A new report from the European Southern Observatory (ESO):

Mysterious Neptune dark spot detected from Earth for the first time

Using ESO’s Very Large Telescope (VLT), astronomers have observed a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot adjacent to it. This is the first time a dark spot on the planet has ever been observed with a telescope on Earth. These occasional features in the blue background of Neptune’s atmosphere are a mystery to astronomers, and the new results provide further clues as to their nature and origin.

Large spots are common features in the atmospheres of giant planets, the most famous being Jupiter’s Great Red Spot. On Neptune, a dark spot was first discovered by NASA’s Voyager 2 in 1989, before disappearing a few years later.

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,”

says Patrick Irwin, Professor at the University of Oxford in the UK and lead investigator of the study published today in Nature Astronomy.

Irwin and his team used data from ESO’s VLT to rule out the possibility that dark spots are caused by a ‘clearing’ in the clouds. The new observations indicate instead that dark spots are likely the result of air particles darkening in a layer below the main visible haze layer, as ices and hazes mix in Neptune’s atmosphere.

Coming to this conclusion was no easy feat because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail. The opportunity came after the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere, including one in the planet’s northern hemisphere first noticed in 2018. Irwin and his team immediately got to work studying it from the ground — with an instrument that is ideally suited to these challenging observations.

Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the researchers were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths, and obtain a 3D spectrum [1]. This meant they could study the spot in more detail than was possible before.

“I’m absolutely thrilled to have been able to not only make the first detection of a dark spot from the ground, but also record for the very first time a reflection spectrum of such a feature,”

says Irwin.

Since different wavelengths probe different depths in Neptune’s atmosphere, having a spectrum enabled astronomers to better determine the height at which the dark spot sits in the planet’s atmosphere. The spectrum also provided information on the chemical composition of the different layers of the atmosphere, which gave the team clues as to why the spot appeared dark.

The observations also offered up a surprise result.

“In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,”

says study co-author Michael Wong, a researcher at the University of California, Berkeley, USA. This rare cloud type appeared as a bright spot right beside the larger main dark spot, the VLT data showing that the new ‘deep bright cloud’ was at the same level in the atmosphere as the main dark spot. This means it is a completely new type of feature compared to the small ‘companion’ clouds of high-altitude methane ice that have been previously observed.

With the help of ESO’s VLT, it is now possible for astronomers to study features like these spots from Earth. “This is an astounding increase in humanity’s ability to observe the cosmos.

At first, we could only detect these spots by sending a spacecraft there, like Voyager. Then we gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground,”

concludes Wong, before adding, jokingly:

“This could put me out of work as a Hubble observer!”

Notes

[1] MUSE is a 3D spectrograph that allows astronomers to observe the entirety of an astronomical object, like Neptune, in one go. At each pixel, the instrument measures the intensity of light as a function of its colour or wavelength. The resulting data form a 3D set in which each pixel of the image has a full spectrum of light. In total, MUSE measures over 3500 colours. The instrument is designed to take advantage of adaptive optics, which corrects for the turbulence in the Earth’s atmosphere, resulting in sharper images than otherwise possible. Without this combination of features, studying a Neptune dark spot from the ground would not have been possible.

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An Infinity of Worlds:
Cosmic Inflation and the Beginning of the Universe

A new report from the European Southern Observatory (ESO):

New type of star gives clues to mysterious origin of magnetars

Magnetars are the strongest magnets in the Universe. These super-dense dead stars with ultra-strong magnetic fields can be found all over our galaxy but astronomers don’t know exactly how they form. Now, using multiple telescopes around the world, including European Southern Observatory (ESO) facilities, researchers have uncovered a living star that is likely to become a magnetar. This finding marks the discovery of a new type of astronomical object — massive magnetic helium stars — and sheds light on the origin of magnetars.

Despite having been observed for over 100 years, the enigmatic nature of the star HD 45166 could not be easily explained by conventional models, and little was known about it beyond the fact that it is one of a pair of stars [1], is rich in helium and is a few times more massive than our Sun.

“This star became a bit of an obsession of mine,”

says Tomer Shenar, the lead author of a study on this object published today in Science and an astronomer at the University of Amsterdam, the Netherlands.

“Tomer and I refer to HD 45166 as the ‘zombie star’,”

says co-author and ESO astronomer Julia Bodensteiner, based in Germany.

“This is not only because this star is so unique, but also because I jokingly said that it turns Tomer into a zombie.“

Having studied similar helium-rich stars before, Shenar thought magnetic fields could crack the case. Indeed, magnetic fields are known to influence the behaviour of stars and could explain why traditional models failed to describe HD 45166, which is located about 3000 light-years away in the constellation Monoceros.

“I remember having a Eureka moment while reading the literature: ‘What if the star is magnetic?’,”

says Shenar, who is currently based at the Centre for Astrobiology in Madrid, Spain.

Shenar and his team set out to study the star using multiple facilities around the globe. The main observations were conducted in February 2022 using an instrument on the Canada-France-Hawaii Telescope that can detect and measure magnetic fields. The team also relied on key archive data taken with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at ESO’s La Silla Observatory in Chile.

Once the observations were in, Shenar asked co-author Gregg Wade, an expert on magnetic fields in stars at the Royal Military College of Canada, to examine the data. Wade’s response confirmed Shenar’s hunch:

“Well my friend, whatever this thing is — it is definitely magnetic.”

Shenar’s team had found that the star has an incredibly strong magnetic field, of 43 000 gauss, making HD 45166 the most magnetic massive star found to date [2].

“The entire surface of the helium star is as magnetic as the strongest human-made magnets,”

explains co-author Pablo Marchant, an astronomer at KU Leuven’s Institute of Astronomy in Belgium.

This observation marks the discovery of the very first massive magnetic helium star.

“It is exciting to uncover a new type of astronomical object,” says Shenar, ”especially when it’s been hiding in plain sight all along.”

Moreover, it provides clues to the origin of magnetars, compact dead stars laced with magnetic fields at least a billion times stronger than the one in HD 45166. The team’s calculations suggest that this star will end its life as a magnetar. As it collapses under its own gravity, its magnetic field will strengthen, and the star will eventually become a very compact core with a magnetic field of around 100 trillion gauss [3] — the most powerful type of magnet in the Universe.

Shenar and his team also found that HD 45166 has a mass smaller than previously reported, around twice the mass of the Sun, and that its stellar pair orbits at a far larger distance than believed before. Furthermore, their research indicates that HD 45166 formed through the merger of two smaller helium-rich stars.

“Our findings completely reshape our understanding of HD 45166,”

concludes Bodensteiner.

Notes

[1] While HD 45166 is a binary system, in this text HD 45166 refers to the helium-rich star, not to both stars.

[2] The magnetic field of 43 000 gauss is the strongest magnetic field ever detected in a star that exceeds the Chandrasekhar mass limit, which is the critical limit above which stars may collapse into neutron stars (magnetars are a type of neutron star).

[3] In this text, a billion refers to one followed by nine zeros and a trillion refers to one followed by 12 zeros.

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