Kyoto, Japan -- Down here on Earth we don't usually notice, but the Sun is frequently ejecting huge masses of plasma into space. These are called coronal mass ejections (CMEs). They often occur together with sudden brightenings called flares, and sometimes extend far enough to disturb Earth's magnetosphere, generating space weather phenomena including auroras or geomagnetic storms, and even damaging power grids on occasion.
Scientists believe that when the Sun and the Earth were young, the Sun was so active that these CMEs may have even affected the emergence and evolution of life on the Earth. In fact, previous studies have revealed that young Sun-like stars, proxies of our Sun in its youth, frequently produce powerful flares that far exceed the largest solar flares in modern history.
Huge CMEs from the early Sun may have severely impacted the early environments of Earth, Mars, and Venus. However, to what extent explosions on these young stars exhibit solar-like CMEs remains unclear. In recent years, the cool plasma of CMEs has been detected by optical observations on the ground. However, the high velocity and expected frequent occurrence of strong CMEs in the past have remained elusive.
In order to resolve this problem, an international team of researchers, including Kosuke Namekata of Kyoto University, sought to test whether young Sun-like stars produce solar-like CMEs.
"What inspired us most was the long-standing mystery of how the young Sun's violent activity influenced the nascent Earth," says Namekata. "By combining space- and ground-based facilities across Japan, Korea, and the United States, we were able to reconstruct what may have happened billions of years ago in our own solar system."
The team's analysis included simultaneous ultraviolet observations by the Hubble Space Telescope and optical observations by ground-based telescopes in Japan and Korea. Their target was the young solar analogue EK Draconis. Hubble observed far-ultraviolet emission lines sensitive to hot plasma, while the three ground-based telescopes simultaneously observed the hydrogen Hα line, which traces cooler gases. These simultaneous, multi-wavelength spectroscopic observations allowed the research team to capture both the hot and cool components of the ejection in real time.
These observations led to the first evidence of a multi-temperature coronal mass ejection from EK Draconis. The team found that hot plasma of 100,000 degrees Kelvin was ejected at 300 to 550 kilometers per second, followed about ten minutes later by a cooler gas of about 10,000 degrees ejected at 70 kilometers per second. The hot plasma carried much greater energy than cool plasma, suggesting that frequent strong CMEs in the past could drive strong shocks and energetic particles capable of eroding or chemically altering early planetary atmospheres.
Theoretical and experimental studies support the critical role that strong CMEs and energetic particles can play in initiating biomolecules and greenhouse gases, which are essential for the emergence and maintenance of life on an early planet. Therefore, this discovery has major implications for understanding planetary habitability and the conditions under which life emerged on Earth, and possibly elsewhere.
The research team noted that the success of this study was achieved through international teamwork and precise coordination between space- and ground-based observatories.
"We were happy to see that, although our countries differ, we share the same goal of seeking truth through science," says Namekata.
