NASA's Sounding Rocket Program has confirmed where X-rays around Earth come from, settling a decades-old debate, but also identified another X-ray group from an unknown source.

In a study published in the Astrophysical Journal, Youaraj Uprety and colleagues detail the results of the flight of the Diffuse X-ray emission from the Local galaxy (DXL) sounding rocket, outlining concrete answers to where X-rays come from.

There were three theories about the source of X-ray emissions in space: that X-rays are a kind of background noise; that they come from a structure called the Local Hot Bubble; and that they are brought about by solar winds interacting with neutral gas pockets.

The first theory was ruled out quickly because the Milky Way has a lot of neutral gas that could absorb X-rays from distant sources. But with X-rays still being detected around Earth, this means that the emissions are originating from somewhere else but still near the solar system.

It was only when the DXL sounding rocket was used that researchers were able to confirm the last two theories. More specifically, solar wind charge exchange accounts for about 40 percent of the X-rays, with the rest coming from the Local Hot Bubble.

According to the researchers, identifying the Local Hot Bubble's contribution to X-ray emissions is necessary for better understanding the structures surrounding the solar system, facilitating the creation of better interstellar material models in the galaxy.

However, the sounding rocket was also able to pick up high-energy X-rays, which researchers have determined could not have come from either of the confirmed sources.

"The temperature of the Local Hot Bubble is not high enough to produce X-rays in this energy range, so we're left with the open question on the source of these X-rays," said Uprety, who is the study's lead author.

The DXL Sounding Rocket

The DXL is the second spacecraft to fly with the goal of measuring X-rays. Unlike its predecessor, a satellite known as ROSAT, it was launched at a time when the planet was passing through the helium-focusing cone, a region in space where neutral helium is denser by several times compared to everything else in the inner solar system. It was flown Dec. 13, 2012 and was in flight for about 15 minutes. It's necessary to observe X-rays from beyond Earth because the planet's atmosphere blocks most of the rays, making observations from the ground impossible.

Another X-Ray Conundrum

NASA's Chandra X-ray Observatory has also detected X-ray emissions from Pluto, the first time that X-rays were observed originating from an object from the Kuiper Belt, a region beyond Neptune believed to be milling with comets, asteroids and others mainly composed of ice.

However, scientists are baffled by the discovery because Pluto supposedly lacks the natural mechanisms necessary for emitting X-rays, being cold and rocky and not having a magnetic field.

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