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.
Sources:
1)
Draft version April 27,
2016
Preprint typeset using LATEX style AASTeX6 v. 1.0
DISCOVERY OF A MAKEMAKEAN MOON
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.
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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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