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Using ground-based and space-based observations, a team of researchers has been monitoring a difficult-to-see comet carefully. It’s called Comet 323P/SOHO, and it was discovered over 20 years ago in 1999. But it’s difficult to observe due to its proximity to the Sun.

They’ve found that the Sun is slowly tearing the comet to pieces.

Comets with the SOHO designation were discovered with the SOHO spacecraft. The Solar and Heliospheric Observatory (SOHO) is a joint ESA/NASA mission launched in 1995. Its mission is to study the Sun, and though the mission was scheduled to last two years, it’s been operating for over 26 years.

As a by-product of its solar observations, the spacecraft has discovered 4,000 comets. Most of these comets are Sungrazers, a class of comets very close to the Sun. Astronomers think most Sungrazers are chunks of a much larger comet that broke apart. 323P isn’t a Sungrazer; it’s a near-Sun comet. Small comets can completely evaporate in one close approach to the Sun, while larger ones can survive many. For most of them, their small perihelions spell their eventual doom.

Comet 323P/SOHO’s perihelion is only 0.04 astronomical units. Its orbital period is just over four years. In 2020, observations with the Subaru telescope showed that the comet had no cometary features as it approached perihelion. But that changed.

“However, in our post-perihelion observations, it developed a long, narrow tail mimicking a disintegrating cometary debris cloud,” the authors write in their paper.

The paper is titled “The Lingering Death of Periodic Near-Sun Comet 323P/SOHO.” The first author is Man-To Hui from the State Key Laboratory of Lunar and Planetary Science, Macau University of Science and Technology. The others are from institutions in the USA, Germany, Canada, and Taiwan.

Sungrazers have perihelions smaller than Mercury’s. Astronomers think they’re main-belt asteroids or short-period comets that were drawn closer to the Sun via the gravitational influence of giant planets or as a consequence of ancient impacts. Sungrazers usually don’t last very long. Their lifetimes seldom surpass 10 million years, according to astronomers, due to their orbit in the inner Solar System. Not only do they have to contend with the Sun’s gravitational power, they frequently cross the paths of the terrestrial planets.

According to the new paper, 323P/SOHO is flirting with destruction. The researchers identified two 20 meter diameter fragments coming from the comet shortly in March 2021. They also found that the comet lost between 0.1-10% of the mass of its nucleus. It looks like its end is near.

The Lowell Discovery Telescope captured this image of comet 323P/SOHO in February 2021. The debris tail is clearly visible. Image Credit: Hui et al. 2022.
The Lowell Discovery Telescope captured this image of comet 323P/SOHO in February 2021. The debris tail is visible. Image Credit: Hui et al. 2022.

The comet’s nucleus is only about 172 meters (560 ft) in diameter. It rotates quickly, at 0.522 hr., the most rapid rotation of every known comet in the Solar System. According to the authors of this paper, that means that the nucleus has high cohesive strength. That strength might help it survive more gravitational encounters with the Sun.

If it seems strange that the researchers only observed the comet’s debris tail after it left perihelion, that’s because of how difficult near-Sun comets are to watch. At perihelion, the Sun’s brightness renders the comets nearly unobservable. They’re observable in any detail outside of perihelion.

There are far fewer observed near-Sun objects than models show there should be. Part of the reason for that is the difficulty in observing them in the Sun’s glare. But the other reason is that astronomers expect them to disintegrate into millimetre-sized particles due to thermal destruction. Even so, observing them being turned to dust is difficult. There’s not much good-quality observational evidence of them breaking apart.

This study is among the first instances of comets like these being observed with ground-based observatories.

This Hubble Wide-Field Camera 3 image shows two 20m diameter fragments coming from Comet 323P/SOHO, marked A and B. Image Credit: Hui et al. 2022.Did you miss our previous article…

Frontier Adventure

Dark Photons Could Be the Key to Both Dark Matter and the Muon Anomaly.




If dark matter exists, then where are the particles?

This single question threatens to topple the standard cosmological model, known as the LCDM model. The CDM stands for cold dark matter, and according to the model makes up nearly 85% of matter in the universe. It should be everywhere, and all around us, and yet every single search for dark matter particles has come up empty. If dark matter particles are real, we know what they are not. We don’t know what they are.

There are lots of ideas, from WIMPs to axions to sterile neutrinos, and none of them have shown up in our detectors. But one of the problems could be that while dark matter particles are everywhere, their particle mass is much higher than we can detect in our particle accelerators and neutrino observatories. If that’s the case, we may never observe them directly. But we might be able to detect the force that allows them to interact.

In particle physics, each fundamental force has one or more carrier bosons. Electromagnetism has the photon, the strong force has the gluons, the weak force has W & Z bosons, the gravitational force the graviton. Dark matter interacts gravitationally, but it also may interact via a dark force, which should have a carrier boson known as the dark photon.

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A hypothetical dark photon interaction. Credit: APS/Alan Stonebraker

Dark photons turn up in a generalization of the standard model of particle physics. According to theory, they would interact with dark matter similar to the way photons interact with charged particles. But just as the weak force and electromagnetism are connected as the electroweak force, this dark force and electromagnetism would be connected as a kind of electrodark force. What this means is that regular photons and dark photons could mix slightly, allowing dark matter to interact with regular matter very slightly. Although photons have no mass, dark photons would have mass. This means they would only interact over very short distances, and could quickly decay into other particles. Like the gluons of the strong force, we can’t observe them directly, but we can observe how they cause particles to interact. This is where a new study on dark photons comes in.

The authors analyze the dark photon model in two ways. The first is to use experimental data to constrain the physical parameters of dark photons, such as their mass and how strongly they mix with regular photons. The second is to compare a particle physics model with and without dark photons to key experimental results. In general, the study finds that the dark photon model is a better fit than the standard model, but it’s a particularly good fit for an experiment known as the anomalous magnetic moment of the gluon, or g-2.

The muon is a heavier sibling of the electron, and like the electron, it has an electric charge and a magnetic moment, or g-factor. The value of the muon g-factor is almost, but not exactly, equal to 2. The “not exactly” part, g – 2, is one of the most precisely measured values in particle physics. It is also one of the most precisely calculated values in particle theory. And they don’t agree.

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Experiment vs theory for g – 2. Credit: Ryan Postel, Fermilab/Muon g-2 collaboration

Experimentally, g-2 = 0.00233184121. Theoretical calculations put g-2 = 0.00233183620. This is known as the g-2 anomaly and is beyond irksome. If you include dark photon interactions, the theoretical result becomes g-2 = 0.00233183939, which is significantly better. Overall, the dark photon model is preferred over the standard model at 6.5 sigma, which is a very strong result.

All of this is very interesting, but we should add a few caveats. The first is

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Frontier Adventure

We Can’t See the First Stars Yet, but We Can See Their Direct Descendants



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If you take a Universe worth of hydrogen and helium, and let it stew for about 13 billion years, you get us. We are the descendants of the primeval elements. We are the cast-off dust of the first stars, and many generations of stars after that. So our search for the first stars of the cosmos is a search for our own history. While we haven’t captured the light of those first stars, some of their direct children may be in our own galaxy.

The first stars were massive. Without any heavier elements to weigh them down, they needed to be about 300 times that of our Sun in order to trigger nuclear fusion in their core. Because of their size, they went through their fusion cycles rather quickly and lived very short lives. But the supernova explosions signaling their deaths scattered heavier elements such as carbon and iron from which new stars formed. Large second-generation stars also died as supernovae and scattered even more heavy elements. As a result, each generation of stars contained more and more of these elements. In astronomy lingo, we say each generation has a higher metallicity.

Of course, which generation a star is in can be fuzzy. Clearly, the very first stars, forming entirely out of primordial hydrogen and helium are first-generation stars, and stars forming entirely out of the remnants of the first generations are true second-generation stars. But stars form at all different sizes, so it’s quite likely that some massive second-generation stars became supernova before some of the smaller first-generation stars. Many early stars could have formed from mostly first-generation material with a touch of second-generation dust, while others formed mostly from second-generation stars with a sprinkling of first-generation heritage. Stars like our Sun are likely a mix of material from multiple generations.

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The distribution of stars in our galaxy. Credit: NASA, ESA, and A. Feild [STScI]

For modern stars, rather than trying to determine their generation, we categorize them into populations based on their metallicity. A star’s metallicity is taken as the ratio of iron to helium [Fe/He] on a logarithmic scale. Population I stars have an [Fe/He] of at least -1, meaning they have 10% of the Sun’s iron ratio or more. Population II stars have an [Fe/He] of less than -1. The third category, Population III, is reserved for true first-generation stars.

In the Milky Way galaxy, most of the stars in the galactic plane are population I stars like the Sun. They formed much later in the history of our galaxy, and are younger with more metals. Older population II stars are generally found in the halo surrounding our galaxy, or in the old globular clusters that orbit the Milky Way. That makes sense since older stars have had more time to drift out of the galactic plane. Given the evolution of our galaxy, it’s quite likely that some of the population II stars in our halo are truly second-generation stars. But how can we distinguish them from other old stars?

That’s the goal of a new study published on the *arXiv*. It looks at both observations of distant quasars and simulations of population III stars to determine the metallicity of truly second-generation stars. The authors found that while second-generation stars would be rare in the Milky Way halo, some could be lurking there. The key to identifying them is not their abundance of iron relative to helium, [Fe/He], but rather the ratios of carbon and magnesium to iron, [C/Fe] and [Mg/Fe].

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Frontier Adventure

This Exoplanet is Probably a Solid Ball of Metal



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We can’t understand nature without understanding its range. That’s apparent in exoplanet science and in our theories of planetary formation. Nature’s outliers and oddballs put pressure on our models and motivate scientists to dig deeper.

Gliese 367 b (or Tahay) is certainly an oddball. It’s an Ultrashort Period (USP) planet that orbits its star in only 7.7 hours. There are almost 200 other USP planets in our 5000+ catalogue of exoplanets, so Gliese 367 b isn’t unique in that regard. But it’s an outlier in another way: it’s also an ultra-dense planet—almost twice as dense as Earth.

That means it has to be almost pure iron.

“You could compare GJ 367 b to an Earth-like planet with its rocky mantle stripped away.”

Elisa Goffo, lead author, University of Turin.

Astronomers found Tahay in TESS (Transiting Exoplanet Survey Satellite) data from 2021. But new research in The Astrophysical Journal Letters is refining the oddball planet’s mass and radius with improved measurements. It also found two siblings for the planet. The research is “Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b: Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days.” The lead author is Elisa Goffo, a Ph.D. student at the Physics Department of the University of Turin.

Artist illustration of NASA's Transiting Exoplanet Survey Satellite (TESS) observing the heavens. TESS found G 367 b, but only barely. The tiny planet was at the limit of TESS's detection ability. (Credit: NASA's Goddard Space Flight Center)
Artist illustration of NASA’s Transiting Exoplanet Survey Satellite (TESS) observing the heavens. TESS found G 367 b, but only barely. The tiny planet was at the limit of TESS’s detection ability. (Credit: NASA’s Goddard Space Flight Center)

TESS found Gliese 367 b in 2021 when it detected an extremely weak transit signal from the red dwarf star named Gliese 367. The signal was at the limits of TESS detection capability, so astronomers knew it was small, like Earth.

As part of the 2021 effort, the researchers used the High-Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory to determine G 367 b’s mass and density. They determined that the planet’s radius is 72% of Earth’s and its mass is 55% of Earth’s. That means that it was likely an iron planet, the leftover core of a once much larger planet.

Fast forward to now and the new research by Goffo and her colleagues.

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The magnificent Milky Way galaxy is radiant over the ESO’s La Silla Observatory in this image. The ESO 3.6-metre telescope is home to an extrasolar planet hunter called the High Accuracy Radial Velocity Planet Searcher (HARPS), a spectrograph with unrivalled precision. Image Credit: ESO/B. Tafreshi (

They also used HARPS to measure the small planet. This time they used 371 HARPS observations of G 367 b. These results show that the planet is even more dense than the 2021 study found. Instead of 55% of Earth’s mass, this new research reveals that the planet is 63% of Earth’s mass. Its radius also shrank from 72% of Earth’s to 70% of Earth’s.

What it boils down to is that G 367 b is twice as dense as Earth.

How did the planet get
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