This Week in Astronomy: The Youngest Planet We Have Ever Found
It has been a good week for results that make you rethink a timeline: a planet so young it is still being built, a tidy explanation for one of Webb's strangest discoveries, a star that died and came back showing us its next act, and simulations explaining why Venus sits alone.
The youngest planet we have ever found
The record for the youngest known exoplanet has just been broken, and not by a little. Elias 2-24 b is less than a million years old. The previous record was a four way tie between the two planets around PDS 70 and the two around WISPIT 2, all of them comfortably past five million years. On a stellar timescale, that is the difference between a newborn and a toddler.
The planet has roughly the mass of Jupiter and orbits about 55 times further from its star than Earth does from the Sun. Its host lies around 450 light years away, in one of the nearest star forming regions to us, in the direction of Ophiuchus. What makes it special is not just its age but its setting: it sits inside a gap in the dusty disc that still surrounds the star, and it is still actively pulling material in. We are watching planet formation rather than inspecting the finished product.
The discovery came out of the archive rather than a new observing run. Andrea Bernardi, a doctoral candidate at Universidad Diego Portales in Chile, reprocessed Keck Observatory coronagraph images from 2018 and found a signal nobody had been confident enough to call a planet at the time. Keck data from 2020, observations from the Very Large Telescope and supporting evidence from ALMA turned a maybe into a yes. As co-author Lucas Cieza put it, the object "sits right at the limit of what current technology can detect."
The wider point matters more than the record. Astronomers have long assumed the gaps carved into protoplanetary discs are the fingerprints of forming planets, but assuming is not the same as seeing. Catching one in the act tightens that link considerably, and it pressures formation models that already struggled to build a Jupiter this quickly. If you have ever imaged the Rho Ophiuchi region with Telescope Live, you have pointed a camera at the neighbourhood where this is happening.
Reference: Newfound "Baby" Planet Smashes Record for Youngest Known World, NASA Science. Published in The Astrophysical Journal Letters.
What Webb's little red dots might actually be
Since Webb started returning deep field data, one class of object has refused to behave: small, extremely red sources in the early Universe that look, confusingly, a bit like growing black holes and a bit like enormous stars. They picked up the nickname "little red dots", and a great deal of argument.
A team led by Sunmyon Chon at the Max Planck Institute for Astrophysics may have an answer, and it came from a computer rather than a telescope. Using Japan's ATERUI III supercomputer, they ran cosmological simulations following gas from galactic scales down to the point of collapse. In their models, intense far ultraviolet light from neighbouring galaxies floods a young gas cloud and suppresses ordinary star formation. The gas cannot fragment into normal stars, so it does something rarer: it collapses into a single supermassive star, which promptly becomes a black hole seed of around a million solar masses.
These seeds do not then sit and wait. Wrapped in dense gas and pushed by the surrounding radiation, they feed at rates well above what we normally think possible, growing dozens of times faster than black holes can today, and the synthetic spectra drawn from the simulations look remarkably like the little red dots Webb has been finding.
If it holds up, it solves a real problem. Supermassive black holes of millions to billions of solar masses exist less than 600 million years after the Big Bang, and building them from ordinary stellar remnants in that window has always been awkward. A seed born large and growing fast sidesteps the difficulty. The honest caveat: this is a simulation matching observations, not an observation confirming a simulation.
Reference: Japanese Supercomputer Explains Webb's Little Red Dots, NAOJ. Published in Nature.
A star that died, came back, and is now a Wolf-Rayet
In 1996 the Japanese amateur astronomer Yukio Sakurai spotted a new object in Sagittarius. It turned out to be far stranger than a nova: a white dwarf, a stellar corpse, reigniting. The star had undergone a very late thermal pulse, a helium shell flash that reheats a dying star and sends it backwards through its own evolution. Only a handful of these "born again" stars are known.
Thirty five years on, a team led by W. Marcolino of the Observatório do Valongo in Rio de Janeiro, with Albert Zijlstra of Jodrell Bank among the co-authors, used the FORS2 spectrograph on the Very Large Telescope to check on it. Sakurai's Object has emerged from the dust that was hiding it, and its spectrum now shows carbon and helium emission lines in a fast stellar wind. It has become a [WC] star, a carbon rich member of the Wolf-Rayet class, with a surface temperature between roughly 27,000 and 36,000 kelvin.
This is what makes the object so valuable. Stellar evolution normally plays out over millions of years, and we study it by taking a census of stars at different stages and inferring the sequence. Sakurai's Object is running that sequence on a human timescale, in front of us. The team found it sits at an earlier stage than the comparable star V605 Aquilae, and that the data favour models built on lower mass remnants over those suppressing convective mixing. Small details, but the sort that calibrate how a Sun-like star ends.
Reference: The emergence of a [WC] star in Sakurai's object, Monthly Notices of the Royal Astronomical Society.
Why Venus has no moon
Venus is nearly Earth's twin in size and mass, and it has no moon at all. Mercury has none either, but Mercury is small and close to the Sun. Venus is the odd one out, and Stephen Kane at the University of California, Riverside has now published a careful look at whether it could ever have kept one.
Kane simulated hypothetical moons ranging from half to ten times the mass of our own and tracked how tides would move them. The answer was consistent to the point of surprising him. Because Venus turns so slowly, taking 243 Earth days for one rotation, tidal forces drag a satellite inward rather than pushing it out as Earth does with the Moon. The moon spirals in and is torn apart by Venusian gravity, somewhere between about 30 million and 1.7 billion years after formation. Larger moons meet that fate faster, not slower.
There is one escape route. Had an early giant impact left Venus spinning quickly, with a day shorter than roughly 12 hours, a lunar mass moon could have survived for billions of years. That absence is therefore evidence about Venus itself, hinting its collision history did not leave it spinning fast. None of this shows Venus ever had a moon, only that if it did, the moon was doomed, and that one giant impact explains the empty sky above Venus without inventing extra catastrophes.
Reference: New Study Explains Why Venus Has No Moon, Sci.News. Published in The Astrophysical Journal.
Until next week
A planet younger than recorded human history, a black hole seed born a million suns heavy, a corpse that came back as a Wolf-Rayet, and a moon that never stood a chance. Clear skies, and see you in seven days.