Makemake

Discovery

Makemake was discovered on March 31, 2005, by a team at the Palomar Observatory, led by Michael E. Brown, and was announced to the public on July 29, 2005. The team had planned to delay announcing their discoveries of the bright objects Makemake and Eris until further observations and calculations were complete, but announced them both on July 29 when the discovery of another large object they had been tracking, Haumea, was controversially announced on July 27 by a different team in Spain.

Despite its relative brightness (it is about a fifth as bright as Pluto), Makemake was not discovered until well after many much fainter Kuiper belt objects. Most searches for minor planets are conducted relatively close to the ecliptic (the region of the sky that the Sun, Moon and planets appear to lie in, as seen from Earth), due to the greater likelihood of finding objects there. It probably escaped detection during the earlier surveys due to its relatively high orbital inclination, and the fact that it was at its farthest distance from the ecliptic at the time of its discovery, in the northern constellation of Coma Berenices.

Besides Pluto, Makemake is the only other dwarf planet that was bright enough that Clyde Tombaugh could have detected it during his search for trans-Neptunian planets around 1930. At the time of Tombaugh's survey, Makemake was only a few degrees from the ecliptic, near the border of Taurus and Auriga, at an apparent magnitude of 16.0. This position, however, was also very near the Milky Way, and Makemake would have been almost impossible to find against the dense background of stars. Tombaugh continued searching for some years after the discovery of Pluto, but he did not find Makemake or any other trans-Neptunian objects.

Name

The provisional designation 2005 FY₉ was given to Makemake when the discovery was made public. Before that, the discovery team used the codename "Easterbunny" for the object, because of its discovery shortly after Easter.

In July 2008, in accordance with IAU rules for classical Kuiper belt objects, 2005 FY₉ was given the name of a creator deity. The name of Makemake, the creator of humanity and god of fertility in the myths of the Rapa Nui, the native people of Easter Island, was chosen in part to preserve the object's connection with Easter.

Orbit and classification

As of December 2015, Makemake is 52.4 AU (7.84×10⁹ km) from the Sun, almost as far from the Sun as it ever reaches on its orbit. Makemake follows an orbit very similar to that of Haumea: highly inclined at 29° and a moderate eccentricity of about 0.16. Nevertheless, Makemake's orbit is slightly farther from the Sun in terms of both the semi-major axis and perihelion. Its orbital period is nearly 310 years, more than Pluto's 248 years and Haumea's 283 years. Both Makemake and Haumea are currently far from the ecliptic—the angular distance is almost 29°. Makemake is approaching its 2033 aphelion, whereas Haumea passed its aphelion in early 1992.

Makemake is a classical Kuiper belt object (KBO), which means its orbit lies far enough from Neptune to remain stable over the age of the Solar System. Unlike plutinos, which can cross Neptune's orbit due to their 2:3 resonance with the planet, the classical objects have perihelia further from the Sun, free from Neptune's perturbation. Such objects have relatively low eccentricities (e below 0.2) and orbit the Sun in much the same way the planets do. Makemake, however, is a member of the "dynamically hot" class of classical KBOs, meaning that it has a high inclination compared to others in its population. Makemake is, probably coincidentally, near the 11:6 resonance with Neptune.

Brightness, size, and rotation

Makemake is currently visually the second-brightest Kuiper belt object after Pluto, having a March opposition apparent magnitude of 17.0 in the constellation Coma Berenices. This is bright enough to be visible using a high-end amateur telescope.

Combining the detection in infrared by the Spitzer Space Telescope and Herschel Space Telescope with the similarities of spectrum with Pluto yielded an estimated diameter from 1,360 to 1,480 km. From the 2011 stellar occultation by Makemake, its dimensions have been initially measured to be (1502 ± 45) × (1430 ± 9) km. However, this analysis of the occultation data was later reanalyzed, which led to the dimension estimate of (7003143400000000000♠1434+48
−18) × (7003142000000000000♠1420+18
−24 km) without a pole-orientation constraint. This means that Makemake is slightly larger than that of Haumea, making it likely the fourth-largest known trans-Neptunian object after Pluto, Eris, and 2007 OR₁₀, though the error bars with the latter overlap. Makemake was the fourth dwarf planet recognized, because it has a bright V-band absolute magnitude of −0.44. Makemake has a high geometrical albedo of 6999810000000000000♠0.81+0.01
−0.02.

The rotation period of Makemake is estimated at 7.77 hours. Its lightcurve amplitude is small, only 0.03 mag. This was thought to be due to Makemake currently being viewed pole on from Earth; however, S/2015 (136472) 1's orbital plane (which is probably orbiting with little inclination relative to Makemake's equator due to tides resulting from its rapid rotation) is edge-on from Earth, implying that Makemake is really being viewed equator-on.

Spectra and surface

Like Pluto, Makemake appears red in the visible spectrum, and significantly redder than the surface of Eris (see colour comparison of TNOs). The near-infrared spectrum is marked by the presence of the broad methane (CH₄) absorption bands. Methane is observed also on Pluto and Eris, but its spectral signature is much weaker.

Spectral analysis of Makemake's surface revealed that methane must be present in the form of large grains at least one centimetre in size. In addition to methane, large amounts of ethane and tholins as well as smaller amounts of ethylene, acetylene and high-mass alkanes (like propane) may be present, most likely created by photolysis of methane by solar radiation. The tholins are probably responsible for the red color of the visible spectrum. Although evidence exists for the presence of nitrogen ice on its surface, at least mixed with other ices, there is nowhere near the same level of nitrogen as on Pluto and Triton, where it composes more than 98 percent of the crust. The relative lack of nitrogen ice suggests that its supply of nitrogen has somehow been depleted over the age of the Solar System.

The far-infrared (24–70 μm) and submillimeter (70–500 μm) photometry performed by Spitzer and Herschel telescopes revealed that the surface of Makemake is not homogeneous. Although the majority of it is covered by nitrogen and methane ices, where the albedo ranges from 78 to 90%, there are small patches of dark terrain whose albedo is only 2 to 12%, and that make up 3–7% of the surface. These studies were made before S/2015 (136472) 1 was discovered; thus, these small dark patches may actually have been the dark surface of the satellite rather than any actual surface features on Makemake. However, another experiment has refuted these studies. Between 2006 and 2013, the Telescopio Nazionale Galileo acquired new visible and near infra-red spectra for Makemake that covered nearly 80% of its surface; this study found that the variation in the spectra were negligible, suggesting that Makemake's surface may indeed be homogenous.

Atmosphere

Makemake was expected to have an atmosphere similar to that of Pluto but with a lower surface pressure. However, on 23 April 2011 Makemake passed in front of an 18th-magnitude star and abruptly blocked its light. The results showed that Makemake presently lacks a substantial atmosphere and placed an upper limit of 4–12 nanobar on the pressure at its surface.

The presence of methane and possibly nitrogen suggests that Makemake could have a transient atmosphere similar to that of Pluto near its perihelion. Nitrogen, if present, will be the dominant component of it. The existence of an atmosphere also provides a natural explanation for the nitrogen depletion: because the gravity of Makemake is weaker than that of Pluto, Eris and Triton, a large amount of nitrogen was probably lost via atmospheric escape; methane is lighter than nitrogen, but has significantly lower vapor pressure at temperatures prevalent at the surface of Makemake (32–36 K), which hinders its escape; the result of this process is a higher relative abundance of methane. However, studies of Pluto's atmosphere by New Horizons suggest that methane, not nitrogen, is the dominant escaping gas, suggesting that Makemake's absence of nitrogen may be more complicated.

Satellite

S/2015 (136472) 1, nicknamed MK 2 by the discovery team, is the only known moon of the dwarf planet Makemake. It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake. Observations leading to its discovery occurred in April 2015, using the Hubble Space Telescope's Wide Field Camera 3 and its discovery was announced on 26 April 2016.

On 26 April 2016, astronomers using observations from the Hubble Space Telescope taken in April 2015 announced the discovery of a moon with a diameter of ~175 km (for an assumed albedo of 4%) orbiting Makemake at a distance of ≥ 21,000 km with a period of ≥ 12 days (the minimum values are those for a circular orbit; the actual orbital eccentricity is unknown). It was given the provisional name S/2015 (136472) 1.

Most other large trans-Neptunian objects have at least one satellite: Eris has one, Haumea has two, Pluto has five, and 2007 OR₁₀ has one satellite. 10% to 20% of all trans-Neptunian objects are expected to have one or more satellites. Because satellites offer a simple method to measure an object's mass, Makemake's satellite should lead to better estimates of its mass.

Observations

A preliminary examination of the imagery suggests that S/2015 (136472) 1 has a reflectivity similar to charcoal, making it an extremely dark object. This is somewhat surprising because Makemake is the second-brightest known object in the Kuiper belt. One hypothesis to explain this is that its gravity is not strong enough to prevent bright but volatile ices from being lost to space when it is heated by the distant Sun.

Further observations will be needed in order to determine S/2015 (136472) 1's orbit. If it is circular, it would suggest that S/2015 (136472) 1 was formed by an ancient impact event, but if it is elliptical, it suggests that it may have been captured.

Alex Parker, the leader of the team that performed the analysis of the images at the Southwest Research Institute, said that S/2015 (136472) 1's orbit appears to be aligned edge-on to Earth-based observatories. This would make it much more difficult to detect because it would be lost in Makemake's glare much of the time and, along with its dark surface, would contribute to previous surveys failing to observe it.

Exploration

It was calculated that a flyby mission to Makemake could take just over 16 years using a Jupiter gravity assist, based on a launch date of 21 August 2024 or 24 August 2036. Makemake would be approximately 52 AU from the Sun when the spacecraft arrives.