Asteroids orbiting the planet Neptune were found in a broad red spectrum, suggesting the presence of two asteroid populations in the region, according to a new study by an international team of researchers. The research was published in the journal Monthly Notices of the Royal Astronomical Society: Letters.

A team of scientists from the USA, California, France, the Netherlands, Chile and Hawaii observed 18 asteroids that share Neptune’s orbit and are known as Neptune Trojans. They range in size from 50 to 100 km and are located at a distance of about 4.5 billion kilometers from the Sun. Asteroids orbiting this far are dim, making it difficult for astronomers to study them. Prior to the new study, only a dozen Neptune Trojans had been studied, which required the use of some of the largest telescopes on Earth.

The new data was collected over two years by the WASP Wide Field Camera at Palomar Observatory in California, GMOS cameras at the Gemini North and South telescopes in Hawaii and Chile, and the LRIS camera at the Keck Telescope in Hawaii.

A few of the 18 Neptune Trojans observed were much redder than most asteroids, comparing them to other asteroids in the group examined in previous studies. Redder asteroids are expected to have formed much further from the Sun; a population of these are known as cool classical trans-Neptune objects located beyond the orbit of Pluto, about 6 billion kilometers from the sun. The newly observed Neptune Trojans also differ from the typically more neutrally colored Jupiter-orbiting asteroids.

The red color of asteroids means they contain higher percentages of ammonia and more volatile ices such as methanol. They are extremely sensitive to heat and can quickly turn into gas if the temperature rises, so they are more stable at greater distances from the sun.

The position of the asteroids at the same orbital distance as Neptune also means that they are stable on time scales comparable to the age of the Solar System. They actually act as a time capsule recording the initial conditions of the solar system.

The presence of redder asteroids among the Neptune Trojans indicates the existence of a transition zone between more neutrally colored objects and redder objects. Redder Neptune asteroids may have formed beyond this transitional boundary before being caught in Neptune’s orbit. The Neptune Trojans would have been caught in the same orbit as the planet Neptune when the ice giant planet migrated from the inner Solar System to where it is now, about 4.5 billion kilometers from the Sun.

From NASA’s Goddard Space Flight Center, lead author Dr. Bryce Bolin said: “In our new work, we have more than doubled the sample of Neptune Trojans studied with large telescopes. It is very exciting to find the first evidence of redder asteroids in this group.”

“As we now have a larger sample of Neptune Trojans with measured colors, we can now begin to see significant differences between asteroid groups. Our observations also show that Neptune’s Trojans also differ in color compared to asteroid groups located further away from the Sun. The explanation may be that treating asteroid surfaces with solar heat may have different effects for asteroids at different solar distances.”