A lesson from Pluto
First posted on homepage: 12 July 2010 (GMT+10)
Re-posted on homepage: 9 June 2021 (GMT+10)
Pluto orbits 40 times further from the sun than Earth, and for over 70 years it has been regarded as the ninth planet of our solar system. Clyde Tombaugh (1906–1997) discovered Pluto in 1930 by comparing photographs of stars taken two weeks apart at the Lowell Observatory in Arizona.1
Because of perceived irregularities in the motion of Uranus, Percival Lowell (1855–1916), the founder of the observatory, believed in the existence of a ninth planet. He dubbed it Planet X and calculated that it would be six times more massive than Earth. He even specified its location.2 Lowell searched for the planet without success from 1906 until he died.
Tombaugh was hired by the observatory in 1929 and discovered the planet near where Lowell suggested. This apparently vindicated Lowell’s predictions so the discovery was appropriately announced on Lowell’s birthday (13th March) and the first two letters of Pluto’s name are his initials.3
Pluto is so faint that it can only be seen with a telescope 30 cm (12 in) or larger, and astronomers were unable to determine its size and mass. Early estimates could rely only on the deviations of the orbits of Neptune and Uranus. The size was quickly revised down from Lowell’s estimate, and eventually astronomers settled on a mass about three quarters that of Earth.
All this changed around 1978, nearly 50 years after Pluto’s initial discovery. The key evidence was found by James Christy of the US Naval Observatory when he realized that Pluto has a moon. He noticed that some of the images from their 1.5 metre telescope showed Pluto slightly elongated but the stars in the same photographs were not. From those images he was able to estimate the diameter of the moon’s orbit and its orbital period. As a result astronomers could calculate the mass of Pluto with far more certainty.4 It is now accepted that Pluto is only 1/500th the mass of the earth. Ongoing observations confirmed Pluto’s moon, and the International Astronomical Union gave it official status in 1985 and named it Charon.5
With such a tiny mass, Pluto could not possibly have affected the orbits of the gas giants Uranus or Neptune. In 1983 astronomers searched the entire sky by the Infrared Astronomical Satellite but no hidden planet was found. It is now generally believed that the perturbations to the orbits of Uranus and Neptune were imaginary, that Lowell’s calculations were wrong, and Tombaugh’s discovery was a coincidence.1
How could so many scientists be so wrong for so long about the mass of Pluto—by a factor of 400? A similar question is often asked when creationists speak of the earth being only 6,000 years old instead of the generally accepted age of 4,600 million years.
The mass of Pluto, like the age of the earth, has not been measured directly. It is calculated from scientific models that are all based on assumptions. All the scientists got the same wrong answers because they all used the same models and the same assumptions. However, ongoing observations of the behaviour of Pluto led to more information that enabled an entirely different approach to the problem, overturning the previous assumptions and coming up with a radically new and soundly-based estimate.
There is another big difference. The mass of Pluto is operational science, where we can continue to make observations in the present using newer and better instruments and technology. But the age of the earth is historical science. We cannot travel back in time to make observations of things that only happened in the past. For information about the past we need reliable reports from eyewitnesses.
Pluto contradicts the nebular hypothesis
Pluto belongs to a class of objects that orbit the sun beyond Neptune, called TNOs (Trans Neptunian Objects). Astronomers regard these as material left over from the gas and dust nebula from which the solar system supposedly formed, supposedly 4.6 billion years ago.
But Pluto is a problem for the nebular hypothesis. First, it does not orbit in the same plane as the other planets (i.e., the ecliptic) but at an angle of 17°. Why not? Second, its axis of rotation is not perpendicular to its orbital plane but tilted so that it points almost directly at the sun at present. How come? Third, Pluto’s orbit is not circular but highly elliptical. In fact, it occasionally comes closer to the sun than Neptune. Why? These features of Pluto contradict the predictions of the nebular hypothesis, so astronomers have had to invent ad hoc secondary stories to explain them. So much for the nebular hypothesis.
Pluto and its moons1 don’t support the idea of billions of years either. Analysis of light from Charon suggests that its surface is covered with active volcanoes of ammonia-rich water spewing out of the moon’s deep interior. Similar conclusions have been reached for many TNO’s. This means there must be a source of internal heat within these objects. But if they are billions of years old they should have been cold and dead billions of years ago.
- Apart from Charon, two smaller moons, Nix and Hydra, were discovered in 2005.
References and notes
- Abell, G.O., Morrison, D. and Wolff, S.C., Exploration of the Universe, 6th edition, Saunders College Publishing, Philadelphia, USA, p. 186, 1993. Return to text.
- Ref. 1, p. 185. Return to text.
- Ref. 1. The name is from classical mythology and was suggested by Venetia Burney, an 11 year old school girl from Oxford, England. Return to text.
- Using Newton’s formulation of Kepler’s third law. Return to text.
- Ref. 1, pp. 288–289. Return to text.