Earth may have lost the Sun’s protective shield millions of years ago

One study from NASA's SHIELD (Solar Wind with Hydrogen Ion charge Exchange and Large-Scale Dynamics) center, one of NASA's DRIVE (Diversify, Realize, Integrate, Venture, Educate) Science Centers, examines how the heliosphere moved through the Milky Way over time. The heliosphere is the vast protective bubble produced by the Sun that surrounds the solar system. Researchers found that changes in the galactic environment around this bubble may have affected conditions on Earth.

A separate study led by a NASA scientist explores another ancient puzzle: how a much dimmer young Sun could have kept Earth warm. The findings suggest that powerful solar eruptions may have helped generate strong greenhouse gases in Earth's early atmosphere.

The Sun's Journey Through the Milky Way

Earth's climate has changed dramatically over tens of millions of years. At various points, major ice ages temporarily lowered global average temperatures by several degrees, while other periods brought repeated swings between warmer and colder conditions.

Scientists have traditionally investigated changes in Earth's orbit, greenhouse gas levels, ice coverage, and other planetary factors to understand these shifts. New research now suggests that changes in the space environment surrounding the Sun may also have played an important role.

The solar system is enclosed within a protective region created by the Sun, somewhat like the way Earth is surrounded by an atmosphere. This region, called the heliosphere, forms as charged particles in the solar wind continuously stream outward from the Sun in every direction.

Reconstructing the Sun's Galactic Path

The entire heliosphere travels around the center of the Milky Way. During the Sun's 4.6-billion-year lifetime, the solar system has moved through many different galactic environments.

In a study published on Aug. 21 in Annual Review of Astronomy and Astrophysics, researchers with NASA's SHIELD center used computer simulations to reconstruct the heliosphere's past trajectory through the galaxy. Their results suggest that some of the regions the solar system encountered may have produced measurable changes on Earth.

Merav Opher, SHIELD's principal investigator at Boston University, and her colleagues simulated encounters between the solar system and extremely cold regions filled with gas and dust. Their work indicates that the Sun passed through such environments at least three times during the past several million years.

During these encounters, enormous interstellar "cold clouds" may have pressed against the heliosphere with enough force to dramatically compress it. According to the simulations, the heliosphere may at times have shrunk to a size smaller than Earth's orbit, temporarily leaving our planet outside the Sun's protective bubble.

When Earth May Have Lost Its Solar Shield

The modeled encounters occurred approximately 2 to 3 million years ago, 6 to 7 million years ago, and 13 to 14 million years ago. If the heliosphere contracted as the simulations suggest, Earth's atmosphere would have been directly exposed to a very different interstellar environment during those periods.

The timing also lines up with geological evidence. Elements commonly associated with interstellar dust have been found in deep-sea sediment cores, Antarctic snow, and samples from the Moon that correspond to these time periods.

These episodes of heliosphere collapse may also help explain some ancient climate patterns.

In the simulations, exposing Earth's atmosphere to a dense, cold cloud of galactic hydrogen increased atmospheric water vapor and changed conditions in the upper atmosphere. Those effects eventually influenced conditions closer to Earth's surface.

The results raise the possibility that the solar system's passages through colder parts of the Milky Way contributed to some long-term climate changes on Earth, potentially including ice ages.

Building a Digital Twin of the Heliosphere

NASA funds SHIELD as one of several centers designed to advance the study of heliophysics. As a DRIVE Science Center, SHIELD brings together researchers with different areas of expertise, methods, and scientific perspectives.

One of the center's goals is to build a detailed model, or "digital twin," of the heliosphere. This model could help scientists better understand how the Sun's protective bubble responds when it encounters features such as dense interstellar clouds.

Studying the history and structure of our habitable solar system could also provide clues about how life evolved on Earth. In the future, that knowledge may help researchers identify other star systems capable of supporting habitable worlds.

The Mystery of the Faint Young Sun

A second study focuses on a different problem involving the Sun's early history.

Vladimir Airapetian, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his collaborators investigated how early Earth managed to remain warm enough for liquid water when the young Sun produced much less energy than it does today.

About three billion years ago, the Sun was only 70% as bright as it is now. Based on that reduced output, Earth should theoretically have been frozen solid.

Geological evidence tells a different story. Stable liquid water existed on Earth long before then. The contradiction between a relatively warm early Earth and a cooler, dimmer Sun is known as the Faint Young Sun paradox.

A Violent Young Sun May Hold the Answer

Scientists may find clues by examining young stars elsewhere in the Milky Way that resemble the early Sun.

These "toddler" stars are far more active than mature stars. Observations from NASA's retired Kepler space telescope show that young Sun-like stars can produce enormous superflares on a daily basis, sending high-energy particles racing outward through space.

If the young Sun behaved in a similar way, Airapetian proposes that these energetic particles could have triggered chemical reactions in Earth's atmosphere that helped warm the planet.

To test that idea, Airapetian's team recreated conditions thought to resemble the atmosphere of early Earth inside a sealed chamber. The researchers combined molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide.

They then bombarded the gas mixture with protons to imitate the stream of energetic particles generated by solar superflares.

Superflares Could Have Created a Powerful Greenhouse Gas

The simulated particle bombardment produced several chemical changes, including the formation of nitrous oxide. Nitrous oxide is a greenhouse gas roughly 300 times more potent than carbon dioxide. The study was published in Astrophysical Journal Letters.

That nitrous oxide could have helped early Earth retain heat.

Not all of the gas would have remained in the atmosphere, however. Intense ultraviolet radiation from the young Sun would have broken down some nitrous oxide molecules, separating them back into nitrogen and oxygen.

Even so, the researchers found that relatively little nitrous oxide may have been needed.

Computer simulations showed that if just 10% of the nitrous oxide produced in the laboratory experiment survived, it could have raised temperatures near Earth's equator to around 41 degrees Fahrenheit (5 degrees Celsius). That would have kept temperatures above the freezing point of water.

Conditions That May Have Favored Early Life

A cooler but unfrozen Earth may have offered another advantage.

The smaller surviving amount of nitrous oxide could have supported prebiotic chemical reactions. Research has found that temperatures only slightly above freezing can be more effective than warmer conditions for assembling complex chains of amino acids.

That means the young Sun's violent activity may have done more than prevent Earth from freezing. It may also have helped create environmental conditions favorable to the chemistry that preceded life.

How the Sun Helped Shape Earth

Together, the two studies highlight how deeply Earth's history is connected to the Sun.

The solar system's movement through the Milky Way may have exposed Earth to changing interstellar environments capable of influencing its climate. Billions of years earlier, intense activity from the young Sun may have helped warm the planet when sunlight alone should not have been sufficient.

Earth is unusual in many ways, but it has never existed in isolation. It formed within a larger star-planet system and has remained closely connected to the changing behavior and environment of the Sun throughout its history.

Understanding that relationship could reveal new clues about Earth's climate, the evolution of life, and the conditions that might make other planetary systems habitable.