Haumea,
formal
designation (136108) Haumea, is a
dwarf planet in the
Kuiper belt. Its mass is one-third the mass of
Pluto.
It was discovered in 2004 by a team headed by
Mike Brown of Caltech
at the
Palomar
Observatory
in the United States and, in 2005, by a team headed
by J. L. Ortiz at the Sierra Nevada
Observatory
in Spain, though the latter claim has been
contested. On September 17, 2008, it was designated a dwarf
planet by the
International Astronomical
Union (IAU) and named after
Haumea, the Hawaiian goddess of
childbirth.
Haumea's extreme elongation makes it unique among known
trans-Neptunian objects (TNOs).
Although its shape has not been directly observed, calculations
from its
light curve suggest it is an
ellipsoid, with its greatest
axis twice as long as its shortest.
Nonetheless, its
gravity is believed
sufficient for it to have relaxed into
hydrostatic equilibrium, thereby
meeting the definition of a dwarf planet. This elongation, along
with its unusually rapid rotation, high density, and high
albedo (from a surface of crystalline water ice), are
thought to be the results of a
giant
collision, which left Haumea the largest member of a
collisional family that includes several
large TNOs and its two known moons.
Classification
Haumea is a
plutoid, a
term used to describe
dwarf planets beyond Neptune's orbit. Its
status as a dwarf planet means it is presumed to be massive enough
to have been rounded by its own gravity but not to have
cleared its neighbourhood of
similar objects. Although Haumea appears to be far from spherical,
its
ellipsoidal shape is thought to result
from its rapid
rotation, in much the same
way that a
water balloon stretches out
when tossed with a spin, and not from a lack of sufficient gravity
to overcome the
compressive
strength of its material. Haumea was initially listed as a
classical Kuiper belt
object (classical KBO) in 2006 by the
Minor Planet Center, but it is no longer
listed as such. The nominal trajectory suggests that it is in a
fifth-order
12:7
resonance with Neptune since the
perihelion distance of 35 AU is near the
limit of stability with Neptune.
Further observations of the orbit will be required to verify its
dynamical status.
Name
Until it was given a permanent name, the Caltech discovery team
used the nickname "
Santa" among
themselves, as they had discovered Haumea on December 28, 2004,
just after
Christmas. The Spanish team
proposed a separate discovery to the
Minor Planet Center (MPC) in July 2005.
On July 29, 2005, Haumea was given its first official label, the
temporary designation , with the "2003" based on the date of the
Spanish discovery image. On September 7, 2006, it was numbered and
admitted into the official minor planet catalogue as .
Following
guidelines
established by the IAU that classical KBOs be given names of
mythological beings associated with creation, in September 2006 the
Caltech team submitted formal names from
Hawaiian mythology to the IAU for both
and its moons, in order "to pay homage to the place where the
satellites were discovered". The names were proposed by
David Rabinowitz of the Caltech team.
Haumea is the matron goddess of the
island of Hawai
i
, where the Mauna Kea Observatory
is located. In addition, she is identified
with
Pāpā, the goddess of the earth
and wife of
Wākea (space), which is
appropriate because is thought to be composed almost entirely of
solid rock, without the thick ice mantle over a small rocky core
typical of other known Kuiper belt objects. Lastly, Haumea is the
goddess of fertility and childbirth, with many children who sprang
from different parts of her body; this corresponds to the swarm of
icy bodies thought to have broken off the dwarf planet during an
ancient collision. The two known moons, also believed to have been
born in this manner, are thus named after two of Haumea's
daughters,
Hi iaka and
Nāmaka.
Discovery controversy
Two teams claim credit for the discovery of Haumea. Mike Brown and
his team at Caltech discovered Haumea in December 2004 on images
they had taken on May 6, 2004. On July 20, 2005, they published an
online abstract of a report intended to announce the discovery at a
conference in September 2005. At around this time, José Luis Ortiz
Moreno and his team at the Instituto de Astrofísica de Andalucía at
Sierra Nevada Observatory in Spain found Haumea on images taken on
March 7–10, 2003. Ortiz emailed the Minor Planet Center with their
discovery on the night of July 27, 2005.
Brown came to suspect the Spanish team of fraud upon learning that
his observation logs were accessed from the Spanish observatory the
day before the discovery announcement. These logs included enough
information to allow the Ortiz team to
precover Haumea in their 2003 images, and they
were accessed again just before Ortiz scheduled telescope time to
obtain confirmation images for a second announcement to the MPC on
July 29. Ortiz later admitted he had accessed the Caltech
observation logs but denied any wrongdoing, stating he was merely
verifying whether they had discovered a new object.
IAU protocol is that discovery credit for a
minor planet goes to whoever first submits a
report to the MPC with enough positional data for a decent
determination of its orbit, and that the credited discoverer has
priority in choosing a name. However, the IAU announcement on
September 17, 2008, that Haumea had been accepted as a dwarf
planet, made no mention of a discoverer. The location of discovery
was listed as the Sierra Nevada Observatory of the Spanish team,
but the chosen name, Haumea, was the Caltech proposal.
Orbit and rotation
Haumea has a typical orbit for a classical Kuiper belt object, with
an
orbital period of 283 Earth years,
a
perihelion of 35
AU, and an
orbital inclination of 28°. It passed
aphelion in early 1992, and is currently
more than 50 AU from the Sun.
Haumea's orbit has a slightly greater
eccentricity than the other members of
its collisional family. This is
thought to be due to Haumea's weak fifth-order 12:7 orbital
resonance with Neptune gradually modifying its initial orbit, over
the course of a billion years, through the
Kozai effect, which allows the exchange of
an orbit's inclination for increased eccentricity.
With a
visual magnitude of 17.3,
Haumea is the
third brightest
object in the Kuiper belt after Pluto and , and easily
observable with a large amateur telescope. However, since the
planets and most
small Solar
System bodies share a
common
orbital alignment from their
formation in the
primordial disk of the Solar
System, most early surveys for distant objects focused on the
projection on the sky of this common plane, called the
ecliptic. As the region of sky close to the
ecliptic became well explored, later sky surveys began looking for
objects that had been dynamically excited into orbits with higher
inclinations, as well as more distant objects, with slower
mean motions across the sky. These surveys
eventually covered the location of Haumea, with its high orbital
inclination and current position far from the ecliptic.
Haumea displays large fluctuations in brightness over a period of
3.9 hours, which can only be explained by a rotational period of
this length. This is faster than any other known equilibrium body
in the
Solar System, and indeed faster
than any other known body larger than 100 km in diameter. This
rapid rotation is thought to have been caused by the impact that
created its satellites and collisional family.
Physical characteristics
Since Haumea has moons, the mass of the system can be calculated
from their orbits using
Kepler's third law. The result is 28% the mass of the Plutonian
system and 6% the mass of the Earth's
Moon.
Nearly all of this mass is in Haumea.
Size, shape, and composition
The size of a Solar System object can be derived from its
optical magnitude, its distance, and its
albedo. Objects appear bright to Earth
observers either because they are large or because they are highly
reflective. If their reflectivity (albedo) can be ascertained, then
a rough estimate can be made of their size. For most distant
objects, the albedo is unknown, but Haumea is large and bright
enough for its
thermal emission to be
measured, which has given an approximate value for its albedo and
thus its size. However, the calculation of its dimensions is
complicated by its rapid rotation. The
rotational physics of
deformable bodies predicts that over as
little as a hundred days, a body rotating as rapidly as Haumea will
have been distorted into the
equilibrium form of a
scalene ellipsoid. It is
thought that most of the fluctuation in Haumea's brightness is
caused not by local differences in albedo but by the alternation of
the side view and end view as seen from Earth.
The rotation and amplitude of Haumea's
light curve place strong constraints
on its composition. If Haumea had a low
density like Pluto, with a thick mantle of
ice over a small
rocky
core, its rapid rotation would have elongated it to a greater
extent than the fluctuations in its brightness allow. Such
considerations constrain its density to a range of
2.6–3.3 g/cm³. This range covers the values for
silicate minerals such as
olivine and
pyroxene, which
make up many of the
rocky
objects in the Solar System. This suggests that the bulk of
Haumea is rock covered with a relatively thin layer of ice. A thick
ice mantle more typical of Kuiper belt objects may have been
blasted off during the impact that formed the Haumean collisional
family.
The denser the object in hydrostatic equilibrium, the more
spherical it must be for a given rotational period, placing
constraints on Haumea's possible dimensions. Fitting its accurately
known mass, its rotation, and its inferred density to an
equilibrium ellipsoid predicts that Haumea is approximately the
diameter of Pluto along its longest axis and about half that at its
poles. Since no observations of
occultations of stars by Haumea or
occultations of the dwarf planet with its moons have yet been made,
direct, precise measurements of its dimensions, like those that
have
been made for Pluto, do
not yet exist.
Several ellipsoid-model calculations of Haumea's dimensions have
been made. The first model produced after Haumea's discovery was
calculated from
ground-based
observations of Haumea's
light curve at
optical wavelengths: it provided a
total length of 1,960 to 2,500 km and a
visual albedo (p
v) greater
than 0.6.This model gives approximate triaxial dimensions of 2,000
x 1,500 x 1,000 km, with an albedo of 0.73.The
Spitzer Space Telescope has
estimated Haumea to have a diameter of 1,150 km and an albedo of
0.84 , from
photometry at
infrared wavelengths of 70 μm.Subsequent
light curve analyses have suggested an equivalent circular diameter
of 1,450 km.These independent size estimates overlap at an
average
geometric mean diameter of
roughly 1,400 km. This makes Haumea one of the largest
trans-Neptunian objects discovered, third or fourth after , , and
perhaps , and larger than , , or .
Surface
In addition to the large fluctuations in Haumea's light curve due
to the body's shape, which affect all
colours equally, smaller independent colour
variations seen in both visible and near-infrared wavelengths show
a region on the surface that differs both in colour and in albedo.
More specifically, a dark red area on Haumea's bright white surface
has been seen, which indicates an area rich in minerals and organic
(carbon-rich) compounds, or possibly a higher proportion of
crystalline ice. Thus Haumea may have a mottled surface reminiscent
of Pluto, if not as extreme.
In 2005,
the Gemini
and Keck
telescopes
obtained spectra of Haumea which showed
strong crystalline water ice features similar to
the surface of Pluto's moon Charon. This is peculiar, because
crystalline ice forms at temperatures above 110 K, while the
surface temperature of Haumea is below 50 K, a temperature at
which
amorphous ice is formed. In
addition, the structure of crystalline ice is unstable under the
constant rain of
cosmic rays and
energetic particles from the Sun that strike trans-Neptunian
objects. The timescale for the crystalline ice to revert to
amorphous ice under this bombardment
is on the order of ten million years, while trans-Neptunian objects
have been in their present cold-temperature locations for
timescales of thousands of millions of years. Radiation damage
should also redden and darken the surface of trans-Neptunian
objects where the common surface materials of
organic ices and
tholin-like compounds are present, as is the case
with Pluto. Therefore, the spectra and
colour suggest Haumea and its family members
have undergone recent resurfacing that produced fresh ice. However,
no plausible resurfacing mechanism has been suggested.
Haumea is as bright as snow, with an albedo in the range of
0.6–0.8, consistent with crystalline ice. Other large TNOs such as
appear to have albedos as high or higher. Best-fit modeling of the
surface spectra suggested that 66% to 80% of the Haumean surface
appears to be pure crystalline water ice, with one contributor to
the high albedo possibly
hydrogen
cyanide or
phyllosilicate clays.
Inorganic cyanide salts such as copper potassium cyanide may also
be present.
However, further studies of the visible and near infrared spectra
suggest a homomorphous surface covered by an intimate 1:1 mixture
of amorphous and crystalline ice, together with no more than 8%
organics. The absence of ammonia hydrate excludes cryovolcanism and
the observations confirm that the collisional event must have
happened more than 100 million years ago, in agreement with the
dynamic studies.The absence of measurable
methane in the spectra of Haumea is consistent with
a warm
collisional history that would
have removed such
volatiles,in contrast to
.
In September 2009, Haumea was discovered to have a large dark
reddish spot, possibly an impact feature, and not to be uniformly
bright as previously believed. While the reason for the color is
unknown, possibilities include crystalline ice or higher
concentrations of minerals and organic compounds than the rest of
the surface.
Moons
small
satellites have been
discovered orbiting Haumea,
Haumea I Hi
iaka and
Haumea II Namaka.
Brown's
team discovered both in 2005, through observations of Haumea using
the W.M.
Keck Observatory.
Hi iaka, at first nicknamed "
Rudolph" by the Caltech team,
was discovered January 26, 2005. It is the outer and, at roughly
310 km in diameter, the larger and brighter of the two, and
orbits Haumea in a nearly circular path every 49 days. Strong
absorption features at 1.5 and 2
micrometres in the
infrared spectrum are consistent with nearly pure
crystalline water ice covering much of the surface. The unusual
spectrum, along with similar absorption lines on Haumea, led Brown
and colleagues to conclude that capture was an unlikely model for
the system's formation, and that the Haumean moons must be
fragments of Haumea itself.
Namaka, the smaller, inner satellite of Haumea, was discovered on
June 30, 2005, and nicknamed "
Blitzen". It is a tenth the mass of Hi
iaka, orbits Haumea in 18 days in a highly elliptical,
non-Keplerian orbit, and as of 2008 is
inclined 13° from the larger moon, which
perturbs its orbit.The relatively
large eccentricities together with the mutual inclination of the
orbits of the satellites are unexpected as they should have been
damped by the
tidal effects. A
relatively recent passage by a (3:1) resonance might explain the
current excited orbits of the Haumea moons.
At present, the orbits of the Haumean moons appear almost exactly
edge-on from Earth, with Namaka periodically
occulting Haumea. Observation of such transits
would provide precise information on the size and shape of Haumea
and its moons, as happened in
the late 1980s with Pluto and
Charon. The tiny change in brightness of the system during
these occultations will require at least a
medium-
aperture
professional telescope
for detection. Hi iaka last occulted Haumea in 1999, a few years
before discovery, and will not do so again for some 130 years.
However, in a situation unique among
regular satellites, Namaka's orbit is being
greatly
torqued by Hi
iaka, preserving the viewing angle of Namaka–Haumea transits for
several more years.
Collisional family
Haumea is the largest member of its
collisional family, a group of
astronomical objects with similar physical and orbital
characteristics thought to have formed when a larger progenitor was
shattered by an impact. This family is the first to be identified
among TNOs and includes—beside Haumea and its moons—
(≈600 km), ( 700 km), (≈500 km), ( 700 km), and
( 700 km). Brown et al. proposed that the family were a direct
product of the impact that removed Haumea's ice mantle, but a
second proposal suggests a more complicated origin: that the
material ejected in the initial collision instead coalesced into a
large moon of Haumea, which was later shattered in a second
collision, dispersing its shards outwards. This second scenario
appears to produce a dispersion of velocities for the fragments
that is more closely matched to the measured velocity dispersion of
the family members.
The presence of the collisional family could imply that Haumea and
its "offspring" might have originated in the
scattered disc. In today's sparsely populated
Kuiper belt, the chance of such a collision occurring over the age
of the Solar System is less than 0.1 percent. The family could not
have formed in the denser primordial Kuiper belt because such a
close-knit group would have been disrupted by
Neptune's migration into the
belt—the believed cause of the belt's current low density.
Therefore it appears likely that the dynamic scattered disc region,
in which the possibility of such a collision is far higher, is the
place of origin for the object that generated Haumea and its
kin.
Because it would have taken at least a billion years for the group
to have diffused as far as it has, the collision which created the
Haumea family is believed to have occurred very early in the Solar
System's history.
Notes
References
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