Earth has a second known ‘Trojan asteroid’ that shares its orbit

A recently found space rock is schlepping along with Earth around the sun. This “Trojan asteroid” is only the second one discovered that belongs to our planet. And it’s probably a visitor.

Trojan asteroids, which are also found accompanying Mars, Jupiter and Neptune, hang out in two locations near a planet where the gravitational pulls of that planet and the sun balance each other (SN: 10/15/21). Because of this balancing act, these locations are stable spots in space. In 2010, astronomers discovered the first known Earth Trojan — called 2010 TK7 — orbiting within one of these two regions, known as L4, tens of millions of kilometers from Earth and leading our planet around the sun (SN: 8/2/11).

Now, researchers have found another one. Dubbed 2020 XL5, this roughly 1-kilometer-wide asteroid is also at L4, astronomer Toni Santana-Ros of the University of Barcelona and colleagues report February 1 in Nature Communications.

The space rock was first spotted in December 2020, and follow-up observations suggested that it might be at L4. To confirm this, Santana-Ros and colleagues observed the asteroid using ground-based telescopes in 2021. Measurements of its brightness let the researchers estimate the asteroid’s size — about three to four times as wide as 2010 TK7. They also scoured archival data and found the object in images dating to 2012.

“There is no doubt this is an Earth Trojan,” Santana-Ros says. That decade-worth of observations let the team calculate the rock’s orbit thousands of years into the future, confirming the asteroid’s nature. It will hang around at L4 for at least 4,000 years, the team predicts. 2010 TK7, for comparison, will stick around for some 10,000 years.

Now that scientists know of two just-visiting Earth Trojans, they can envision more. The fact that the team discovered a second object means that 2010 TK7 isn’t a rarity or loner, Santana-Ros says. “It might be part of a family or population.”