[meteorite-list] Hubble Discovers Moon Orbiting the Dwarf Planet Makemake

[Ron Baalke]

https://www.nasa.gov/feature/goddard/2016/hubble-discovers-moon-orbiting-the-dwarf-planet-makemake

Hubble Discovers Moon Orbiting the Dwarf Planet Makemake
April 26, 2016

Peering to the outskirts of our solar system, NASA's Hubble Space Telescope 
has spotted a small, dark moon orbiting Makemake, the second brightest 
icy dwarf planet - after Pluto - in the Kuiper Belt.

The moon - provisionally designated S/2015 (136472) 1 and nicknamed 
MK 2 - is more than 1,300 times fainter than Makemake. MK 2 was seen 
approximately 13,000 miles from the dwarf planet, and its diameter is 
estimated to be 100 miles across. Makemake is 870 miles wide. The dwarf 
planet, discovered in 2005, is named for a creation deity of the Rapa 
Nui people of Easter Island.

The Kuiper Belt is a vast reservoir of leftover frozen material from the 
construction of our solar system 4.5 billion years ago and home to several 
dwarf planets. Some of these worlds have known satellites, but this is 
the first discovery of a companion object to Makemake. Makemake is one 
of five dwarf planets recognized by the International Astronomical Union.

The observations were made in April 2015 with Hubble's Wide Field Camera 
3. Hubble's unique ability to see faint objects near bright ones, together 
with its sharp resolution, allowed astronomers to pluck out the moon from 
Makemake's glare. The discovery was announced today in a Minor Planet 
Electronic Circular.

The observing team used the same Hubble technique to observe the moon 
as they did for finding the small satellites of Pluto in 2005, 2011, and 
2012. Several previous searches around Makemake had turned up empty. "Our 
preliminary estimates show that the moon's orbit seems to be edge-on, 
and that means that often when you look at the system you are going to 
miss the moon because it gets lost in the bright glare of Makemake," 
said Alex Parker of Southwest Research Institute, Boulder, Colorado, who 
led the image analysis for the observations.

A moon's discovery can provide valuable information on the dwarf-planet 
system. By measuring the moon's orbit, astronomers can calculate a mass 
for the system and gain insight into its evolution.

Uncovering the moon also reinforces the idea that most dwarf planets have 
satellites.

"Makemake is in the class of rare Pluto-like objects, so finding a companion 
is important," Parker said. "The discovery of this moon has given 
us an opportunity to study Makemake in far greater detail than we ever 
would have been able to without the companion."

Finding this moon only increases the parallels between Pluto and Makemake. 
Both objects are already known to be covered in frozen methane. As was 
done with Pluto, further study of the satellite will easily reveal the 
density of Makemake, a key result that will indicate if the bulk compositions 
of Pluto and Makemake are also similar. "This new discovery opens a 
new chapter in comparative planetology in the outer solar system," said 
team leader Marc Buie of the Southwest Research Institute, Boulder, Colorado.

The researchers will need more Hubble observations to make accurate measurements 
to determine if the moon's orbit is elliptical or circular. Preliminary 
estimates indicate that if the moon is in a circular orbit, it completes 
a circuit around Makemake in 12 days or longer.

Determining the shape of the moon's orbit will help settle the question 
of its origin. A tight circular orbit means that MK 2 is probably the 
product of a collision between Makemake and another Kuiper Belt Object. 
If the moon is in a wide, elongated orbit, it is more likely to be a captured 
object from the Kuiper Belt. Either event would have likely occurred several 
billion years ago, when the solar system was young.

The discovery may have solved one mystery about Makemake. Previous infrared 
studies of the dwarf planet revealed that while Makemake's surface is 
almost entirely bright and very cold, some areas appear warmer than other 
areas. Astronomers had suggested that this discrepancy may be due to the 
sun warming discrete dark patches on Makemake's surface. However, unless 
Makemake is in a special orientation, these dark patches should make the 
dwarf planet's brightness vary substantially as it rotates. But this 
amount of variability has never been observed.

These previous infrared data did not have sufficient resolution to separate 
Makemake from MK 2. The team's reanalysis, based on the new Hubble observations, 
suggests that much of the warmer surface detected previously in infrared 
light may, in reality, simply have been the dark surface of the companion 
MK 2.
There are several possibilities that could explain why the moon would 
have a charcoal-black surface, even though it is orbiting a dwarf planet 
that is as bright as fresh snow. One idea is that, unlike larger objects 
such as Makemake, MK 2 is small enough that it cannot gravitationally 
hold onto a bright, icy crust, which sublimates, changing from solid to 
gas, under sunlight. This would make the moon similar to comets and other 
Kuiper Belt Objects, many of which are covered with very dark material.

When Pluto's moon Charon was discovered in 1978, astronomers quickly 
calculated the mass of the system. Pluto's mass was hundreds of times 
smaller than the mass originally estimated when it was found in 1930. 
With Charon's discovery, astronomers suddenly knew something was fundamentally 
different about Pluto. "That's the kind of transformative measurement 
that having a satellite can enable," Parker said.

The Hubble Space Telescope is a project of international cooperation between 
NASA and the European Space Agency. NASA's Goddard Space Flight Center 
in Greenbelt, Maryland, manages the telescope. The Space Telescope Science 
Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. 
STScI is operated for NASA by the Association of Universities for Research 
in Astronomy in Washington, D.C.

For images and more information about Makemake's moon MK 2 and Hubble, 
visit:

* http://www.nasa.gov/hubble
* http://hubblesite.org/news/2016/18

For additional information, contact:

Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou at nasa.gov

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver at stsci.edu / villard at stsci.edu

Alex Parker
Southwest Research Institute, Boulder, Colorado
360-599-5346
alex.parker at swri.org