MK 2: Makemake's hidden moon

How Hubble found a dark moon in the glare of a bright dwarf planet — and why it matters for measuring Makemake.

Published 2026-10-23

For a decade after its discovery, Makemake appeared to be alone. That changed with Hubble Space Telescope images taken on April 27, 2015, which showed a faint point of light right next to the dwarf planet.^[4][3]

Finding a moon in the glare

The moon, provisionally designated S/2015 (136472) 1 and nicknamed “MK 2,” is more than 1,300 times fainter than Makemake. NASA reported it at roughly 13,000 miles (about 21,000 km) from the dwarf planet.^[4] Hubble's Wide Field Camera 3 was sharp enough to separate it from Makemake's glare; the discovery, by Alex Parker, Marc Buie, William Grundy and Keith Noll, was announced in April 2016.^[4][3]

Part of the challenge was geometry: when the moon was discovered, its orbit was seen nearly edge-on from Earth, so it often appeared to pass in front of or behind Makemake.^[3]

A dark companion

MK 2 is thought to be very dark — in contrast to Makemake's bright, methane-frosted surface. NASA estimates its radius at about 80 km; other analyses suggest a diameter around 175 km. It appears to follow a roughly circular orbit of about 18 days.^[1][3]

Why one small moon matters

A moon's orbit reveals the mass of the body it circles. Tracking MK 2 has allowed astronomers to estimate Makemake's mass at about 2.69 × 10²¹ kg and its density at about 1.76 g/cm³ — consistent with a mix of rock and ice.^[3] If Makemake's spin axis is aligned with the moon's orbit, the dwarf planet would be steeply tilted and experience extreme seasons.^[3]

Numbered citations link to the sources list on the Makemake overview page.