Comet Pan-STARRS is here! Catch a glimpse tonight at twilight — it’s kind of a big deal

During the next few weeks, Comet Pan-STARRS will grace our skies as a naked-eye comet. As its name indicates, astronomers discovered this comet on June 6, 2011, using the Panoramic Survey Telescope and Rapid Response System, a planned array of telescopes in Hawaii. This array’s primary mission is to detect near-Earth objects (comets and asteroids with orbits near Earth’s) that pose a risk of impact. Of the four telescopes planned for the array, only the first, PS1, is yet operational. This is the telescope used to discover the comet.

Viewers south of the equator have been observing Pan-STARRS since February, as the geometry of its orbit has favored southern observers until now. However, as Pan-STARRS approached perihelion late on March 9, it also comes up through the plane of Earth’s orbit, making it visible from right here in Houston during mid-March 2013.

via Discover Magazine
Image via Discover Magazine

What are comets?

Comets are made of ice and dust and are often called ‘dirty snowballs.’ They are believed to be left over from the formation of the solar system. Long-period comets such as this one originate in the Oort Cloud, located about 50,000 times as far from the Sun as the Earth.  As comets approach the sun, ice changes into gas and the dust embedded within the ice is released. A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be 100,000 miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust away from the comet’s head into a tail spreading millions of miles behind the comet and pointed away from the Sun. Comets have bluish gas tails and yellowish dust tails.

Where should I look?

Face to the west during late evening twilight, which is about 8 p.m. once Daylight Saving Time begins. On March 12, a thin crescent Moon will be to the lower right of the comet. Pan-STARRS will shift towards the north (to the right as you face west) each night. This shift towards the north means that Pan-STARRS always sets soon after the Sun and is visible only during evening twilight. You will therefore need a clear, unobstructed horizon to the west when observing right at dusk. Note that by the time Pan-STARRS appears in our sky, it is already receding from the Sun, and thus getting a little dimmer each day. Most likely, Pan-STARRS will fade from view by April. How long can you follow it?

What should I look for?

Look for a fuzzy spot, not a single point of light. Observers south of the equator have reported the comet’s total brightness as a little brighter than the Big Dipper’s stars. However, the comet always appears low to the horizon and in twilight; the thicker atmosphere and brighter background will make it dimmer. While Pan-STARRS is visible, the Moon goes from crescent to full, which will dim the comet. Past comets, such as Hale-Bopp in 1997, have shown long, extended tails.

However, Australian observers in early March reported a compact appearance for the comet, with a tail visible only up to two or three degrees from the head of the comet. (Your fist held at arm’s length blocks 10 degrees.) This appearance concentrates more of the comet’s brightness in a small area of sky, making it easier to see a against a twilight sky. Comet tails have a fainter look, comparable in brightness to the Milky Way band. Binoculars may help you locate the tail if you can’t make it out with the naked eye in the twilight.

Will I ever see this again?

No one has ever seen Pan-STARRS before, and no one alive today will see Pan-STARRS again.  By all indications, this comet has traveled from the Oort Cloud into the inner solar system for the first time ever.  And once it’s gone, it won’t return for another 110,000 years.  However, Pan-STARRS could be just the ‘warm-up act’ for a much bigger and brighter comet in November and December 2013 — comet ISON.

Comets are notoriously unpredictable, though; there’s no telling if ISON will meet expectations.  It’s better to take advantage of clear skies forecast for this week and look for Pan-STARRS. When you see it, you’ll be looking at one of the oldest, most pristine objects of our solar system.

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?