This week in astronomy: shattered planets, cosmic hydrogen and a newborn magnetar
A quiet week for rockets turned out to be a loud one for results. Four pieces of work landed between 28 September and 1 October, and between them they span most of the subject: the violent end of baby planets, the radio hum of hydrogen in the young cosmos, the probable birth of a magnetar, and the rocky heart of a dwarf planet past Neptune.
Webb assembles the first proper catalogue of planetary demolition
Rocky planets are built by collisions, and the last few are enormous. Our own Moon is thought to be the wreckage of an impact between the young Earth and a Mars-sized body, some 100 million years after the Sun formed. The trouble with testing that picture is that it happened once, here, four and a half billion years ago. What astronomers want is a sample.
A team led by Kate Su of the Space Science Institute in Boulder has now built one. Writing in The Astrophysical Journal, they present mid-infrared spectra of 21 extreme debris disks: systems carrying unusually large quantities of warm dust close in to their star, in the zone where rocky planets sit in our own system. Sixteen were observed with the James Webb Space Telescope, twelve spectroscopically for the first time, with five more from the Spitzer archive.
The spectra split the sample cleanly in two. Roughly a third are rich in silica, the glassy material you get when rock is vaporised and then condenses again in a hurry, which points to genuinely violent impacts between bodies the size of Mars. The remaining two thirds are silica-poor, suggesting gentler encounters between smaller, Moon-sized objects that cracked and ground each other down rather than melting outright.
The ages are the interesting part. Silica-rich disks turn up only around stars younger than about 300 million years, while silica-poor ones persist across a far wider span of ages. Read as a sequence, that is the closing act of planet building: a short, brutal phase of giant impacts, then a long drizzle of lesser collisions. It is the closest thing yet to watching our own Moon-forming impact play out elsewhere.
Reference: NASA's Webb Provides Crash Course on Planet-Shattering Collisions, NASA, 1 October 2026.
CHIME maps the cosmos using hydrogen alone
Neutral hydrogen makes up most of the ordinary matter in the Universe, and it radiates at a wavelength of 21 centimetres. Map the sky in that signal and you have a wholly independent way to trace how the expansion rate has changed. The trouble is that the signal is desperately faint and the foregrounds are deafening, so for years the only way to dig it out was to cross-correlate with a catalogue of galaxy positions from optical surveys.
That crutch has now been kicked away. A team led by Arnab Chakraborty at the University of Toronto, working with the CHIME collaboration, reports in The Astrophysical Journal the first detection of the cosmological 21 centimetre signal in CHIME's own data, with no external survey required. The measurement rests on 94 nights of observing from 2019 with the fixed cylindrical array near Penticton in British Columbia, and it probes the era when the Universe was around five billion years old, when only about two per cent of hydrogen remained neutral and atomic.
Mark Halpern of the University of British Columbia called it "a completely new technique for probing the cosmos", and the phrase is fair: a standalone detection means the method can be pushed into epochs where no galaxy catalogue exists. CHIME has nearly seven years of data in hand, and the collaboration expects to reach back to when the Universe was about three billion years old. If the technique holds up, intensity mapping becomes a third pillar alongside supernovae and galaxy clustering for testing whether dark energy is really constant.
Reference: Canadian telescope directly maps earliest glow of hydrogen, opening a new window on the universe, University of British Columbia, 28 September 2026.
A ten-minute X-ray flash, and probably a magnetar
When two neutron stars merge, the usual signature is a brief gamma-ray burst followed by a fading glow. What gets left behind is harder to pin down. Sometimes the merged object collapses straight into a black hole; sometimes, if it is light enough and spinning fast enough, it survives a while as a magnetar, a neutron star whose magnetic field dominates everything around it. Telling those endings apart is hard, because the candidates fade before they give anything away.
An event caught by the Einstein Probe satellite may have broken that pattern. A team led by An Li at Beijing Normal University reports in Science Bulletin a gamma-ray burst lasting about half a second, followed by a prompt X-ray flash that ran on for nearly ten minutes, the longest ever recorded from a neutron star merger. Follow-up with the Very Large Telescope, using X-Shooter and FORS2, placed the host at a redshift of 0.6610, so the light has been travelling more than six billion years, and showed absorption from iron and magnesium.
Two things make the merger interpretation stick. No supernova appeared, which rules out the collapse of a single massive star, and the X-ray flash lasted far too long to be powered by the debris alone. Something was still injecting energy, and a newborn magnetar, spinning down and pouring its rotational energy into its surroundings, fits neatly. The authors call the evidence compelling rather than conclusive, and they are right to: earlier candidates faded before giving a decisive answer. Even so, it is the clearest look yet at the moment a magnetar is made.
Reference: Astronomers link mysterious cosmic flashes to collisions of dead stars, EurekAlert, 30 September 2026.
Haumea turns out to have a rocky heart
Haumea is one of the stranger objects in the solar system: a dwarf planet beyond Neptune, spun so fast that it has been stretched into something like a rugby ball, attended by two moons and a thin ring. Those moons, Hi'iaka and Namaka, are a gift, because the way their orbits shift over time depends on how Haumea's mass is arranged inside.
Benjamin Proudfoot and colleagues have exploited exactly that, in a paper accepted by The Astrophysical Journal Letters and posted on 28 September. Using two decades of Hubble Space Telescope astrometry, they fitted the satellite masses and Haumea's dynamical oblateness, the quantity known as J2, simultaneously rather than assuming one to extract the other. The answer is a dense rocky core beneath an ice-rich mantle. Haumea is differentiated: rock sank, ice floated, and the body sorted itself out early.
The authors are frank about the limits: the data cannot fix the number of internal layers or the density of each. The implication is what carries weight. If Haumea differentiated, dense rocky cores are probably common across the trans-Neptunian dwarf planets, and buried liquid water out there becomes rather more plausible than it looked a week ago.
For anyone tempted to go looking, Haumea sits at around 17th magnitude: never more than a slow-moving point of light in a patient remote imaging run, but within reach, which is a pleasing thought for a world with a rocky heart and a ring.
Reference: The Dense, Rocky Core of Haumea Revealed by Satellite Dynamics, Proudfoot et al., arXiv, 28 September 2026.
Four results, four very different scales, from a dwarf planet's interior to the expansion of the Universe. Clear skies until the next one.