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    Categories: Solar System

Dwarf Planet Facts: Makemake

Image Credit: NASA/JPL-Caltech

The image above shows the dwarf planet 136472 Makemake, which with a diameter roughly 60% that of Pluto is possibly the biggest object in the “classical population” of the Kuiper belt. Upon its discovery by a team under the leadership of Caltech astronomer Michael E. Brown on March 31, 2005, the object was dubbed “Easterbunny”, in honor of the fact it was first observed soon after Easter.  According to the International Astronomical Union’s rules for naming classical Kuiper belt objects, the object was later named “Makemake” in 2008, after the fertility god and creator of humanity in the myths of the native Rapa Nui people of Easter Island. Interestingly, the first European explorers to make contact with Easter Island did son on Easter Sunday of 1722.

Quick Facts

• Aphelion: 52.840 AU
• Perihelion: 38.590 AU
• Eccentricity: 0.15586
• Orbital period: 309.09 years (112,897 days)
• Equatorial rotation velocity: 7.771 ± 0.003 hours
• Sidereal rotation period: 4.419 km/sec
• Volume: 1.7 ± 0.1 × 109 cubic kilometres
• Mass: < 4.4 × 1021 kilograms
• Surface area: 6.9 ± 0.3 × 106 square kilometres
• Mean density: 1.4 – 3.2 gram/cubic centimetre
• Apparent magnitude: +17.0 (at opposition)
• Satellites: 1 (Designated S/2015 (136472) 1)

Visibility

Although Makemake is the second-brightest object in the Kuiper belt, it has an apparent visual magnitude of only 17.0 at opposition, which means that a large, high-end amateur telescope is required to see it.

Physical Properties

Being a classical Kuiper belt object, Makemake’s orbit is removed far enough from the planet Neptune to remain stable over the expected lifetime of the solar system. Unlike plutinos (dynamic group of trans-Neptunian objects) that are in a 2:3 resonance with Neptune, and can therefore occasionally cross Neptune’s orbit, classical Kuiper belt objects generally have their perihelia far enough from the Sun not to be perturbed by Neptune. As a result, classical objects have nearly circular orbits, and orbit the Sun in much the same way that the major planets do.

However, Makemake falls into a class of classical objects called “dynamically hot” objects, which typically have higher orbital inclinations than “normal” classical objects, meaning there is a high likelihood that Makemake is currently in, or near an 11:6 orbital resonance with Neptune.

Spectra

In visible light frequencies, Makemake has a distinctly red hue much like Pluto, but significantly more so than the surface of Eris. In infrared light, Makemake’s spectrum shows wide methane absorption bands that are significantly stronger than those in the spectra of Pluto and Eris, and spectral studies suggest that the methane on Makemake must be present in the form of large grains or clumps that are at least 1 cm in size

Apart from methane, spectral studies have also revealed the presence of large quantities tholins ethylene, ethane, acetylene, and high mass alkanes (similar to propane) on the dwarf planet’s surface, which is likely present due to the process of photolysis of methane by solar radiation. While the origin of the visible red hue of the dwarf planet is not absolutely certain, it is suspected to be the result of the presence tholins. Nitrogen (mixed in with other ices) is also present on Makemake, but rather surprisingly, the total amount of nitrogen is vastly less than that on Pluto and Triton where it accounts for more than 98% of those bodies’ crusts, which suggests that Makemake’s nitrogen supply had somehow become depleted over the age of the solar system.

Surface

Contrary to expectations, initial observations of Makemake’s surface by both the Herschel and Spitzer space telescopes in the far infrared (24–70 µm) and submillimeter (70–500 µm) frequencies showed that the dwarf planet’s surface is not homogeneous. While most of the object’s surface is covered by nitrogen and methane ices that range in albedo (reflectivity) from 78% to more than 90%, several small, dark patches were observed that had albedo’s ranging between only 2% and about 12% that collectively take up between 3% and about 7% of the objects total surface area.

Since these studies were performed before Makemake’s moon was discovered, most investigators now believe that these dark areas were actually the dark surface of the dwarf planet’s satellite, as opposed to being real differences in reflectivity. Nonetheless, recent observations made between 2008 and 2013 with the Telescopio Nazionale Galileo have shown that real, objective differences in the reflectivity of Makemake’s surface are negligible at best, which suggests that the dwarf planet’s surface may in fact be homogeneous, after all.

Atmosphere

Although Makemake was generally expected to have an atmosphere similar to that of Pluto, albeit with a substantially lower surface pressure, observations made in April of 2011 when the dwarf planet occulted an 18th-magnitude star showed that the object lacks a substantial atmosphere. These observations suggest that the surface pressure of the very tenuous atmosphere on Makemake is unlikely to exceed about 12 nanobar.

Satellite

To date, the object designated S/2015 (136472) 1 is the only known satellite of Makemake, which as it turns out, is nearly as black as charcoal, which was an unexpected finding in view of the fact that Makemake is the second-brightest object in the Kuiper belt. While the reason for the satellite’s low albedo remains unknown, some investigators think that the moon’s gravity is too low to prevent bright, volatile material from escaping to space when the surface ice on the moon sublimates.

Nonetheless, Makemake’s moon is thought to be about 175 km (110 mi) in diameter, and its distance from the dwarf planet along its orbital semi-major axis is estimated to be at least 21,000 km (13,000 mi). While the satellite’s exact orbital eccentricity is not known, it is thought to orbit Makemake once every 12 days or so. Note that the stated orbital period assumes a circular orbit. If the moon’s orbit turns out to be circular, this would suggest that it was formed during a violent impact event; however, if the moon’s orbit is shown to be eccentric, it is likely an object captured by the dwarf planet.

Peter Christoforou :