The search for the Solar System’s ninth planet began in 1906, when a wealthy Bostonian funded the “Planet X” project that, during the first year of the Great War, yielded the first dim images of what an eleven-year-old schoolgirl would eventually name Pluto. It wouldn’t be until almost fifteen years later that Planet X’s Clyde Tombaugh would obtain photographs confirming the celestial body’s movement, and the discovery of a ninth planet in our Solar System would be announced to the world.
And now, a century on from Pluto’s first faint appearance on the Lowell Observatory’s surveys, its discoverer’s ashes are flying within 8,000 miles of it aboard an unmanned NASA probe that promises to lift the veil on the Solar System’s most divergent and controversial known object.
“We’re going to turn points of light
into a planet and a system
of moons before your eyes.”
– Alan Stern, NASA New Horizons
into a planet and a system
of moons before your eyes.”
– Alan Stern, NASA New Horizons
Since its discovery, Pluto has grown ever smaller in the eyes of humanity. Estimates of its mass and size have dwindled enormously over the decades, from around the same mass as Earth in 1930 to what we now know to be just 0.002 Earths (just under a fifth the mass of the Moon, if that’s easier to imagine). This makes Pluto a golf ball when compared to a football Earth, and, as it orbits the Sun forty times farther out than Earth, one that’s always between fifty and eighty miles away. Until now, studying it has been like trying to read a 12-point font on a piece of paper miles away. Squinting isn’t the half of it.
Moreover, following the discovery of what was initially reported as “The Tenth Planet”, Eris, in 2005, the International Astronomical Union (“IAU”) opted to withdraw Pluto’s planetary status rather than grant the same to Eris, in so doing stealing away not only our new tenth planet, but our long-established ninth too. They accomplished this by introducing a three-fold test for planethood: firstly, a planet must orbit its sun; secondly, it must have sufficient mass for gravity to have forced it into a spheroid or ellipsoid shape; and thirdly, it must have “cleared the neighbourhood around its orbit”. In practical terms, this means that it has to have become sufficiently gravitationally dominant to have either cleared away all bodies of comparable size from its orbit zone, or drawn them into its own orbit. Eris, Pluto and the other trans-Neptunian objects in the crowded Kuiper belt cannot satisfy this final criterion, and so find themselves labelled “dwarf planets” or “plutoids”.
Pluto’s reclassification has divided both the astronomical community and lay folk alike. With potentially hundreds of other dwarf planets awaiting discovery in the Kuiper belt, it’s easy to see why the IAU drew a line in the sand when and where they did. Conversely, tearing apart the “Classical Solar System” and thus ruining Robert Holmes’ seminal Doctor Who serial The Sun Makers was unquestionably going to piss people off - particularly when the contrived third criterion focuses on orbital characteristics as opposed to intrinsic ones, and potentially even casts doubt on other planets’ classifications (there’s quite an asteroid belt in Mars’ neighbourhood...) The ensuing contention hasn’t been helped by a lack of clarity when it comes to classifying dwarf planets either; only five are recognised presently, despite claims by astronomers such as Mike Brown, discoverer of Eris, that many other celestial bodies meet the IAU’s criteria.
None of this pigeon-holing, though, should undermine the significance of Pluto in our Solar System or the magnitude of the New Horizons probe’s exploration of it. After all, with a confirmed diameter of 1,473 miles, Pluto is still the largest and second most massive known dwarf planet in the Solar System. The distinction between “largest” and “most massive” might be lost on some readers, but it’s quite literally as it sounds: Pluto is physically larger than Eris (more voluminous, if you will), but Eris is more dense (it has around 27% greater mass). Its axis is tilted too, so like Uranus it seems to spin on its side. It also also boasts an impressive array of moons – Styx, Nix, Kerberos and Hydra – as well as Charon, which is either another satellite or a binary dwarf planet depending on who you ask. Many contend that Pluto and Charon are so close that their gravitational interaction causes them to orbit about a common centre of mass, rather than Charon orbiting Pluto as the Moon does Earth. They are even thought to be tidally locked, so if you could stand on Pluto, Charon would seem to hang perpetually in the sky. Accordingly, I’m particularly interested to see what light New Horizons can shed on Charon and its unusual relationship with the Classical ninth planet.
Probably my favourite thing about Pluto though is its chaotic orbit around the Sun, which differs from all of the other planets in the Classical Solar System. The eight planets’ orbits are broadly circular, while Pluto’s is elliptical, allowing it to come closer to the Sun than Neptune, effectively overtaking it for twenty years at a time. That’s not bad going for a planet that takes a leisurely quarter-millennium to orbit its star. Such a sweeping orbit must have repercussions for its atmosphere too – as the world moves further away from the Sun, its atmosphere probably freezes, with the ice turning back to gas as it draws nearer again. This being the case, what New Horizons shows us will probably only be a snapshot of Pluto as it is at this particular phase of its orbit, but it promises to be one from which many inferences can reasonably be drawn.
Torturously, we’ll have to wait sixteen months for NASA to receive all of the New Horizons probe’s priceless data as not only does it take the data the better part of five hours to traverse the three billion or so miles separating it from Earth, but even NASA’s two-hundred-foot-wide radio dishes can only receive it at a rate of about a kilobyte per second – a rate of transfer that my mid-1990s modem would have been embarrassed about. It certainly pulls into sharp focus just how near your closest broadband exchange really is.
However, even though we’re still hours away from receiving the first data from the (fittingly) plutonium-powered probe’s fleeting flyby, we already know far more about the last unexplored world orbiting the Sun than we did a week ago. The icy plutoid isn’t the greyscale sphere that many had assumed; it’s a gorgeous tapestry of red, orange and even black – quite fitting, really, for a world named after the god of the underworld. Its terrain is correspondingly varied, encompassing mountains and valleys that are redolent of Mars together with craters which evoke familiar images of our own planet’s moon. Other areas appear smooth, perhaps betraying the sphere’s relative youth or geological activity; maybe even both. What I found most interesting, though, is the notion that the dwarf planet’s thin nitrogen atmosphere allows for the possibility of snow, some three billion miles away from the nearest snowman…
Enjoy the coming days and months, space fans, because once New Horizons passes through the Kuiper belt, it’s going to be many lifetimes before it reaches another world, let alone one with all the quirks and caprices of Pluto.