What Galileo Almost Saw

Throughout this International Year of Astronomy, 2009, we have been thinking back on Galileo Galilei and the historic discoveries he made with is telescope back in 1610. However, it’s also interesting to reflect on a discovery that Galileo almost made–the planet Neptune.

europa 606
Galileo Galilei
Creative Commons License photo credit: dizarillo

Astronomers did not become aware of  Neptune until 1846.  On September 23 of that year, Johann Galle of the Berlin Observatory received a letter from Urbain Le Verrier in Paris.  Le Verrier had been trying to understand why Uranus was not quite where people expected it to be.

When William Herschel announced the discovery of Uranus in 1781, astronomers went to work calculating its orbit around the Sun.  In 1821, Alexis Bouvard noticed that his tabulated positions of Uranus, based on Newton’s laws, did not quite match up with Uranus’ real positions.  He suggested that an eighth planet beyond Uranus was perturbing Uranus’ orbit.  Urbain Le Verrier painstakingly calculated where in the sky this planet might be in order to affect Uranus’s orbit in just the observed way and mailed his predictions to Galle.  Galle, assisted by a student, Heinrich d’Arrest, found Neptune in his telescope the same day he received Le Verrier’s data.  (John Couch Adams of England made similar observations and calculations over the same period.)

Galle and d’Arrest were the first to recognize Neptune, but not the first to see it.  At magnitude 7.9, Neptune is too dim to be seen with the unaided eye, but it does show up as a point of light in simple telescopes and even in binoculars.  From the moment of its discovery, astronomers wondered if earlier telescope users might have seen Neptune without realizing it.

01 The Solar System PIA10231, mod02
Creative Commons License photo credit: Image Editor

In the winter of 1612-1613, Jupiter began to align with Neptune from Earth’s point of view.  The alignment was so complete that on January 4, 1613, Jupiter’s disk actually blocked (occulted) Neptune’s.  Galileo, having discovered four moons around Jupiter in January 1610, was still observing Jupiter three years later.  He made careful drawings of Jupiter, its moons, and any background stars in his telescope’s field of view.  Upon comparing the background stars in Galileo’s drawings to the positions Neptune would have had that winter, astronomers have concluded that Galileo drew Neptune as a background ‘star’ in drawings he made on December 28, 1612, and on January 27 and 28, 1613.

Galileo’s simple telescope was not powerful enough to resolve Neptune into a disk.  (You need a telescope at least 10-12 inches in diameter to do this).  In order to recognize it as a planet, Galileo would have needed to see Neptune change position against background stars. Since it orbits about 30 times as far from the Sun as Earth does, Neptune takes 146 years to go around the Sun once.  As a result, its motion against the background stars is harder to notice.  Once a year, Earth comes around to Neptune’s side of the Sun.  This makes Neptune seem to slow down, stop, and reverse direction against the background stars.  (This is called ‘retrograde’ motion.)  As it turns out, in December 1612, Earth was just coming around to Neptune’s side of the Sun, and Neptune was virtually stationary and about to begin retrograde motion.  Neptune’s motion against the background stars would have been all but unobservable in December 1612.

Neptune
The Roman god Neptune, for whom
the planet is named.
Creative Commons License photo credit: OliBac

By January 1613, however, Neptune was in full retrograde motion.  On January 27 and 28, Galileo did notice that one of his background stars had slightly changed position compared to another.  According to University of Melbourne physicist David Jamieson, this indicates that Galileo knew he had found a new planet.  However, we see no sign that he attempted a second observation of that mysterious star, or that he reported the finding of a new planet. Thus Galileo, first to see Neptune, does not get credit for discovering it.

Others who saw Neptune in their telescopes and mistook it for a star include Jerome Lalande of the Paris Observatory, whose staff conducted a detailed survey of the sky in 1795, and William Herschel’s son John, who happened to see it in 1830.

Uranus is another planet seen before its formal discovery.  In fact, at visual magnitude 5.6, Uranus is right at the threshold of visibility to the naked eye.  This means that if you’ve been out on a clear night with no clouds or light pollution, and Uranus happened to be up, you’ve probably seen it.  And so have countless observers across the globe throughout history who looked up in pristine skies.  Uranus moves so slowly (taking 84 years to orbit the Sun once) and blends in so well with the stars in its general direction, that our eyes pass right over it.  That’s why it took William Herschel’s telescope in 1781 to recognize Uranus for what it is.  When John Flamsteed, the very first Astronomer Royal of the United Kingdom, prepared a catalog of visible stars, he misidentified Uranus as a star, designating it ’34 Tauri’ (the 34th star of the constellation Taurus).

As 2009 ends, Jupiter is once again approaching Neptune in our sky.  As I write this (late November 2009), Jupiter is by far the brightest thing in the south-southwest at dusk (unless the Moon is out).  Neptune is just under 4 degrees to Jupiter’s upper left (three fingers held together at arm’s length block about 5 degrees).  Since Jupiter is orbiting much faster than Neptune, we see Jupiter gain on Neptune’s position  during the next few weeks.  Unlike in 1613, Jupiter will not align with Neptune exactly; the two planets are just over half a degree apart at closest approach on December 21.  (One half of one degree is about the apparent size of the Moon’s disk.)  Jupiter then pulls ‘ahead’ of Neptune and is just over two degrees away by New Year’s.  Here is a  finder chart to help you identify which point of light among the stars is Neptune.  This holiday season, then, you have the chance to repeat Galileo’s observations from the winter of 1612-1613.  But you, unlike Galileo, will know exactly what you’re seeing.

Science Doesn’t Sleep (9.2.08)

Touchdown! The Tigers Win the Game!
He’s excited because he’s getting smarter.
Creative Commons License
photo credit: foundphotoslj

So here’s what went down after you logged off.

Crew aboard the International Space Station had a bit of excitement over the long weekend (on top of the presumably high levels created by living in space) – as they had to fire the station’s thrusters in a “debris avoidance maneuver.” This is a fancy way of saying they were about to be hit with space trash.

Not really a “team player?” No worries – even watching sports improves brain function.

The Rodney Dangerfield of the solar system: Astronomer Heidi Hammel wants you to know why the Icy Giants deserve more respect.

Even geniuses make mistakes: Einstein made at least 23 of them.

He was only 18 when he died, but King Tut may already have been a father – of twins.

Rap + Physics = awesome. A rap video about the science behind CERN’s Large Hadron Collider has been viewed over 600,000 times. It’s no dramatic hamster – but for a video about science, that’s pretty solid.

Meltdown: The Houston Chronicle weighed in on climate change today – what are your thoughts?

Eight is Enough?

Creative Commons License photo credit: CommandZed

Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?