Why haven't more large planetary objects
been discovered in the Kuiper belt for the past 15 years?
As is well known to all astronomy enthusiasts, in the remote region of the solar system known
as the "Kuiper Belt," several large objects have been discovered in the past years,
especially since 2002. Those who followed the events
of those years, however, will also remember that after a period of exciting discoveries,
culminating with that of Eris, hailed as "the tenth planet," the flow of good news suddenly
ceased in 2007. So much so that it seemed to some that it was the 2006 Prague Conference,
which had sanctioned Pluto's forfeiture of planetary status,
that put an end to the discoveries, demotivating researchers and effectively turning the search
for new planets into the less glamorous and lucrative "search for dwarf planets."
From then on, in fact, only routine discoveries and no new Eris on the horizon...
What would you say, then, if we tried to shed light
on the real reasons that led to the abrupt end of that era of exciting finds 15 years ago?
Said the American astronomer Fred Whipple that if one wants to continue to consider the solar system
according to the familiar representation still used by many popular textbooks,
with the Sun at the center of a system of tidy,
coplanar ellipses, it becomes really difficult to put the remote, chaotic and mysterious
regions beyond Neptune into the context... If we strive instead to see it as a complex,
continuously interacting physical system, with the Sun at the center of an enormous sphere
of influence extending to tens of thousands of astronomical units away, populated by a myriad
of exotic objects and only a small minority of familiar objects located in its immediate vicinity
(the known planets and satellites), then here is where our view of the system changes profoundly.
And if it is visionaries such as Kenneth Edgeworth, Gerard Kuiper, Jan Oort, and Fred
Whipple himself who are credited with having hypothesized in the past the existence
of a "something that was not yet there," it is astronomer Michael Brown, professor of planetary
astronomy at the "California Institute of Technology," who is credited with having
contributed in recent years to filling "the empty box" of those faraway regions with real objects.
Until thirty years ago, in fact, the existence of the Oort Cloud, as well as that of the nearest
Kuiper belt, were only working hypotheses, and until the early years of this century, the region
beyond Neptune and Pluto seemed inhabited only by a sparse representation of ordinary asteroids.
In 2000, Varuna was found, a major object (whose diameter, however, was soon scaled down by the
combined observations of the Herschel and Spitzer space telescopes); but it wasn’t until 2002-2007,
when Michael Brown arrived on the scene, that things really started to get serious.
Almost always in collaboration with Chad Trujillo-the discoverer in 1992
of the first object found in the Kuiper belt-Brown ignited the imaginations of astronomers
and ordinary enthusiasts alike by populating those regions with near-planet-size objects,
including the largest ever, Eris, far more massive than Pluto itself.
"Formidable those years," one would say! But from 2007 onward
it seemed that the exciting string of discoveries suddenly stopped
when people were already beginning to hope for the discovery of something even more sensational...
Someone half-mouthed also speculated that the decline in researchers' performance had to do
with the findings of the Prague Conference that had dethroned Pluto from planetary status in 2006.
But did things really turn out that way?
Well, not really, and Brown himself explains it. Basically, Brown says that being able to
find all those objects in such a short time was certainly not the result of randomness, but of
methodical observational surveys that had covered both the boreal and southern celestial hemisphere,
leaving only the galactic plane region and the poles of the ecliptic unexplored.
It was certainly possible, Brown reports, that a few brilliant KBOs (Kuiper Belt Objects
are called by this acronym) had thus managed to escape the search, but numerous statistical
studies gave 70 to 90 percent efficiency for those observing campaigns, which
suggested a fairly small number of residual KBOs. At that point, organizing a survey to cover the
missing areas would have been too burdensome (and expensive) a task for such small expected results,
and it is for that reason alone that the discoveries seemed to suddenly cease...
It was simply not deemed worthwhile to initiate new research programs.
The researchers noticed, however, that all the objects discovered in so few years
had also been recorded in the photographic "plates" of
much "older" surveys, such as in those of the Palomar of the late 1950s...
Of course, at that time, apart from Tombaugh's mighty search campaign
all dedicated to the hunt for Pluto, no one would have known how to identify as non-stellar objects
certain bright dots of inappreciable proper motion that appeared among thousands of stars.
It would have been necessary to compare plates taken days apart to realize this...
However, the fact that all the large KBOs found up to that point could also be traced in old archival
images raised the conviction of the researchers that instead of starting new observational
programs, the existence of already formed archives could be exploited programmatically.
"Hey, guys, just a moment before we continue... BE sure to join the Insane Curiosity Channel...
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A dataset with ideal characteristics was soon identified in the one made by the Catalina Sky
Survey, a U.S. project that makes use of three telescopes to discover and monitor so-called
Near-Earth Objects (NEOs). Two of these instruments (1.5 and 0.7 meters in diameter)
are in Arizona; the third (0.5 m) is in Australia, at the Siding Spring Observatory,
where it has contributed to the Siding Spring Survey in the southern celestial hemisphere.
NEOs, as they are known, are asteroids,
comets, and meteoroids that may pose an impact hazard to our planet, which is the
reason for a project dedicated to scanning the sky from palm to palm to discover them.
Brown and associates, of course, were not at all interested in Neos, but only in the images
that the Catalina Sky Survey had produced in years and years of continuous research.
Those images were perfect for their purposes: in fact, the survey had covered as many as
19,700 square degrees of sky (remember that the entire celestial vault is made up of
41,253 square degrees) between declination from -25° to +70°, while the Siding Spring Survey had
put on file 14,100 square degrees from -80° to 0°. Taking into account the areas of overlap,
the total useful coverage was about 30,000 square degrees, three-fourths of the entire sky!
The limiting magnitudes for the two surveys were +19.4 and +18.9, respectively,
sufficient to allow the detection of large objects in the Kuiper belt.
Each observation area had then been imaged several times over 7 years from 2005 to 2012,.
At that point, however, there was the question of how to proceed. It had been calculated, in fact,
that in the 30,000 square degrees about 3.9 billion transient events had been photographed
(about 130,000 per square degree!), where a "transient event" means a dot of light
on the slab that does not correspond to any known object, but for that very reason can be anything:
a supernova, a flare, an artifact, a belt asteroid, a small NEO, a cosmic ray...
How then to distinguish the dots that identify KBOs from everything else?
The thing, as you can imagine, is neither easy nor immediate. With a skimming made through a digital
"blink comparator," you can eliminate all those dots that remain at a distance of time without
manifesting the smallest displacement... In addition, we know that a KBO that is on average
more than 25 AU can not manifest in any condition a proper motion higher than 4.9 seconds of arc per
hour. Anything that moves faster (NEOs, band asteroids, artifact pairs, etc.) is discarded.
Well, even at the end of this double selection, many dots remained: 3.5 million!
At this point the so-called "Keplerian filter" comes to the rescue, i.e., all the dots that night
after night seem to indicate the motion of an object are examined by software, which each time
is like asking the question: does this succession of dots, meet the criteria of an object moving
with Keplerian motion? If the answer is reasonably positive within set limits, and if the magnitude
also does not manifest suspicious overhangs, then the case is classified as a "probable KBO."
At the end of the whole rigmarole, which was also complicated by the application of more
complex "identify or remove" algorithms, Brown and his team found themselves grappling with 1192
"probable KBOs" in the Northern Hemisphere and 5515 in the Southern Hemisphere. A small enough
number (of course, many of these cases were the result of multiple observations
related to the same object) to allow the evaluation of each entry individually.
And so it was that the initial thick daisy remained with only eight petals...
Which meant, out of metaphor, that thanks to the Catiline Survey,
Brown and colleagues had succeeded in identifying with certainty
the existence of eight large planetary objects belonging to the Kuiper belt!
A successful experiment, then? Well, not really... It's almost funny to say, but the eight KBOs that
survived the sorting were objects that had been discovered and cataloged for several years,
and almost all of them were found by Brown himself!
Read their names and you will quickly understand: Makemake,
Eris, Orcus, Huya, 2002 VE95, 2002 TX300, and Nereid (yes, Neptune's own moon!)...
Basically, neglecting the almost humorous side of it, we can summarize the whole affair this way:
the analysis on the Catiline survey had largely fulfilled its purpose, and had proved so good
that it detected with 100% efficiency the presence of all those KBOs that were below the instrumental
limiting magnitude in the years when the archive was formed (except Pluto, which was immersed in
the Milky Way at that time, and for Quaoar, located at the edge of an uncovered region)!
And the fact that the survey did not show the existence of even a completely
new KBO can only mean one thing: Below that magnitude, there is nothing more to discover!
Or at least, very little. Actually in the article, Brown leaves some hope, in the sense that as we
said before the Catiline survey left uncovered the regions around the pole of the ecliptic,
which correspond to 20% of the total sky. That, added to the tenuous possibility that
something might be moving in the unexplored (and unexplored) areas occupied by the Galactic plane
leads the American astronomer to calculate a 32 percent probability that there is still
one large KBO yet to be discovered, and 10 percent that there are two.
Can we set our hearts at rest, then? Have we already found all the dwarf
planets in the solar system? The answer is no, for at least two good reasons.
The first lies in the small door left open by the incomplete coverage of the survey
(indeed, let us recall that Eris itself possesses a strongly inclined orbit on the ecliptic,
which entitles us to hope that the regions north of the ecliptic
may still hold surprises), while the second reason is that we have not yet searched
everywhere. It remains to explore the Oort Cloud, or at least, its innermost part, where
numerous simulations lead us to speculate the existence of planets the size of Mars or Earth.
Because of their distance from the Sun, however,
these still hypothetical objects are too dim to be discovered with current technologies.
We will have to be patient, and hope for that 32 percent calculated by Brown.
A very arduous task, however... that will require all the observational power of the
new generation telescopes; like the James Webb, which will be operational in a very
few days (on July 12 it should deliver us the images of its "first light") or like ESO's
39-meter Extremely Large Telescope, under construction in the Chilean Atacama Desert.
So let's be patient, and let's all keep our fingers crossed.