The European Southern Observatory's (ESO's) New Technology Telescope (NTT) has peered inside an asteroid.

Using scrupulous measurements researchers found the asteroid Itokawa has varied densities throughout its structure, an ESO news release reported. The finding could help researchers gain insight into how asteroids and planets form

The team measured the rate at which the Near Earth asteroid spins; and how that movement changed over time.  The asteroid is small and shaped like a peanut .

In order to determine the object's inner structure researchers looked at the brightness variation that occurred when it rotated and combined it with other observations.

"This is the first time we have ever been able [to] determine what it is like inside an asteroid," Stephen Lowry of the University of Kent, UK, said in the news release. "We can see that Itokawa has a highly varied structure - this finding is a significant step forward in our understanding of rocky bodies in the Solar System."

The spin of asteroids can be affected by light from the Sun; the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect when sunlight forms heat on the surface of the object, when the heat is not radiated evenly it can impose torque on the object.

The team found the YORP effect was causing Itokawa to spin at an accelerated rate, but the change is only 0.045 seconds per year. This finding suggests two parts of the asteroid have different densities from each other.

This is the first-ever evidence that asteroids have varied internal structures. One theory on how this occurred is that two asteroids bumped into each other and merged together.

"Finding that asteroids don't have homogeneous interiors has far-reaching implications, particularly for models of binary asteroid formation. It could also help with work on reducing the danger of asteroid collisions with Earth, or with plans for future trips to these rocky bodies," Lowry said.