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Until mid-2006, textbooks around the world taught that the Solar System had nine planets. Since August of that year, the number has been eight.
An important point to state immediately: The Solar System did not lose any celestial body. Pluto is still exactly where it has always been, still completing an orbit around the Sun every 248 years. What changed was the definition of the word "planet."
How Pluto was discovered
Clyde Tombaugh discovered Pluto in 1930 at the Lowell Observatory in Arizona, after months of comparing pairs of photographs of the night sky to find a point of light that shifted position.
This search stemmed from a flawed calculation. Percival Lowell believed that a "Planet X" was disturbing the orbits of Uranus and Neptune. It was later determined that these irregularities were not real—they originated from an inaccurate mass value for Neptune. Pluto is too small to cause any measurable disturbance. In other words, Tombaugh found Pluto while searching for something that did not exist, in the exact region of the sky that the incorrect calculation had pointed toward.
The name Pluto—the god of the underworld in Roman mythology—was proposed by Venetia Burney, an 11-year-old student in Oxford.
The problem gradually emerged
From the start, Pluto was an unusual member. It is smaller than Earth's Moon. Its orbit is an elongated ellipse, tilted 17 degrees relative to the ecliptic plane, and at times it comes closer to the Sun than Neptune—its distance fluctuates between 30 and 49 AU.
Since the 1990s, better telescopes began detecting a series of other bodies in the same region: the Kuiper belt, which extends from Neptune's orbit out to about 50 AU. Pluto is not a lonely planet at the edge of the system; it is one of many similar objects.
The tipping point arrived in 2005, when Mike Brown's team discovered Eris—a Kuiper belt object about 27% more massive than Pluto.
Astronomers were forced to choose. If Pluto is a planet, then Eris must also be a planet, as would subsequent objects discovered—the list of planets would grow indefinitely and lose all meaning.
The 2006 definition
In August 2006, the IAU general assembly in Prague voted to adopt a definition: a celestial body is a planet when it
- orbits the Sun,
- has enough mass for its gravity to pull it into a nearly spherical shape,
- has cleared the neighborhood around its orbit.
Pluto satisfies the first two conditions but fails the third. The difference is not slight: Earth is 1.7 million times more massive than the sum of all other bodies in its orbital zone, whereas Pluto accounts for only a tiny fraction of the mass of the Kuiper belt.
The assembly simultaneously established the dwarf planet category for bodies that satisfy the first two conditions. This group currently has five members recognized by the IAU: Pluto, Eris, Ceres, Haumea, and Makemake.
This definition is not without its detractors. The third condition depends on location: Earth itself, if placed in Pluto's orbit, would not be able to clear its neighborhood. This is why some planetary scientists, including the principal investigator of the New Horizons mission, still do not accept this classification.
What New Horizons found
The New Horizons spacecraft flew past Pluto on July 14, 2015, and what it sent back dispelled the old image of a dead ball of ice.
- Sputnik Planitia — a nitrogen ice plain about 1,000 km wide, with a surface divided into polygonal cells caused by convection, and almost entirely free of impact craters. This is a sign of a surface that is still being renewed, potentially less than 10 million years old.
- Water-ice mountains as high as 3,500 m, hard enough to stand firm at temperatures around −230 °C.
- A multi-layered, thin atmosphere consisting mainly of nitrogen, with dozens of photochemical haze layers stacked on top of one another.
- Signs suggesting a liquid water ocean beneath the icy crust.
- Five moons, one of which, Charon, is so large that the common center of mass of the Pluto–Charon pair lies outside Pluto—they orbit a point in empty space.
An object with geological activity, a layered atmosphere, and possibly an underground ocean, nearly 5 billion km from the Sun. The fact that it is no longer called a planet makes it no less fascinating.
Naming convention in Vietnamese: the original name Pluto should be used. The name "Sao Diêm Vương" places it in the same mold as Mars (Sao Hoả) or Jupiter (Sao Mộc), whereas it belongs to a different group—and within that group, the other members bear the names Eris, Ceres, Haumea, and Makemake.
Edited from the article Tại sao Pluto không còn là hành tinh? by Đặng Vũ Tuấn Sơn, published by the Vietnam Astronomy Society (VACA). The copyright of the original content belongs to VACA.
Further reading
References
- [1]Tại sao Pluto không còn là hành tinh? — Thiên văn Việt Nam (VACA) — Đặng Vũ Tuấn Sơn (2012)
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker — Wikimedia Commons, Public domain.
