Off to dwarf planets

The demotion of Pluto to a dwarf planet has made space explorers more curious about the celestial body. Will this mean we will soon need a Pluto Orbital Probe, wonders C Sivaram.

The New Horizon spacecraft launched nine years ago, in January 2006, will closely fly by the puzzling dwarf planet Pluto, its closest approach within 12,000 km of the planet’s surface to take place on July 14. Meanwhile, the Dawn spacecraft is closing in on another dwarf planet Ceres, its arrival being expected on March 6.

While we are all fairly familiar with planets - an integral component of our universe, not many have heard of what are called dwarf planets. Let us go back a little in time. Pluto had been regarded as one of the regular planets of our solar system. Textbooks referred to it as the farthest planet (from Earth) and the last of the major planets. But in 2006, the International Astronomical Union (IAU) took a final (and long pending) decision to demote Pluto to the ‘dwarf planet status’.

This new category of objects, now includes four other celestial objects apart from Pluto. These are Eris, Makemake, Haumea and Ceres. Apart from Ceres, which was earlier classified as an asteroid, the other three are farther than Pluto. The discovery of Eris, an object bigger than Pluto was a major factor in creating this new class of dwarf planets.

Pluto was discovered just 85 years ago, in February 1930, by Clyde Tombaugh working at the Lowell observatory in Arizona, USA. Tombaugh happened to compare two photographs he had taken - one at the end of January that year, and hen he compared the photo taken at the end of January with the one taken six days earlier, he found that the faint fifteenth magnitude speck had moved. His discovery was finally announced on March 13, 1930.

Actually, astronomers Percival Lowell and William Pickering had predicted the existence of this new planet, and dubbed it Planet X. In the mid 19th century, deviations of the orbit of Uranus from Newtonian predictions led to astronomers Urbaine Le Verrier and John Couch Adams postulating the existence of Neptune (then unknown planet) whose gravitational perturbations on Uranus could explain the anomalies in its orbit. When the telescope was turned to the predicted position, there was Neptune. After some decades, anomalies started appearing again in the orbit of Uranus, as also that of Neptune.

Lowell (and also Pickering) now predicted the existence of another new planet (X) to account for these deviations and predicted its position. In fact, Tombaugh found Pluto within six degrees from the predicted location, which despite its faintness, seemed to confirm its planetary status. Lowell had dedicated his observatory to look for this new trans-Neptunian planet which was predicted to have a mass six times that of the Earth.

As Tombaugh recalled later, the observatory naturally championed the idea that this was Lowell’s fulfillment. It was really hot news. The astronomical community within a few months accepted the new object as a planet of full rank (although pointed questions were often raised about its mass).

Only with the advent of larger telescopes and better instruments could Pluto’s diameter be measured accurately. Meanwhile in 1978, a moon or satellite orbiting Pluto was discovered and named Charon (after the ferryman who transported souls across the river Styx in the underworld, Pluto being the God of the underworld). Now Charon’s orbital period and distance from Pluto, enabled the mass of Pluto to be determined.

Shockingly, it was less than one-five hundredth of that of the Earth. Even our moon is five times bigger than Pluto. Pluto’s diameter was only 2300 km, barely spanning the breadth of the United States. It is outsized by seven of the solar system’s natural satellites. Pluto is hardly the massive perturber predicted prior to its discovery. The calculations involved to predict its presence are now known to be erroneous and so its discovery was just a lucky accident.

Pluto’s orbit is very eccentric; for a few decades, it was nearer to Earth, closer than Neptune even, coming closest in 1989. The Hubble space telescope has taken clearly resolved features on Pluto. Its surface temperature was measured to be forty degrees Kelvin. Charon has a temperature of fifty degrees Kelvin. The lower temperature of Pluto is attributed to the heat taken away by sublimation of methane ice from the surface. This is one reason why the NHS was rusted to Pluto in about ten years as Pluto is now receding away from the Sun and its atmospheric constituents may soon condense on its surface. This would make it difficult to detect the gaseous elements present. A minimum energy trip would have taken forty years to reach Pluto.

Rediscovery of Ceres

Let us now come to Ceres. This is the largest of the asteroids, which are innumerable minor objects orbiting between Mars and Jupiter. Ceres was the first asteroid to be discovered on January 1, 1801, by Giuseppe Piazzi. It was ‘lost’ for one year and rediscovered in 1802 by Wilhelm Olbers, who was also the discoverer of Vesta, the second largest asteroid. Ceres is just 950 km across. Although they both have now been classified as dwarf planets, Pluto is ten times more massive than Ceres.

Ceres is of much interest as there are indications that there may be a large water reservoir (in its interior). Its low density suggests that one-third of it may be water ice.

Ironically, it turns out, that Ceres and Pluto may be close cousins. Ceres has a high ice to rock ratio (unlike other asteroids like Vesta) but this ratio closely matches that of Pluto and other such objects of the outer solar system. Ceres and other icy chunks may have originated in the same realm as Pluto, but later migrated inwards due to some early massive upheaval which rearranged the early solar system in its formative period.

Ceres appears to have ammonium-rich clay surface unlike that of asteroid fragments found on Earth. The Dawn spacecraft could indeed shed light on whether Ceres is a wayward cousin of Pluto. Pluto’s moon Charon is nearly half the size of Pluto, another example of a twin planet-moon system (like the Earth). Moreover four other smaller moons have been found orbiting Pluto.

The New Horizon’s spacecraft would just pass by Pluto. Perhaps we require a Pluto Orbital Probe (POP) to explore in more detail the fascinating outer planet with its oversize moon. A POP with an elongated orbit ranging from just above Pluto’s surface to the orbit of Charon, would have an orbital period of about two days. It can explore both Pluto and Charon in detail. Perhaps ISRO should take the lead (too many spacecrafts have already explored Mars, Venus, Jupiter, Saturn in detail) and pioneer in launching POP.