Neptune trojan facts for kids

Neptune's L4 trojans with plutinos for reference.

Neptune trojans (selection)   · 2001 QR322   · 2005 TN53   · 2007 VL305

Plutinos   · Pluto   · Orcus   · Ixion

Neptune trojans are small space rocks that orbit the Sun in a special way, very close to Neptune. They are found in stable areas called Lagrangian points. Think of them like companions to Neptune, always following roughly the same path around the Sun.

These trojans have almost the same orbital period as Neptune. This means they take about the same amount of time to go around the Sun. Scientists have found twenty-eight Neptune trojans so far. Most of them (24) are ahead of Neptune, in a spot called the Sun–Neptune L₄ Lagrangian point. The other four are behind Neptune, in its L₅ region. They are called 'trojans' because they are similar to the Jupiter trojans, which are also found in stable points near Jupiter.

When scientists found one trojan, 2005 TN₅₃, orbiting at a steep angle (more than 25 degrees), it was a big deal! This suggested that the cloud of Neptune trojans is "thick," meaning it spreads out quite a bit. This "thick" cloud hints that these trojans were likely captured by Neptune's gravity a long time ago, rather than forming right where they are now. Some scientists even think there might be many more large Neptune trojans (about 100 kilometers wide) than Jupiter trojans.

In 2010, the first trojan behind Neptune was discovered. It was named 2008 LC₁₈. It's harder to find objects in Neptune's trailing L₅ region because it's in the direction of the center of our Milky Way galaxy. This part of the sky is full of bright stars, making it tough to spot smaller, dimmer objects.

Discovering Neptune's Companions

The first Neptune trojan, 2001 QR₃₂₂, was found in 2001. It was located in Neptune's L₄ region. This discovery added a fifth known group of stable small bodies in our Solar System.

In 2005, the discovery of 2005 TN₅₃, with its high-angle orbit, showed that Neptune trojans exist in "thick" clouds. This helped scientists understand more about how these objects might have formed.

On August 12, 2010, the first L₅ trojan, 2008 LC₁₈, was announced. It was found during a special search that looked at areas where dust clouds block the light from stars near the Galactic Center. Finding this L₅ trojan suggested that large trojans are just as common behind Neptune as they are ahead of it. This helps scientists create better models about where these trojans came from.

The New Horizons spacecraft could have looked for L₅ Neptune trojans when it flew past this area on its way to Pluto in 2014. One of the dusty areas that blocks starlight was right along New Horizons's path. The trojan 2011 HM₁₀₂, which has the steepest orbit known, was even bright enough for New Horizons to see it in late 2013. However, the mission team decided to focus on preparing for the Pluto flyby instead.

How They Orbit and Where They Came From

An animation showing the path of six of Neptune's L₄ trojans in a rotating frame with a period equal to Neptune's orbital period. Neptune is held stationary. (Click to view.)

The orbits of Neptune trojans are very stable. It's thought that Neptune might have kept up to half of its original trojan population since the Solar System formed. Both the L₄ and L₅ regions of Neptune can hold stable trojans equally well. Neptune trojans can "swing" back and forth up to 30 degrees from their stable points over about 10,000 years. If a Neptune trojan leaves its stable orbit, it might end up in an orbit similar to a centaur, which is another type of small icy body.

The discovery of trojans with steep orbits is very important for understanding how the whole group formed. These steep orbits suggest that the trojans were captured by Neptune during a time when the planets were moving around in the early Solar System. This is different from them forming right where they are now, or from collisions. The fact that there are similar numbers of large L₅ and L₄ trojans means that there wasn't much gas drag when they were captured. This points to a common way they were all captured.

The capture of Neptune trojans during planetary movement is similar to how Jupiter trojans were captured in the "Nice model." This model suggests that as Uranus and Neptune moved closer to each other, their gravity could have caused some trojans to become unstable and then get captured into new stable orbits. This process could also allow new trojans to be captured as the planets continued to move. For trojans with steep orbits to be captured, the planets must have moved slowly, or the trojans must have already had those steep orbits.

Colors of Neptune Trojans

The first four Neptune trojans discovered have similar colors. They are a bit reddish, slightly redder than the gray objects found in the Kuiper belt. However, they are not as extremely red as some other very red Kuiper belt objects. Their colors are similar to those of centaurs, Jupiter trojans, and the irregular moons of gas giants. This suggests that these different groups of small bodies might have formed in similar ways.

It's hard to study the exact surface materials of Neptune trojans with today's technology because they are too dim. This means many different types of surface materials could match the colors we see.

Some Neptune trojans have been observed to be very red, similar to the cold objects in the Kuiper belt.

Naming Neptune Trojans

In 2015, the IAU, which names objects in space, decided on a new way to name Neptune trojans. They are now named after Amazons. The Amazons were a group of female warriors from Greek myths who fought alongside the Trojans in the Trojan War. This naming rule applies to trojans in both the L4 and L5 regions.

As of 2019, two Neptune trojans have been officially named:

• 385571 Otrera: Named after Otrera, who was the first queen of the Amazons in Greek mythology.
• 385695 Clete: Named after Clete, an Amazon who was a companion to Queen Penthesilea. Penthesilea led the Amazons in the Trojan War.

How Many Are There?

Even though we've only found a small number of Neptune trojans, many of them have steep orbits. This suggests that there might be many more high-inclination trojans than low-inclination ones. Scientists estimate that for every one low-inclination trojan, there are about four high-inclination ones.

Based on estimates, there could be around 400 Neptune trojans with a radius larger than 40 kilometers in Neptune's L₄ region. This would mean that large Neptune trojans are 5 to 20 times more common than Jupiter trojans. There might be fewer smaller Neptune trojans because they could break apart more easily. Scientists believe that large L₅ trojans are just as common as large L₄ trojans.

Some trojans, like 2001 QR₃₂₂ and 2008 LC₁₈, show signs of being a bit unstable in their orbits. This could mean they were captured more recently, or they could be long-term members that just aren't perfectly stable.

As of April 2022, 33 Neptune trojans are known. Most of them (28) are in the L₄ region ahead of Neptune. Four are in the L₅ region behind Neptune. One special trojan, 316179_{2010 EN}, orbits on the opposite side of Neptune (L₃) but often moves between the L4 and L5 points.

Here is a list of some known Neptune trojans:

Some objects, like 2005 TN₇₄ and (309239) 2007 RW₁₀, were first thought to be Neptune trojans. But later observations showed they weren't. 2005 TN₇₄ is now believed to be in a special orbital relationship with Neptune. (309239) 2007 RW₁₀ is currently following a temporary "quasi-satellite" loop around Neptune.

See also

In Spanish: Asteroide troyano de Neptuno para niños

• Nice model
• Nice 2 model
• (309239) 2007 RW₁₀, a temporary quasi-satellite of Neptune.