Our solar system is our home base in space and the only place in the universe that we know contains life. Yet many key questions about its origins remain unresolved.
The signature of the radioactive isotope aluminum-26 in ancient meteorites has perplexed scientists for decades, for example, because it hints that our system might have been jump-started by nearby star explosions or energetic massive stars that showered this unstable isotope into our nascent home, profoundly affecting its development.
Now, scientists have shown that a star-forming region in the constellation Ophiuchus appears to be experiencing the same radioactive enrichment of aluminum-26, offering a glimpse of what our own embryonic solar system might have looked like some five billion years ago.
Led by John Forbes, an astrophysicist at the Flatiron Institute’s Center for Computational Astrophysics, the team combined several datasets to demonstrate that budding stars in Ophiuchus “may serve as analogues for the emerging solar system” because they are being “inundated with [aluminum-26],” according to a study published on Monday in Nature Astronomy.
“Aluminum-26 is this long-standing puzzle in the solar system that people have been working on since it was discovered [in meteorites] in the ’70s,” said Forbes in a call.
“The fact we could see it, and that it seems to be closely tied with this region that is actively forming stars, Ophiuchus, is just really tantalizing,” he added. “You can see an example of how something like that might have happened for the solar system.”
Lots of interesting ingredients were baked into the solar system, but few are as consequential as aluminum-26, because of its pyrotechnic origin and its influence on planetary formation.
The isotope must have been seeded into our solar system via gamma rays from some kind of explosive stellar event that occurred near the molecular cloud that eventually evolved into the Sun, as well as planets like Earth.
“The key thing to know about aluminum-26 is that it decays very rapidly; its half-life is about 700,000 years,” Forbes said, “That's extremely fast on the timescale of the galaxy’s star formation, and it means that the solar system had to have been born near a source of aluminum-26.”
Scientists have speculated that this source could have been neighboring Wolf-Rayet stars, which are extremely hot and massive stars with strong stellar winds, or perhaps a nearby supernova (or multiple supernovae) that directly triggered the formation of our system with its shockwaves.
In other words, it may be that the birth of our solar system depended on the death of some other star system, or that our cosmic home was midwived by Wolf-Rayet stars that have long since died.
It’s a mind-boggling idea that has been hotly debated for decades, in part because it could help explain some of the most fundamental properties of the solar system’s planets. The radioactive properties of aluminum-26 made it a powerful early heat source for developing rocky bodies, possibly even limiting how much water they end up with, which has implications for understanding the habitability of star systems given that water is essential to all known life on Earth.
“The main reason we care about aluminum-26 is mostly understanding how the solar system formed in the first place,” Forbes said. “But interestingly, it also matters for planets. We’ve actually known for a long time that aluminum-26 is very important in the early life of a solar system. It's one of the main heat sources, so if you have some aluminum-26 in your protoplanets, dust grains, and asteroid-size objects, that actually causes some melting in those objects and it will also tend to get rid of ice on those objects.”
“There's this idea now that the more aluminum-26 you have in a forming system, the drier the planets are, and the less water they end up with,” he added. “That's another direction that's very, very interesting—understanding how much aluminum-26 you typically end up with in extrasolar planets.”
Gamma ray observations of the star-forming region in Ophiuchus, which is located about 400 light years from Earth, reveal that aluminum-26 is flowing into this nest of baby stars from another region called the Upper Scorpius association. By combining these older observations with newer positional datasets from the European Space Agency’s Gaia satellite, Forbes was able to produce a sophisticated new model that predicted the likely sources of the aluminum-26 enrichment at Ophiuchus.
The results of all of this integrated data reveal that there is a 59 percent chance that Ophiuchus is receiving aluminum-26 from supernovae, and a 68 percent chance that the isotope enrichment is coming from multiple sources, including supernovae.
“Very little here is new data from us that we took,” Forbes explained. “We brought together the evidence from the meteorites, the evidence from gamma rays, dust data from [ESA’s] Herschel [Space Observatory], near-infrared observations from the VISIONS Survey in Europe, and the distances from Gaia.”
“It's all those things together, along with the latest and greatest statistical techniques and theoretical yield calculations in our understanding of supernovae and Wolf-Rayet stars, that we're trying to bring together,” he continued.
The percentages in the new study shed light on the conditions that might have accompanied the birth of the solar system, heavily impacting its eventual properties. The research also establishes Ophiuchus as a place where star systems that are similar to our own might be emerging, providing a rare and powerful new lens through which we can view our own cosmic history.
“This business with the aluminum-26 might really change things up in what we understand to be a solar analogue,” said Forbes. “We've shown that the solar system level of aluminum-26 enrichment is well in line with what we're seeing in Ophiuchus.”
“But there's a huge spread, and there are plenty of regions where there are no massive stars—where it's a quiet backwater, maybe you could say—so those systems are just not going to have much aluminum-26 at all,” he concluded. “Maybe, that has substantial implications for the sort of planets that will form there.”