Thursday, 12 May 2016

A New Moon in the Solar System - Makemake’s Moon

A New Moon in the Solar System - Makemake’s Moon

Helena Puumala’s SF Romance and Adventure series features a planet, Kordea, with seven moons.  This rather unusual setting gives me the opportunity to talk about some of the remarkable moons in our solar system.  In some earlier blogs, I covered a lot of the more significant moons of the solar system, primarily the larger ones.  But, very recently a new moon was discovered, orbiting the minor planet Makemake.  That discovery was by the Hubble Space Telescope, this April.

Makemake is one of the many dwarf planets beyond the orbit of Pluto, and perhaps the biggest of the Kuiper belt objects (KBOs).  It’s about two-thirds the size of Pluto, with an orbit of between 38.6 and 52.8 AU (astronomical units, the distance from the Earth to the Sun).  That’s about 5.8 billion to 8.0 billion km.  For comparison, Pluto’s orbit is between 29.7 and 49.3 AU.  By the way, Makemake is named after a mythological figure of the residents of Easter Island.
Obviously, it is cold out there, about 30 degrees Kelvin.  Pluto’s average temperature is somewhat higher, at about 44 degrees Kelvin.  We now know that Pluto is a much more dynamic and interesting body than had long been assumed, due to the observations of the New Horizons spacecraft in July 2015.  Perhaps Makemake and its moon will surprise us too, once we make it out there, with a spacecraft.
So far, the new moon has the evocative name S/2015 (136472).  Makemake’s moon is quite small, about 160 km in diameter.  It orbits Makemake at about 21,000 km.  Not much else is known about it, as of now, though a few things can be inferred.
Interestingly, it ought to look about the same size in the sky of Makemake as our moon does from Earth.  A little trigonometry gives its angular dimension in Makemake’s sky as about half a degree (about 26 minutes):
Makemake’s moon’s  radius: 80 km
It’s Moon’s orbital distance: 21000 km
Tangent of corresponding angle: opp/adj = 80/21000 = 0.003809524
ArcTan = (0.003809524) = 0.003809505 radians
= 0.003809505*(180/3.14159) degrees
=  0.21 degrees
= 13.05 minutes
= 26.1 minutes (add both half-angles)
As a check, we can do the same with Earth’s moon, Luna:
Luna’s radius: 1740 km
Luna’s orbital distance: 380,000 km
Tangent of corresponding angle: opp/adj = 30/3800000 = 0.004578947
ArcTan = (0.004578947) = 0.004578915 radians
= 0.004578915*(180/3.14159) degrees
=  0.26 degrees
= 15.7 minutes
= 31.4 minutes (add both half-angles)
Wiki gives Luna’s angular measure in the sky as 29 to 34 arcminutes, so that checks out for the Earth-moon system.  In both cases, the angular measure of the moon, as seen from the planet, can vary throughout the orbit, for various reasons.
Naturally, Makemake’s moon would not be appear to be as bright as our moon, in the Makemake sky, even though it would appear to be about the same size as our moon looks from Earth.  That’s because both the planet and the moon are a lot farther away from the sun, so there will be less sunlight to reflect off the moon back to Makemake.
From the Earth, the apparent magnitude of Makemake is about 17.0, while the apparent magnitude of its moon is about 25.1.  That’s a combination of Makemake’s moon being much smaller than the planet, as well as its low albedo (reflectivity).   It turns out that Makemake is very bright (albedo of about 0.8 to 0.9, indicating an icy surface, possibly a bit reddish due to tholins), while its moon is very dark (similar to charcoal, about 0.04).
Here are my calculations for that result.  As seen from the Earth, there is an 8.1 difference in apparent magnitude.  Some of this is due to the smaller size of Makemake’s moon:
Planet radius/Moon Radius: 720/80  = 9
Square that to compare areas: = 81
So, Makemake is 81 times brighter than its moon, due to the difference in size.  That corresponds to a magnitude difference of 4.78 (a difference of 100 times brightness is 5 magnitudes).
That leaves 3.32 magnitudes to be accounted for by the difference in albedo, which corresponds to a factor of 21 times brightness (2.512 raised to the power 3.32).  If the albedo of Makemake is around .9, then the albedo of its moon must be around .04 (.9/.04=22.5).
The difference in albedos leads some researchers to think that the moon may be a captured object, rather than being born at the same time as Makemake.  Otherwise, the compositional materials would be expected to be similar, and the albedos not so far apart.  However, there is still support for the idea that both bodies are the result of some giant collision in the distant past.
Earlier observations of Makemake had indicated a mottled surface, mostly bright, but with some very dark zones.  Now that this moon has been discovered, it is probable that the dark spots were actually the moon passing between the telescope and the planet.
The fact that Makemake’s moon had not been discovered up until now, is thought to be due to its orbit, relative to Earth.  If the planet’s moon’s orbital plane is edge on from Earth, it would either be in front of Makemake, or behind it, for a significant amount of the time.  It is thought to be just positioned in the right place in the most recent observations, and thus detected.
Makemake’s rotation period has been measured at 7.77 hours, while its moon is probably of the order of somewhat over a dozen days.  So, a lunar month on Makemake would probably be close to 40 Makemake days, not all that different from the situation on Earth.  But, the precise orbital parameters for Makemake’s moon have not yet been measured.
Finally, it is worth noting that Makemake’s absence of a moon was a bit of an anomaly, as the other large trans-Neptunian dwarf planets (Pluto, Eris, Haumea) were known to have satellites.  This fixes that little mystery.


1)    Draft version April 27, 2016
Preprint typeset using LATEX style AASTeX6 v. 1.0

Alex H. Parker1 and Marc W. Buie
Southwest Research Institute
1050 Walnut St., Suite 300
Boulder, CO 80302, USA
Will M. Grundy
Lowell Observatory, Flagsta_, AZ, USA.
Keith S. Noll
NASA Goddard Space Flight Center, Greenbelt, MD, USA.
 arXiv:1604.07461v1 [astro-ph.EP] 25 Apr 2016

2)    Wiki, Makemake Entry,
3)    New Scientist website: Never-before-seen moon spotted around dwarf planet Makemae, Daily News 27 April 2016.


Now, here’s a moon-based pitch for Helena Puumala’s latest SF Romance series, The Witches’ Stones:

Book one, Rescue from the Planet of the Amartos
Featuring plenty of futuristic cold-war type action, the novel concerns a struggle between three galactic powers (The Terran Confederation, the Organization, and the Witches of Kordea) over one talented young Terran woman, Sarah McKenzie, whose psychic abilities could tip the balance of power in the galaxy.

The Witches’ Stones Book 2: Love and Intrigue under the Seven Moons of Kordea.
Book 2 of the series features the planet Kordea, home to a race of beautiful and powerful psychic aliens, known as the Witches of Kordea.  Sarah continues to be the object of intrigue and conflict, and Terran Confederation agent Coryn Leigh must continue to ensure her safety, as the Organization plots her abduction.  Love and romance complicate matters.
The planet has seven moons, an extraordinary arrangement for a terrestrial sized planet in its star's habitable zone, which features prominently in the story.  A terrestrial planet with seven moons would be cool (though it would probably be a very unstable arrangement, which is noted and explained in the novel).  It also features an artificial moon at the L2 point, which could destroy the planet, should it be dislodged from that point.

The Witches’ Stones Book 3: Revenge of the Catspaw
The Organization, also known as the Neotsarians, have captured agent Coryn Leigh and are using him as bait to get to Sarah.  But matters aren’t simple, where Catspaws are involved, who can turn the tables on their masters.
Available on Amazon at the end of May 2016.  Check her author page for all her books, including this new one.


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