Sedna: The story of the Transneptunian object who was a planet for a day
Sedna is not that big a deal. From the point of view of physical
characteristics, it is just one of many transneptunian objects
discovered in the past years. Not the smallest, but not the largest either.
Over time, its diameter, estimated after the discovery at an optimistic 1800 km,
has dropped to half that, and today it is believed to be similar in size to Ceres,
the queen of the main asteroid belt. And so, one would be forgiven for wondering-why
deal with it, beyond the bare minimum of information owed to any celestial object?
As we are about to see, the reasons are there, but we want to leave the final judgment to you!
Think about it! We are making so much progress in our knowledge of the universe,
in the study of galaxies millions and billions of light-years away, yet we still know very little
about the planetary system that hosts us. No one, in fact, can yet answer this simple question,
"where does our Solar System "end" ?" By the early 1990s, our celestial
cradle seemed to have no more secrets for us Earthlings. The Mariner and Venera probes had
unveiled the surfaces of Mercury, Venus, and Mars, while the Pioneer and Voyager
missions had later succeeded in reaching and photographing the great outer planets as well.
Thus, having long since vanished the "ghosts" chased in the past
(such as the intramercurial planet "Vulcan" and "Planet X"), in the years that followed
the belief was almost reached that every corner of our cosmic cradle, from Mercury to Pluto,
had already been explored. One star, nine planets, many moons, thousands of asteroids,
the occasional comet - what more could there still be to discover?
Then in 1992 came the surprise discovery of the first object orbiting beyond Pluto,
soon followed by many other medium-sized asteroids (two or three hundred kilometers in diameter):
the first real inhabitants of that Kuiper Belt whose existence, until then,
had only been hypothesized... and it was as if suddenly the solar system doubled in volume.
So it was really true... beyond the orbit of Neptune stretched a belt 10 times larger than
that of the asteroids, with a huge number of rocky and icy objects orbiting the sun
at distances between 35 and 55 Astronomical Units
(remember that a so-called Astronomical Unit is a unit of measurement equal to the Earth-Sun
distance, which is about 150 million kilometers). And the digital revolution was also making
it possible to get to find fainter and fainter objects.
In November 1998, Chaos, a 600-km-diameter transneptunian 46 Astronomical Units away,
was found; then in November 2000, it was the turn of Varuna, 680 km in diameter and 43
Astronomical Units away, followed in May 2001 by Ixion, 700 km in diameter and 40 AU away.
Then entered the scene the staff of Michael Brown, a Caltech astronomer who had set
up a revolutionary wide-field search survey with Palomar's Oschin Telescope.
And it was during that campaign that, on the night of Nov. 14, 2003, an object was detected and given
the provisional designation of 2003 VB12. From the very first data available,
it becomes clear that we are in the presence of something extremely distant: an initial analysis
suggests that it may be about 100 AU... But not only that... from initial cursory assessments
it would be a really large object, with a diameter that could approach 2,000 km!
The November 14 detections are confirmed by other observers, and,
thanks to subsequent observations, an attempt is made to speculate on a first possible orbit.
But the available data are still too sparse to return sufficiently acceptable results:
Indeed, to calculate the characteristics of an orbit, it is necessary to have observations
covering a time span of several years. There is a loophole, however, and that is the so-called
"precovery" observations (short for pre-discovery recovery). That is, one hopes that the object - by
chance - may have been photographed months or years earlier during some other survey; so as to
extend the observational span as much as possible And that is exactly what happens with 2003 VB12!
The object is also identified in Palomar-Quest images taken on September 29 and August 30,
2003. The new astrometric data not only allow a more accurate orbit to be defined but also allow
the object to be identified on images taken by Palomar's Schmidt dating back to October 2002.
And fate is even more benevolent: the new dates are able to go back even further,
discovering the object's presence on images taken by the Near-Earth Asteroid Tracking
program in September and October 2001. Now the available data are adequate and the
orbit can be defined with sufficient accuracy. Thus it turns out that at the time of discovery
the object was 90 AU from the Sun, but also that 2003 VB12 traverses a very elliptical, elongated
orbit... with perihelion (the closest point to the Sun) at 76 AU and aphelion (the farthest point) as
high as 937 AU, or 30 times farther than Neptune! This is an incredibly large orbit,
so much so that the object takes a whopping 11,400 years to travel the entire orbit.
No other known object discovered beyond Neptune reached such distances.
And so, suddenly, the solar system found itself growing tenfold in size!
When the official news of the discovery came in February 2004,
it caused a sensation and was interpreted by many as if a new gold rush had opened at that moment,
with the Kuiper Belt instead of the Klondike. There was a real belief that very soon very
large planets would be found in those regions. And right away the newcomer is being hailed by all the
media as "the tenth planet in the solar system!" Because of its location in those icy regions,
the discoverers proposed that 2003 VP12 be assigned the name of Sedna, a deity of the Inuit
(the original inhabitants of the Arctic regions of North America and Siberia)
believed to be the mistress of the deep sea and mother of all creatures that inhabit the ocean.
But one question lurked: how could a planet-sized object have such a strange orbit? Indeed,
Sedna's celestial path is very different from that
which characterizes the objects that populate the Kuiper Belt.
Rather, it resembles the orbit of a very long-period comet, one of those icy bodies
stationed even farther out in the so-called Oort cloud, a spherical region of space surrounding
our Solar System that would be a kind of "reservoir" of objects of a cometary nature.
It is believed that cometary nuclei populating those remote regions may sometimes experience
gravitational influences capable of deflecting them to the innermost parts of the Solar System.
And this may be the case with Sedna. The difference
is that this time we are dealing with an object with dimensions unimaginable for a comet...
It is also impossible to think that Sedna's orbit is the original one: the high eccentricity is a
clear sign of heavy dynamical interventions. Not to mention that it would also be very difficult to
clarify how such a large amount of material could have aggregated at such a distance from the Sun.
Undoubtedly Sedna had to have been formed elsewhere and then placed on its present
orbit by some powerful dynamical mechanism; such as the passing in
remote times of a star roughly the size of the Sun at a distance of about 800 AU.
According to simulations, such an event could not only have originated in such
elongated orbits but would also have been able to explain the origin of Sedna itself.
A hypothesis that among other things led to an even more intriguing scenario. In fact,
other numerical simulations led to the hazard that the perturbing star might not even have come from
outer space, but might be one of the many stars formed with the Sun within a small star cluster!
Suggestive hypothesis, but obviously very difficult to prove with so little data available.
However… it is not such a strange orbit that has pushed Sedna overwhelmingly into the spotlight.
It is its size that interests the general public.
The images that had enabled its discovery obviously did not allow
very precise assessments of its diameter. One had to rely on apparent luminosity and distance, which
were two known values. But then one had to make assumptions about the reflectivity of the surface.
From a combination of considerations, they were able to estimate a diameter of 1800 km,
so much so that, as we have already seen, they immediately cried out for the "tenth
planet in the solar system"; it was in fact the largest transneptunian object after Pluto!
And from there began that long process, unscientific and very political,
that would lead to the decision to decay Pluto from planetary status.
You all know how that went, don't you? Until the 1800s there was really no need to give
a definition of a planet. The only tenants of the Solar System were comets, satellites, and,
indeed, planets: those known since antiquity to which Uranus and Neptune had been added.
A demarcation so obvious that there was really no need to codify it into a definition.
Common sense was more than enough. Then came the discovery of Ceres in 1801,
followed shortly by that of numerous other objects confined between Mars and Jupiter.
At first, Ceres was there to acquire the title of "planet", but within a few years, the number of
potential new "planets"(which we now know to be mere asteroids) began to grow alarmingly.
And it was their large number that ruled out the possibility that those specks of light
could all be classified as planets. Two centuries later, the same thing
was repeated in the Kuiper Belt. The discovery of Sedna, first believed to be much larger than true,
and then that of Eris prompted astronomers to fear that soon the skies would be
invaded by "dozens of planets." And so, instead of facing an
embarrassing situation, he found it more convenient to downgrade Pluto,
then giving it the "dwarf planet" status. Michael Brown himself, then, in recent years
has proposed that the existence of planetoids as distant and with such an elongated orbit as Sedna,
could be evidence for the presence of a planetary-sized disruptive body:
the famous Planet Nine, about which so much has been heard in recent years!
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So, what do you say? Did Sedna deserve, or does it still deserve, all the attention we are giving it?
If your answer is positive, then you will be pleased to know that
you are in full agreement with those who are planning a space mission to the planetoid.
Sedna will indeed reach perihelion in the biennium 2075-2076.
It will arrive at the minimum distance from the Sun of 76 AU,
and this event will be an opportunity that will not be repeated until about 11400 years from now!
. Reaching Sedna with a spacecraft will not be an easy task.
For comparison, Neptune is at about 30 AU and the Voyager missions, launched in 1977,
are at these times at 156 and 130 Astronomical Units, respectively.
And just as the Voyagers had done, which took advantage of Jupiter's buoyancy to
make their tour of the Solar System, it will take similar gravitational slingshots
to make the journey to Sedna manageable. A team of scientists led by Vladislav Zubko
of the Space Research Institute of the Russian Academy of Sciences thus recently studied several
possible trajectories, favoring a launch date of 2029 as the most feasible option.
The 2029 trajectory would indeed take the spacecraft first to Venus, then back to
Earth (twice) before crossing Jupiter on the way to Sedna with a flight duration
of 20 years and more likely 30. A 30-year flight plan would mean
flying over Sedna more slowly, allowing more time to collect data on the object.
Choosing this option would give the probe a relative speed of 13.70 km/s as it passes Sedna,
comparable to the speed at which New Horizons approached Pluto in 2015.
And just as Voyager had done, which took advantage of Jupiter's buoyancy
to accomplish their tour of the Solar System, it will take similar gravitational slingshots
to make the journey to Sedna manageable. A team of scientists led by Vladislav Zubko
of the Space Research Institute of the Russian Academy of Sciences thus recently studied a number
of possible trajectories, favoring a launch date of 2029 as the most feasible option.
The 2029 trajectory would indeed take the spacecraft first to Venus,
then back to Earth (twice) before crossing Jupiter on the way to Sedna with a flight
duration of 20 years and more likely 30. A 30-year flight plan would mean flying
over Sedna more slowly, allowing more time to collect data on the object. Choosing this
option would give the probe a relative speed of 13.70 km/s as it passes Sedna,
comparable to the speed at which New Horizons approached Pluto in 2015.
The close study of Sedna-needless to say-would represent a unique opportunity
to analyze the most distant celestial body in the solar system
and understand whether it developed in the Oort cloud and whether its composition
and geophysical properties differ from celestial bodies in the Kuiper belt.
We really hope the mission will happen, even if it will be a long, long wait for its arrival.