Gaia unravels the ancient threads of the Milky Way

 

ESA’s Gaia space telescope has further disentangled the history of our galaxy, discovering two surprising streams of stars that formed and wove together over 12 billion years ago.

The two streams, named Shakti and Shiva, helped form the infant Milky Way. Both are so ancient they likely formed before even the oldest parts of our present-day galaxy’s spiral arms and disc.

“What’s truly amazing is that we can detect these ancient structures at all,” says Khyati Malhan of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, who led the research. “The Milky Way has changed so significantly since these stars were born that we wouldn’t expect to recognise them so clearly as a group – but the unprecedented data we’re getting from Gaia made it possible.”

Using Gaia observations, the researchers were able to determine the orbits of individual stars in the Milky Way, along with their content and composition. “When we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition,” adds Khyati. “We named them Shakti and Shiva.”

Truly ancient fragments

Each stream contains the mass of about 10 million Suns, with stars of 12 to 13 billion years in age all moving in very similar orbits with similar compositions. The way they’re distributed suggests that they may have formed as distinct fragments that merged with the Milky Way early in its life.

Both streams lie towards the Milky Way’s heart. Gaia explored this part of the Milky Way in 2022 using a kind of ‘galactic archaeology’; this showed the region to be filled with the oldest stars in the entire galaxy, all born before the disc of the Milky Way had even properly formed.

"We think that our galaxy formed as multiple long, irregular filaments of gas and dust coalesced, all forming stars and wrapping together to spark the birth of our galaxy as we know it. It seems that Shaki and Shiva are two of these components – and future Gaia data releases may reveal more."

“The stars there are so ancient that they lack many of the heavier metal elements created later in the Universe’s lifetime. These heavy metals are those forged within stars and scattered through space when they die. The stars in our galaxy’s heart are metal-poor, so we dubbed this region the Milky Way’s ‘poor old heart’,” says co-author Hans-Walter Rix, also of MPIA and the lead ‘galactic archaeologist’ from the 2022 work.

“Until now, we had only recognised these very early fragments that came together to form the Milky Way’s ancient heart. With Shakti and Shiva, we now see the first pieces that seem comparably old but located further out. These signify the first steps of our galaxy's growth towards its present size.”

A complex family tree

While very similar, the two streams are not identical. Shakti stars orbit a little further from the Milky Way’s centre and in more circular orbits than Shiva stars. Fittingly, the streams are named after a divine couple from Hindu philosophy who unite to create the Universe (or macrocosm).

Some 12 billion years ago, the Milky Way looked very different to the orderly spiral we see today. We think that our galaxy formed as multiple long, irregular filaments of gas and dust coalesced, all forming stars and wrapping together to spark the birth of our galaxy as we know it. It seems that Shaki and Shiva are two of these components – and future Gaia data releases may reveal more.

Khyati and Hans-Walter also built a dynamical map of other known components that have played a role in our galaxy’s formation and were discovered using Gaia data. These include Gaia-Sausage-Enceladus, LMS1/Wukong, Arjuna/Sequoia/I’itoi, and Pontus. These star groups all form part of the Milky Way’s complex family tree, something that Gaia has worked to build over the past decade.

“Revealing more about our galaxy’s infancy is one of Gaia’s goals, and it’s certainly achieving it,” says Timo Prusti, Project Scientist for Gaia at ESA. “We need to pinpoint the subtle yet crucial differences between stars in the Milky Way to understand how our galaxy formed and evolved. This requires incredibly precise data – and now, thanks to Gaia, we have that data. As we discover surprise parts of our galaxy like the Shiva and Shakti streams, we’re filling the gaps and painting a fuller picture of not only our current home, but our earliest cosmic history.”

Research Scientists Reveal How the First Cells Could Have Formed on Earth

Roughly 4 billion years ago, Earth was developing conditions suitable for life. Origin-of-life scientists often wonder if the type of chemistry found on the early Earth was similar to what life requires today.

They know that spherical collections of fats, called protocells, were the precursor to cells during this emergence of life. But how did simple protocells first arise and diversify to eventually lead to life on Earth?

Now, Scripps Research scientists have discovered one plausible pathway for how protocells may have first formed and chemically progressed to allow for a diversity of functions.

The findings, published online on February 29, 2024, in the journal Chem, suggest that a chemical process called phosphorylation (where phosphate groups are added to the molecule) may have occurred earlier than previously expected. This would lead to more structurally complex, double chained protocells capable of harboring chemical reactions and dividing with a diverse range of functionalities. By revealing how protocells formed, scientists can better understand how early evolution could have taken place.

“At some point, we all wonder where we came from. We’ve now discovered a plausible way that phosphates could have been incorporated into cell-like structures earlier than previously thought, which lays the building blocks for life,” says Ramanarayanan Krishnamurthy, PhD, co-corresponding senior author and professor in the Department of Chemistry at Scripps Research. “This finding helps us better understand the chemical environments of early Earth so we can uncover the origins of life and how life can evolve on early Earth.”

Krishnamurthy and his team study how chemical processes occurred to cause the simple chemicals and formations that were present before the emergence of life in prebiotic Earth. Krishnamurthy is also a co-leader of a NASA initiative investigating how life emerged from these early environments.

In this study, Krishnamurthy and his team collaborated with the lab of soft matter biophysicist Ashok Deniz, PhD, co-corresponding senior author and professor in the Department of Integrative Structural and Computational Biology at Scripps Research. They sought to examine if phosphates may have been involved during the formation of protocells. Phosphates are present in nearly every chemical reaction in the body, so Krishnamurthy suspected they may have been present earlier than previously believed.

Scientists thought protocells formed from fatty acids, but it was unclear how protocells transitioned from a single chain to a double chain of phosphates, which is what allows them to be more stable and harbor chemical reactions.

The scientists wanted to mimic plausible prebiotic conditions—the environments that existed prior to the emergence of life. They first identified three likely mixtures of chemicals that could potentially create vesicles, spherical structures of lipids similar to protocells. The chemicals used included fatty acids and glycerol (a common byproduct of soap production that may have existed during early Earth). Next, they observed the reactions of these mixtures and added additional chemicals to create new mixtures. These solutions were cooled and heated on repeat overnight with some shaking to promote chemical reactions.

They then used fluorescent dyes to inspect the mixtures and judge if vesicle formation had taken place. In certain cases, the researchers also varied the pH and the ratios of the components to better understand how these factors impacted vesicle formation. They also looked at the effect of metal ions and temperature on the stability of the vesicles.

“The vesicles were able to transition from a fatty acid environment to a phospholipid environment during our experiments, suggesting a similar chemical environment could have existed 4 billion years ago,” says first author Sunil Pulletikurti, postdoctoral researcher in Krishnamurthy’s lab.

It turns out that fatty acids and glycerol may have undergone phosphorylation to create that more stable, double chain structure. In particular, glycerol derived fatty acid esters may have led to vesicles with different tolerances to metal ions, temperatures, and pH—a critical step in diversifying evolution.

“We’ve discovered one plausible pathway for how phospholipids could have emerged during this chemical evolutionary process,” says Deniz. “It’s exciting to uncover how early chemistries may have transitioned to allow for life on Earth. Our findings also hint at a wealth of intriguing physics that may have played key functional roles along the way to modern cells.”

Next, the scientists plan to examine why some of the vesicles fused while others divided to better understand the dynamic processes of protocells.