Hello Nature readers,
Today we are boggled by signs of a ‘forbidden’ black-hole collision, hear that researchers are trying to hoodwink peer-reviewers’ AI tools, and discover the rarest blood group on Earth.

Organoids: now with blood vessels

Researchers have made heart, gut, liver and lung organoids that can grow their own blood vessels. Most organoids lack the vessels necessary to transport blood, oxygen and nutrients, which has restricted their size, function and ability to mature. The research teams grew vascularized organoids by coaxing stem cells to form vessels as the rest of the tissue was growing, rather than trying to add blood vessels later as previous attempts have.

Biggest black hole collision ever detected

Scientists have tracked a gravitational-wave signal to a surprising source: the most massive black hole merger ever detected. “Black holes this massive are forbidden through standard stellar evolution models,” says astrophysicist Mark Hannam. Not only that: the celestial objects were also spinning at speeds close to the limit allowed by Einstein’s theory of general relativity. “It presents a real challenge to our understanding of black hole formation,” says Hannam.

Papers contain messages for AI’s eyes only

Researchers have been sneaking secret messages into their papers in an effort to trick artificial intelligence (AI) tools into giving them a positive peer-review report. Nature has independently found 18 computer science preprint studies containing such hidden messages — in which text is specifically tailored to manipulate large language models (LLMs). Many publishers ban the use of AI in peer review, but there’s evidence that some researchers do use LLMs to evaluate manuscripts or help draft review reports. This creates a vulnerability that others now seem to be trying to exploit, says forensic metascientist James Heathers.

‘Gwada-negative’ is a whole new blood type

Many of us know our blood types as A, B, AB or O, but there are 47 officially recognized blood-group systems — or at least there were. Now there are 48, write transfusion-medicine researchers Martin Olsson and Jill Storry. A woman in Guadeloupe has blood that has been characterized as ‘Gwada-negative’; and because she’s the only person known to have it, she can’t receive a blood transfusion from anyone. The new category, like the other 47, reflects differences in the proteins and other molecules coating the red blood cells.

What we’re still learning from Pluto

Ten years ago, New Horizons became the first, and only, spacecraft to take a close look at Pluto. “Since the flyby, we have basically rewritten the textbooks,” says planetary scientist Kelsi Singer. Among other discoveries, the mission revealed that the dwarf planet is graced with a remarkable heart-shaped feature partially filled with a sea of mysterious polygons. New Horizons is still going strong, gathering data on the outer Solar System — but the mission is threatened by drastic cuts to NASA proposed by the administration of US President Donald Trump.