Pluto & Charon - Double Planet Meets Triple Star
Sept.
2, 2002
A rare celestial phenomenon involving the distant planet Pluto
has occurred twice within the past month. Seen from the Earth,
this planet moved in front of two different stars on July 20
and August 21, respectively, providing observers at various
observatories in South America and in the Pacific area with
a long awaited and most welcome opportunity to learn more about
tenuous atmosphere of that cold planet. On the first date, a
series of very sharp images of a small sky field with Pluto
and the star was obtained with the NAOS-CONICA (NACO) adaptive
optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope
at the Paranal Observatory.
With a diameter of about 2300 km, Pluto is about six times smaller
than the Earth. Like our own planet, it possesses a relatively
large moon, Charon, measuring 1200 km across and circling Pluto
at a distance of about 19,600 km once every 6.4 days. In fact,
because of the similarity of the two bodies, the Pluto-Charon
system is often referred to as a double planet.
At the current distance of nearly 4,500 million km from the
Earth, Pluto's disk subtends a very small angle in the sky,
0.107 arcsec. It is therefore very seldom that Pluto - during
its orbital motion - passes exactly in front of a comparatively
bright star. Such events are known as "occultations", and it
is difficult to predict exactly when and where on the Earth's
surface they are visible.
Stellar Occultations
When Pluto moves in front of a star, it casts a "shadow" on
the Earth's surface within which an observer cannot see the
star, much like the Earth's Moon hides the Sun during a total
solar eclipse. During the occultation event, Pluto's "shadow"
also moves across the Earth's surface. The width of this shadow
is equal to Pluto's diameter, i.e. about 2300 km.
One such occultation event was observed in 1988, and two others
were expected to occur in 2002, according to predictions published
in 2000 by American astronomers Steve W. McDonald and James
L. Elliot (Massachussetts Institute of Technology [MIT], Cambridge,
USA). Further refinements provided by other observers later
showed that the first event would be visible from South America
on July 20, 2002, while a second one on August 21 was expected
to be observable in the Pacific basin, from the western coast
of North America down to Hawaii and New Zealand.
A stellar occultation provides a useful opportunity to study
the planetary atmosphere, by means of accurate photometric measurements
of the dimming of the stellar light, as the planet moves in
front of the star. The observed variation of the light intensity
and colour provides crucial information about the structure
(atmospheric layers) and composition of different gases and
aerosols.
The July 20 Occultation
In order to profit from the rare opportunity to learn more about
Pluto and its atmosphere, a large campaign involving more than
twenty scientists and engineers from the Paris Observatory and
associated institutions was organized to observe the July 20,
2002, event involving an occultation of a star of visual magnitude
11 (i.e., about 100 times fainter than what can be perceived
with then unaided eye), referred to as "P126" in McDonald and
Elliot's catalogue.
 | | Photo
shows the path of Pluto's shadow (grey region) during
the July 20, 2002 occultation. The shadow has a diameter
of about 2300 km and moves from right to left; the timings
along the central line are indicated in one-minute intervals
(Universal Time - UT). The width of the gray area corresponds
to the regions where more than 50% of the light from the
star P126 A was attenuated by Pluto's atmosphere. The
dotted lines indicate where the stellar flux was attenuated
by more than 10%. The maximum duration of the occultation
(for observers located at the middle of the shadow track)
was about 3 min. The plot is based on astrometric measurements
posted at the MIT site. Once completely analyzed, the
VLT NACO images will provide significantly better accuracy
on the location of this track and therefore a solid basis
for the interpretation of the photometric observations
obtained with other telescopes. |
In May 2002, preparatory observations showed that star to be
double, with the brighter component of the system ("P126 A")
being likely to be occulted by Pluto, as seen from South America.
However, because of the duplicity, the predictions of exactly
where the shadow of Pluto would sweep the ground were uncertain
by about 0.1 arcsec in the sky, corresponding to more than 2000
km on the ground.
The NACO Images
In the end, the close approach (an "appulse" in astronomical
terminology) of Pluto and P126 A was indeed observed from various
sites in South America, with several mobile telescopes and also
including major facilities at the ESO La Silla and Paranal Observatories.
In particular, unique and very sharp images were obtained with
the NAOS-CONICA (NACO) adaptive optics (AO) camera mounted on
the ESO VLT 8.2-m YEPUN telescope. These images were made during
the final adjustments of the NACO instrument and in anticipation
of the upcoming science verification observations.
 | | Above
photo shows one of the images obtained with the NAOS-CONICA
(NACO) adaptive optics (AO) camera mounted on the ESO
VLT 8.2-m YEPUN telescope at the Paranal Observatory in
connection with a stellar occultation by Pluto on July
20, 2002. The star was found to be triple - the three
components (A, B and C), as well as Pluto and its moon,
Charon, are indicated in for easy orientation. |
The
NACO image shown was obtained in infrared light (in the K-band
at wavelength 2.2 micron) on July 20, 2002, some 45 min before
Pluto's shadow passed north of Paranal.
The orientation is such that North is up, and East is left.
The small sky field measures about 7 x 7 arcsec2. The pixel
size is 0.027 arcsec, and the achieved image sharpness corresponds
to the theoretical limit (the diffraction limit) with a telescope
of this size and at this wavelength (0.07 arcsec).
The object at the centre is the star P126 A of K-magnitude
9.5, while the brighter object at the right is the companion
star P126 B, 2.25 arcsec away. As P126 B is very red (of stellar
type M), it appears brighter than P126 A at this infrared
wavelength - the opposite is true in visible light. The intensity
of the left part of the image has been multiplied by a factor
of approximately 35 in order to better display Pluto and its
moon Charon, located some 0.5 arcsec to the lower left (SE)
of the planet. Note also the faint star "P126 C", at this
moment very close to Pluto; it is probably a (physical) member
of the P126 system.
A closer inspection of the original image shows that the disk
of Pluto (with a diameter of 0.107 arscec and covering 16
NACO pixels) is (barely) resolved.
Many images were taken by NACO prior to the occultation. They
will allow to retrace the precise motion of Pluto relative
to P126 A, thereby improving the mapping of the motion of
Pluto's shadow on the ground. This is important in order to
apply the correct geometrical circumstances for the interpretation
of the photometric observations. A first evaluation of the
NACO data indicates that the Paranal site "missed" the upper
layers of Pluto's atmosphere by a mere 200 km or so - this
is equivalent to no more than one hundredth of an arcsec as
projected on the sky.
Appendix: Stellar occultations and Pluto's atmosphere
Stellar occultations are presently the only way to probe Pluto's
tenuous atmosphere. When the star moves behind the planet,
the stellar rays suffer minute deviations as they are refracted
(i.e., bent and defocussed) by the planet's atmospheric layers.
This effect, together with the large distance to the planet,
manifests itself as a gradual decline of observed intensity
of the stellar light, rather than an abrupt drop as this would
be the case if the planet had no atmosphere.
Pluto's atmosphere was first detected on August 19, 1985,
during a stellar occultation observed from Israel and then
studied in more detail from Australia and from the Kuiper
Airborne Observatory (KAO) during another such event on July
9, 1988. However, Pluto's atmosphere is still not well understood.
It appears to be mostly composed of a dominant gas of atomic
weight 28, probably molecular nitrogen (N2). Near-IR solar
reflection spectra have since shown a small presence of methane
(CH4), probably at a level of about 1% relative to nitrogen.
The 1988 occultation clearly revealed two different layers
in Pluto's atmosphere, a rather smooth and isothermal outer
part, and a more complex one near the planet's surface, with
the possible presence of an inversion layer (in which the
temperature increases with altitude) or possibly haze of photochemical
origin.
The present observations aimed at discriminating between the
current theoretical models of Pluto's atmosphere by means
of detailed measurements of the changing intensity and colour
of the stellar light, as the star is seen through progressively
lower layers of the planet's atmosphere.
Another important issue is the question of whether Pluto's
atmosphere has changed since 1988. In the intervening 14 years,
the planet has moved away from the Sun in its elliptic orbit,
whereby there has been a change in the insolation (solar flux)
of about 6%. This effect might possibly have caused changes
in the surface temperature and in the overall atmospheric
structure of Pluto. However, any firm conclusions will have
to await a complete and careful evaluation of all available
observations.
European
Southern Observatory
|
