Scientists have discovered hidden “nanoflares” on the Sun, small explosions that we think may explain one of our star’s biggest mysteries.

Known as the solar corona mystery, the answer has long eluded scientists. The corona, which is the upper part of the solar atmosphere, is millions of degrees hotter than the surface, known as the photosphere. This is confusing – shouldn’t the greatest heat be nearest the source?

A few years ago, scientists proposed nanoflares as a possible solution. Million to billions of times weaker than solar flares, they had largely escaped detection. But after their subtle brightening was spotted on the Sun, it was suggested they may supply the missing energy needed to heat the corona. The only problem was we weren’t seeing enough of them to fully explain it.

Now a team led by Shin-nosuke Ishikawa from the Japan Aerospace Exploration Agency (JAXA) has an answer. Publishing their findings in Nature Astronomy, the researchers discovered hidden nanoflares that are only visible via their X-ray emissions, which may provide the missing energy.

“We found a signature of small flares without any brightening intensity increase [in visible light],” Ishikawa told IFLScience. “It’s the first time we’ve ever seen them without brightening.

To make the finding, the team used a NASA sounding rocket – which is one that flies briefly into suborbit before returning to Earth. Called FOXSI-2 (Focusing Optics X-ray Solar Imager) and launched in 2014, it used a highly sensitive X-ray detector to spot X-ray emissions on the Sun, which were likely coming from nanoflares.

It was one of two sounding rockets launched by the team over the last few years, the other being FOXSI-1, launched in 2012. The scientists now plan to launch another in summer 2018, called (you guessed it) FOXSI-3, to get more data on nanoflares.

“X-ray imaging is especially difficult,” Ishikawa said, commenting on why this discovery hasn’t been made before. “The Sun is too bright [for other instruments like NASA’s NuSTAR telescope], but our sounding rocket experiment is optimised for the Sun.”

The main issue at the moment is we don’t know how many nanoflares the Sun is producing. If it’s a lot, it’s probable that they are responsible for heating the solar corona. This latest study, however, was only able to see the overall X-ray emissions from nanoflares. It could not spot them individually.

That may all change in the future. Ishikawa and his colleagues have two proposals in the works for advanced satellites that would hunt for nanoflares. One, called FOXSI, is a NASA proposal that would launch in 2021. The other, Phoenix, is a proposal to JAXA with a launch in 2025.