Led by BU astrophysicist Merav Opher, the $2.4 million NSF-funded COSMIC center will investigate whether the Sun’s encounters in deep space influenced Earth millions of years ago

Millions of years ago, as the Sun traveled through the Milky Way, it may have encountered clouds so dense that they compressed the enormous protective bubble surrounding our solar system—leaving Earth exposed to interstellar space.

Did those encounters change the planet’s atmosphere and climate?

A new National Science Foundation–funded center led by BU astrophysicist Merav Opher is investigating that question.

Opher, a professor of astronomy and director of the NASA-funded SHIELD DRIVE Science Center, has received $2.4 million through the NSF’s Growing Convergence Research program to establish COSMIC, the Center On Solar Wind–Milky Way Impacts on Complex Earth Systems.

COSMIC will bring together experts in astronomy, heliophysics, atmospheric science, and paleoclimate to explore how the Sun’s galactic environment may have influenced Earth over millions of years. By studying the galaxy, Sun, heliosphere, and Earth as parts of one interconnected system, the center will test whether encounters with cold interstellar clouds have driven changes in Earth’s history. Its findings could also help scientists understand the habitability of Earth-like planets elsewhere in the galaxy.

“We want to understand how the location of the Sun in the Galaxy affected the heliosphere and Earth’s history,” Opher says. “We’re connecting galactic scales all the way to Earth. This is super new.”

When Earth May Have Lost Its Protective Cocoon

At the center of the research is the heliosphere, a vast bubble created by the solar wind, the continuous stream of charged particles flowing from the Sun. Stretching far beyond Pluto, the heliosphere envelops the planets and helps protect them from galactic cosmic rays and material traveling through interstellar space. But its size and shape change as the Sun moves through different galactic environments.

Opher has been studying the heliosphere for more than two decades. In 2015, she and her collaborators challenged the established view of the heliosphere as a smooth, comet-shaped bubble with a single long tail. Their computer models suggested instead that the Sun’s magnetic field organizes the heliosphere into two elongated streams of solar material, giving it a shape frequently compared to a croissant.

In 2019, Opher received $15 million from NASA to establish SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics), a multi-institutional effort to better understand the heliosphere as it exists today. In 2021, the team determined that hydrogen particles from interstellar space might cause the heliosphere to bend and break apart rather than form a smooth tail.

As a 2021–2022 fellow at Harvard’s Radcliffe Institute for Advanced Study, Opher began asking how the heliosphere had changed as the Sun moved through the galaxy—and what those changes might have meant for Earth.

That work led to a 2024 paper published in Nature Astronomy, suggesting that the Sun encountered an unusually dense interstellar cloud approximately two to three million years ago. The pressure from the cloud may have compressed the heliosphere so dramatically that Earth was temporarily left outside its protective reach, exposing the planet to interstellar material and increased radiation—with possible consequences for its atmosphere and climate.

Opher and her collaborators have since identified another possible encounter six to seven million years ago. During these periods, they hypothesize, Earth may have been exposed to increased radiation and large amounts of hydrogen, potentially altering its atmospheric chemistry and affecting its climate.

COSMIC will bring together researchers from several disciplines to determine whether these encounters occurred, how they changed the heliosphere, and whether they left measurable traces on Earth.

“Instead of looking at the heliosphere today, we’re looking at the bigger picture,” Opher says. “It’s a really new area. The center was my crazy idea. That’s why I’m like, ‘Oh my God, we got funded.’ It’s so wild.”

Searching for Evidence on Earth

COSMIC researchers will use data from the European Space Agency’s Gaia mission to reconstruct the Sun’s path through the Milky Way and identify interstellar clouds it may have encountered. They will then model how those encounters affected the heliosphere—and whether the resulting increases in hydrogen and radiation altered Earth’s atmosphere and climate.

The team will compare its predictions with evidence preserved in ancient ice cores and marine sediments. These geological records contain indicators of past climate and radiation exposure, including beryllium-10, an isotope produced when energetic particles interact with Earth’s atmosphere, as well as the ratio of Oxygen-18 to Oxygen-16. Researchers affiliated with the NSF Center for Oldest Ice Exploration, or COLDEX, will contribute exceptionally old Antarctic ice records dating back millions of years. Other researchers will study Beryllium -10 as well as Oxygen 18/16 ratio  in ocean sediments that can reveal the temperatures and radiation at which they formed.

“You can learn through isotopes, both in ice and in marine sediments, about past climates,” Opher said. “We will use that data to explore how our predictions compare to what the past climate was.” 

“We’re predicting what you’re going to see,” Opher says. “So it’s not just us modeling. We have data that we’re going to try to back it up.”

Scientists have long understood that changes in Earth’s orbit and axial tilt can influence climate over tens of thousands of years. But those astronomical cycles do not fully explain shifts that unfold over millions of years.

“Now we are coming with a new idea,” Opher says. “Maybe, over millions of years, you have changes due to where the Sun is located in the Galaxy.”

Becoming Students Again

Answering that question will require astronomers, heliophysicists, atmospheric scientists, and paleoclimate researchers to learn enough about one another’s disciplines to ask questions and interpret evidence together.

Opher experienced that process while working with climate scientists on a previous project. She and her research scientist organized weekly meetings with experts on different layers of climate, reading climate research, and learning the language and tools of an unfamiliar field.

“I became a student again,” she says. “You have to leave your ego aside to learn a new discipline. I’ve been inserting myself, reading papers on climate, so I know what to ask. I’m doing a fast-track PhD on climate. It’s nice because we don’t know everything, and we are learning more.”

That kind of deep integration is central to the NSF’s Growing Convergence Research program, which supports teams combining expertise, methods, and perspectives from different disciplines to address complex scientific problems.

“You’re not just writing papers where you stick on coauthors,” Opher says. “You’re learning enough that you can pretend you are a climate scientist or a paleoclimate scientist.”

COSMIC will also train students, postdoctoral scholars, and early-career researchers to conduct team-based, transdisciplinary science. Plans include professional development, communication and leadership training, webinars, a summer school, and research opportunities for undergraduates.

Opher will draw on her experience leading SHIELD. She says BU’s support for that center helped build the collaborative and administrative foundation that made COSMIC possible.

“I really want to acknowledge how supportive the leadership of BU has been,” she says. 

From Earth to Other Worlds

The center’s findings could have implications far beyond Earth’s history.

Other stars also travel through changing galactic environments and produce their own protective cocoons, known as astrospheres. Determining what happened when—or if—Earth lost the heliosphere’s protection could help scientists evaluate conditions on planets orbiting other stars.

“If we can map this for Earth, we’ll be able to say that when exoplanets go through these crossings, something like this may happen as well,” Opher says.

The work could eventually inform research in fields ranging from biology and planetary science to the search for life elsewhere in the galaxy. First, however, Opher and her collaborators must determine whether the crossings occurred—and whether they left traces that can still be detected millions of years later.

“It has huge implications, not just for how life developed on Earth, but for exoplanets,” Opher says. “This is just the beginning. I’m super, super excited.”