Makemake

Makemake (minor-planet designation 136472 Makemake) is a dwarf planet orbiting in the dynamically hot classical Kuiper belt and the second-brightest trans-Neptunian object visible from Earth after Pluto, shining at an opposition apparent magnitude near V ≈ 17.0. Discovered in March 2005 by Michael E. Brown, Chad Trujillo, and David L. Rabinowitz at Palomar Observatory, Makemake has a mean equatorial diameter of approximately 1,430 km—roughly three-fifths the diameter of Pluto or Eris—and a high visual geometric albedo of 0.81. In planetary astronomy, Makemake occupies an intermediate volatile-retention regime between smaller, volatile-depleted Kuiper belt objects (like crystalline-ice-covered Haumea) and larger volatile-rich dwarf planets (like Pluto and Eris): while Makemake has lost nearly all of its hypervolatile molecular nitrogen (N₂) to Jeans atmospheric escape over the age of the solar system, its surface remains coated in centimeter-sized slabs of solid methane (CH₄) and photolytic ethane (C₂H₆) frosts undergoing active radiolytic processing and possible Deuterium/Hydrogen (D/H) fractionation from interior hydrothermal outgassing.

Makemake was discovered on March 31, 2005, by Michael E. Brown, Chad Trujillo, and David L. Rabinowitz using the 48-inch (1.22 m) Samuel Oschin Schmidt Telescope at Palomar Observatory in California during the Caltech Palomar Trans-Neptunian Survey—the same survey that yielded Haumea, Eris, and 90377 Sedna. Because the discovery occurred just days after Easter in 2005, the survey team initially referred to the object by the informal codename "Easterbunny" before its public announcement on July 29, 2005, under the provisional designation 2005 FY₉.

Why had a 17th-magnitude dwarf planet—only slightly fainter than Pluto (V ≈ 14–15)—escaped earlier surveys such as Clyde Tombaugh's Lowell Observatory search? At the time of Tombaugh's 1930 campaign and subsequent ecliptic surveys, 2005 FY₉ was positioned at high ecliptic latitude (~29° north of the ecliptic plane) in the constellation Coma Berenices, far outside the narrow zodiacal band where astronomers traditionally searched for outer planets.

On July 11, 2008, the International Astronomical Union officially designated 2005 FY₉ as the fourth recognized dwarf planet in the solar system (and third plutoid) and named it Makemake, after the creator of humanity and god of fertility in the Rapa Nui mythology of Easter Island—preserving a thematic link to its Easter discovery date while honoring Polynesian island cosmology.

Makemake revolves around the Sun on a non-resonant, dynamically hot classical Kuiper belt trajectory with a semi-major axis of 45.43 AU, a perihelion of 38.10 AU, an aphelion of 52.76 AU, an orbital eccentricity of 0.1613, and a high ecliptic inclination of 28.98°, completing one orbit every 306.21 Earth years. In contrast to Pluto (locked in a 2:3 resonance with Neptune) and Haumea (in an intermittent 7:12 resonance), numerical integrations in orbital mechanics confirm that Makemake is free of low-order mean-motion resonances with Neptune and stable over multi-billion-year timescales.

On April 23, 2011, José Luis Ortiz and colleagues observed Makemake pass directly in front of the 18th-magnitude star NOMAD 1181-0235723 across seven telescopes in Chile and Brazil. The abrupt disappearance and reappearance of the star without refractive atmospheric twilight tails provided two foundational results:

1. Size and Spheroidal Shape: Assuming a nearly pole-on or moderately inclined spheroidal figure, Makemake's projected chord ellipse measures 1,430 ± 9 km across its equatorial diameter and 1,420 ± 18 km across its polar axis (or up to ~1,502 × 1,430 km when incorporating infrared thermal constraints from Spitzer and Herschel), confirming a high visual geometric albedo of 0.81.
2. Upper Limit on a Global Atmosphere: Unlike Pluto, which supports a global ~10 microbar nitrogen-methane atmosphere, Makemake lacks any global envelope above a surface pressure ceiling of 4 to 12 nanobars (less than one-thousandth of Pluto's surface pressure), although localized, transient sublimation plumes over warm dark patches (~40 K) remain possible.

Photometric monitoring with the Tess, Kepler (K2), and ground-based observatories shows that Makemake rotates with a period of 22.8266 hours and a remarkably tiny optical light-curve amplitude of only Δm ≈ 0.03 magnitudes, indicating either a nearly pole-on viewing aspect or a longitudinally uniform frost distribution outside the unresolved dark terrain needed to explain mid-infrared thermal emissions.

Visible and near-infrared spectra of Makemake resemble those of Pluto and Eris, yet reveal distinct microphysical differences driven by the near-depletion of molecular nitrogen (N₂). On Pluto and Eris, CH₄ is dissolved as a dilute solid solution within a dominant matrix of crystalline N₂ ice, which shifts the methane near-infrared absorption bands slightly blueward by ~15–20 Å. On Makemake, by contrast, the CH₄ absorption bands from 0.7 to 2.5 μm sit at their unshifted pure-methane wavelengths and are extraordinarily broad and saturated, indicating that pure solid methane (CH₄) exists in remarkably large optical path lengths—coarse crystals or sintered slabs 0.5 to 1.0 cm in grain diameter covering the bright terrain (~30–35 K).

Solar ultraviolet photons and cosmic-ray protons bombarding this pure methane surface dissociate CH₄ molecules and drive free-radical recombination reactions without the buffering effect of N₂, coating Makemake in higher-order hydrocarbons and reddish tholins:

• Ethane (C₂H₆): Detected via strong absorption features near 2.27–2.46 μm, produced directly by 2 CH₄ → C₂H₆ + H₂ photolysis.
• Ethylene (C₂H₄) and Acetylene (C₂H₂): Intermediate unsaturated hydrocarbons identified in near- and mid-infrared spectra that polymerize into refractory red-brown organic macromolecules responsible for Makemake's reddish visible slope (B − V ≈ 0.83).

In 2024, high-resolution near- and mid-infrared spectroscopy from the James Webb Space Telescope (JWST NIRSpec and MIRI) made two breakthrough discoveries on Makemake. First, JWST measured the Deuterium-to-Hydrogen (D/H) ratio in Makemake's surface methane via monodeuterated methane (CH₃D) bands at 3–5 μm, finding D/H = (2.9 ± 0.6) × 10⁻⁴—significantly lower than primordial cometary methane (such as comet 67P/Churyumov–Gerasimenko) and matching abiotic methane synthesized via water-rock serpentinization or radiolytic hydrothermal reactions inside a warm rocky core. Second, JWST MIRI thermal emission spectra between 15 and 28 μm revealed an unexpected mid-infrared excess (~140–150 K brightness temperature over a tiny fraction of the disk, or warm fine-grained carbon/hydrocarbon dust rings or cryovolcanic hot spots), indicating that Makemake's differentiated rocky interior may have sustained recent or ongoing geothermal activity.

For a decade after Makemake's discovery, searches failed to find a binary companion, leaving Makemake's mass and bulk density unmeasured. In April 2015, Alex H. Parker, Marc W. Buie, Will M. Grundy, and Keith S. Noll used the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope to discover a faint satellite—designated S/2015 (136472) 1 and nicknamed MK 2—roughly 1,300 times fainter than Makemake (ΔV ≈ 7.80 magnitudes).

MK 2 had previously eluded detection because its orbit is viewed nearly edge-on from Earth, causing it to spend most of its time projected directly against Makemake's bright glare. Subsequent Hubble and ground-based adaptive optics astrometry established that MK 2 orbits on a nearly circular trajectory at a semi-major axis of ~21,100 to 22,250 km (well outside Makemake's Roche limit) with an orbital period of 12.4 to 13.0 days. Applying Kepler's third law yields a system mass of (3.1 ± 0.1) × 10²¹ kg and a bulk density of 2.06 ± 0.16 g/cm³, confirming a rock-rich differentiated interior (~65–70% silicate rock by mass). Assuming a dark charcoal-like albedo (~0.04), MK 2 is approximately 175 km in diameter, which partially accounts for the warm, dark thermal component observed alongside bright Makemake in earlier Spitzer and Herschel radiometry.

Why does Makemake have almost no nitrogen ice while Pluto is covered in nitrogen glaciers?

With a diameter of 1,430 km and a surface gravity roughly two-thirds that of Pluto, Makemake sits right inside the "volatile retention gap" of the Kuiper belt. Because molecular nitrogen (N₂) has a much higher vapor pressure at 30–40 K than methane (CH₄), thermal Jeans escape over 4.5 billion years stripped away almost all of Makemake's volatile N₂ envelope into space while leaving behind the less volatile methane frost.

Why didn't Clyde Tombaugh discover Makemake when he found Pluto in 1930?

Although Makemake is the second-brightest Kuiper belt object (magnitude ~17.0) and easily within the reach of 1930s photographic plates, its orbit is tilted by nearly 29° to the ecliptic plane. In 1930, Makemake was located high in the northern sky in Coma Berenices—where dense foreground Milky Way stars were sparse, but far outside the narrow ecliptic strip where Tombaugh concentrated his search—whereas Pluto happened to be passing near the ecliptic node.

What did the James Webb Space Telescope discover about Makemake's methane?

In 2024, JWST measured the ratio of heavy hydrogen (deuterium) to normal hydrogen in Makemake's surface methane (CH₃D / CH₄) and found it to be much lower than in pristine comets. This low D/H ratio indicates that Makemake's methane did not simply survive unchanged from the protoplanetary nebula, but was synthesized or equilibrated inside a warm, differentiated rocky core through geochemical reactions with liquid water before outgassing to the surface.

1. Brown, M. E., Barkume, K. M., Blake, G. A., Schaller, E. L., Rabinowitz, D. L., Roe, H. G., & Trujillo, C. A. (2007). Methane and ethane on the bright Kuiper belt object 2005 FY9. The Astronomical Journal, 133(1), 284–289. https://doi.org/10.1086/509734
2. Ortiz, J. L., Sicardy, B., Braga-Ribas, F., Alvarez-Candal, A., Lellouch, E., Duffard, R., Pinilla-Alonso, N., Ivanov, V. D., Littlefair, S. P., Camargo, J. I. B., Assafin, M., Unda-Sanzana, E., Jehin, E., Morales, N., Tancredi, G., Gil-Hutton, R., de La Cueva, I., Colque, J. P., Da Silva Neto, D. N., ... & Thirouin, A. (2012). Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation. Nature, 491(7425), 566–569. https://doi.org/10.1038/nature11597
3. Parker, A. H., Buie, M. W., Grundy, W. M., & Noll, K. S. (2016). Discovery of a Makemakean moon. The Astrophysical Journal Letters, 825(1), L9. https://doi.org/10.3847/2041-8205/825/1/L9
4. Schaller, E. L., & Brown, M. E. (2007). Volatile loss and retention on Kuiper belt objects. The Astrophysical Journal Letters, 659(1), L61–L64. https://doi.org/10.1086/516709
5. Glein, C. R., Grundy, W. M., Lunine, J. I., Wong, I., Protopapa, S., Pinilla-Alonso, N., Stansberry, J. A., Holler, B. J., Cook, J. C., & Souza-Feliciano, A. C. (2024). Moderate D/H ratios in methane ice on Eris and Makemake as evidence of hydrothermal or metamorphic processes in their interiors: Geochemical analysis. Icarus, 412, 115999. https://doi.org/10.1016/j.icarus.2024.115999