Our understanding of the Milky Way just got a little bit sweeter.
For the first time, scientists have spotted sugar in interstellar space, providing an important clue about the origins of sugar on Earth and possibly the rise of life, according to a new paper published on Monday in the journal Nature Astronomy.
“This is a real, bona fide sugar,” said Brett McGuire, an astrochemist at M.I.T. who was not involved in the study. “It’s just incredibly exciting.”
Sugar’s origins on Earth are mysterious. Scientists know it must have been present very early on, because it’s a necessity for life to arise. But lab experiments to recreate the necessary chemical conditions have repeatedly failed to create these molecules.
So how did sugar get here? Scientists think it could have been delivered to Earth by asteroid and comet impacts early in the planet’s history, because several kinds of sugars, including glucose and ribose, have been found on asteroids and meteorites.
But the question of where they came from before that went unanswered.
“People had a lot of interest in trying to find these molecules,” said Izaskun Jiménez-Serra, an astrochemist at the Center for Astrobiology in Spain who led the new study. In the early 2000s, when the search for sugar was spinning up but not yielding results, “I didn’t actually have much hope,” she said.
But in recent years, as researchers detected other large organic molecules in nebulae, she became more optimistic.
The Milky Way’s interstellar medium was a likely spot for sugar. The interstellar medium is all the dust and gases in between solar systems, and despite it having extreme conditions, it’s “an impressive chemical factory,” the authors of the new study wrote. Hundreds of molecules, including some building blocks of the cellular messenger RNA, have been found there. And laboratory experiments suggested that sugars could form from chemical reactions in ices in the interstellar medium.
So if they were going to spot sugar, it would make sense to find it in there.
Dr. Jiménez-Serra and her collaborators used two radio telescopes to peer deep into the center of the Milky Way, collecting data on the radio frequencies the interstellar medium emitted. As molecules in space spin and move, they produce different frequencies. By comparing the patterns of frequencies they found in space to the patterns that molecules produced in labs, the researchers could see what molecules were out there.
At last, they found the sweet spot. One of the patterns from a nebula near the center of the Milky Way matched up with that of a sugar called erythrulose. Erythrulose is made of four carbon atoms, eight hydrogen atoms and four oxygen atoms. It’s found on Earth in raspberries.
“It was this very beautiful match,” Dr. Jiménez-Serra said, adding that when she saw it, “my heart started beating very, very fast.”
It was an exciting moment, but she wanted to be sure it was sugar. The team checked, and checked again, that it wasn’t some other molecule or simply a mistake. But the results held up.
“Their data and analysis support their conclusion that the molecule is there,” Dr. McGuire said. “They went to extraordinary lengths to account for all possible interlopers.” The results also won the approval of Yoshihiro Furukawa, an astrochemist at Tohoku University in Japan who was not involved in the study. His work led to the discovery of sugars on the asteroid Bennu a few years ago.
The new finding confirms that sugar can form without life in the interstellar medium, and even before stars and planets have formed. That’s a critical first step to forming RNA and DNA, and to explaining how life arose on Earth.
It also makes it more likely that life could have formed somewhere else.
“If the interstellar medium is capable of forming these ingredients, it could also be found in other molecular clouds across the galaxy, enhancing the chances for life to develop elsewhere,” Dr. Jiménez-Serra said.
And this sugar is just the beginning. Dr. Jiménez-Serra is eager to look for larger sugars, such as ribose and deoxyribose, which build RNA and DNA.
The researchers estimated that 0.5 to 50 million tons of this sugar could have been delivered to Earth during the critical early part of its life.
Equally interesting is what they didn’t find: a slightly smaller sugar, one with three carbon atoms. Since they found the more complex version, it’s surprising that they didn’t see the simpler version.
“It defies expectations, in some ways, based on the chemistry we understand,” Dr. McGuire said. “I’m looking forward to seeing the community sink its teeth into this and say: ‘Wow, this was weird. This was wild. This was unexpected. What does it mean?’”
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