet light and X-rays and, although it is hard to separate the UV light coming from the exoplanet and the UV light coming from the star, X-ray emissions should be resolvable.

So, using observations from NASA’s Chandra X-ray Observatory, the researchers looked at Fomalhaut b in soft X-rays and were able to put some pretty strong constraints on whether or not this object really could be a neutron star. As it turned out, Chandra didn’t detect X-rays (within its capabilities). This doesn’t necessarily mean that it isn’t a neutron star, it constrains what kind of neutron star it could be. Interestingly, it also reveals how far away this object could be.

Assuming it is a neutron star with a typical radius of 10 kilometers, and as no X-ray emissions within Chandra’s wavelength range were detected, this object would be a neutron star with a surface temperature cooler than 90,000 Kelvin — revealing that it is over 10 million years old. For this hypothesis to hold, the neutron star would actually lie behind the Fomalhaut system, around 44 light-years (13.5 parsecs) from Earth.

Further studies are obviously needed and, although the researchers point out that Fomalhaut b is still most likely an exoplanet with an extensive ring system (just with some strange and as-yet unexplained characteristics), it’s interesting to think that it could also be a neutron star that isn’t actually in the Fomalhaut system at all. In fact, it could be the closest neutron star to Earth, providing a wonderful opportunity for astronomical studies of these strange and exotic objects.