Neptune trojan

Types of Distant Minor Planets

Cis-Neptunian objects Centaurs Neptune trojans Trans-Neptunian objects (TNOs)‡ Kuiper belt objects (KBOs) Classical KBOs (Cubewanos) Resonant KBOs Plutinos (2:3 Resonance) Scattered disc objects (SDOs) Resonant SDOs Detached objects Oort cloud objects (OCOs)

‡ Trans-Neptunian dwarf planets are "plutoids"

Neptune's L₄ trojans with plutinos for reference.

An animation showing the path of the 6 L₄ Neptune trojans in a rotating frame with a period equal to Neptune's orbital period. (Neptune is held stationary.)

Neptune trojans are bodies in orbit around the Sun, have the same orbital period as Neptune, and follow roughly the same orbital path. Eight Neptune trojans are currently known, of which six orbit in the elongated, curved region around the Sun–Neptune L₄ Lagrangian point 60° ahead of Neptune. The other two orbit near Neptune's L₅ region. The Neptune trojans are termed 'trojans' by analogy with Jupiter's Trojan asteroids.

The discovery of 2005 TN₅₃ in a high-inclination (>25°) orbit was significant as it suggested a “thick” cloud of trojans (Jupiter trojans have inclinations up to 40°), which is indicative of freeze-in capture instead of in situ or collisional formation. It is suspected that large (radius ≈ 100 km) Neptune trojans could outnumber Jupiter trojans by an order of magnitude.

In 2010, the discovery of the first known L₅ Neptune trojan, 2008 LC₁₈, was announced. Neptune's trailing L₅ region is currently very difficult to observe because it is along the line-of-sight to the center of our galaxy, an area of the sky crowded with stars.

It may be possible for the New Horizons spacecraft to investigate L₅ Neptune trojans discovered by 2014, when it passes through this region of space en route to Pluto.

Discovery and exploration

In 2001, the first Neptune trojan was discovered, 2001 QR₃₂₂, near Neptune's L₄ region, and with it the fifth known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN₅₃ has indicated that the Neptune trojans populate thick clouds, which has constrained their possible origins (see below).

On August 12, 2010, the first L5 trojan, 2008 LC₁₈, was announced. It was discovered by scanning regions where the light from the stars near the galactic center is obscured by dust clouds. This has indicated that large L5 trojans are as common as large L₄ trojans, to within uncertainty, further constraining models about their origins (see below).

It may be possible for the New Horizons spacecraft to investigate L₅ Neptune trojans discovered by 2014, when it passes through this region of space en route to Pluto. Some of the patches where the light from the galactic center is obscured by dust clouds are along New Horizons's flight path, allowing detection of objects that the spacecraft could image.

Dynamics and origin

The orbits of Neptune trojans are highly stable; Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System. Neptune cannot currently efficiently capture trojans even for short periods.

It is possible for Neptune trojans to librate up to 30° from their associated Lagrangian points with a 10,000-year period.

The existence of high-inclination Neptune trojans points to 'freeze-in' capture or variations on this process, or during a slow, smooth migration, instead of in situ or collisional formation, as the origin of Neptune trojans. The captured population already had to be dynamically excited for high-inclination trojans to exist. While resonant trans-Neptunian objects are thought to have been captured by sweeping resonances during planet migration, this process would cause the escape of Neptune trojans. Coarse or irregular planetary migration would result in the depletion of the associated trojan reservoir. The estimated equal number of large L₅ and L₄ trojans indicates that there was no gas drag during capture and points to a common capture mechanism for both L₄ and L₅ trojans.

Colors

The first four discovered Neptune trojans have similar colors. They are modestly red, slightly redder than the gray KBOs, but not as extremely red as the high-perihelion cold classical Kuiper belt objects. This is similar to the colors of the blue lobe of the centaur color distribution, the Jupiter Trojans, the irregular satellites of the gas giants, and possibly the comets, which is consistent with a similar origin of these populations of small Solar System bodies.

The Neptune trojans are too faint to efficiently observe spectroscopically with current technology, which means that a large variety of surface compositions are compatible with the observed colors.

Members

The ratio of high-inclination to low-inclination Neptune trojans is estimated to be about 4:1. Relatively fewer high-inclination Neptune trojans are known due to observational biases. Assuming albedos of 0.05, 400+250
−200 Neptune trojans with radii above 40 km are expected to reside in Neptune's L₄. This would indicate that large Neptune trojans are 5 to 20 times more abundant than Jupiter Trojans, depending on their albedos. The size distribution of Neptune trojans may be quite steep, indicating relatively fewer smaller Neptune trojans, which may be because these fragment more readily. Large L5 trojans are estimated to be as common as large L₄ trojans.

As of July 2011, eight Neptune trojans are known, which are listed in the following table. It is constructed from information obtained from the List Of Neptune Trojans maintained by the IAU Minor Planet Center and with diameters from Sheppard and Trujillo's paper on 2008 LC₁₈, unless otherwise noted.

Provisional Designation	Lagrangian Point	Perihelion (AU)	Aphelion (AU)	Inclination (°)	Absolute magnitude	Diameter (km)	Discovery year	Reference
2001 QR322	L4	29.428	31.349	1.3	8.2	~140	2001
2004 UP10	L4	29.351	31.259	1.4	8.8	~100	2004
2005 TN53	L4	28.253	32.284	25.0	9.1	~80	2005
2005 TO74	L4	28.733	31.824	5.2	8.5	~100	2005
2006 RJ103	L4	29.345	31.005	8.2	7.5	~180	2006
2007 VL305	L4	28.131	32.171	28.1	8.0	~160	2007
2008 LC18	L5	27.547	32.468	27.5	8.4	~100	2008
2004 KV18	L5	24.566	35.657	13.6	8.9	56	2011

2005 TN₇₄ and 2007 RW₁₀ were believed to be Neptune trojans at the time of their discovery, but further observations have disconfirmed their membership.^{[citation needed]} 2005 TN₇₄ is currently thought to be in a 3:5 resonance with Neptune.

See also

• Nice model

External links

• Planetary Trojans – the main source of short period comets? (arXiv:1007.2541 : 15 Jul 2010)