New images from Pluto break our hearts, excite our imaginations

Pluto has a heart. A big, icy heart surrounded by a sea of red. It was more of a fluke of photographic composition, a perfect angle for the shot that astronomers and engineers waited for more than nine years to receive, but nonetheless the image has served to anthropomorphize the dwarf planet enough to make us fall in love with it all over again.


Identified by Clyde Tombaugh in 1930 as the ninth planet from the sun, Pluto fell from grace in 2006 when the International Astronomical Union contentiously demoted it to a dwarf planet, but it never fell from our imaginations. Pluto occupies the same region of space as peers of similar or greater size, including the more massive dwarf planet Eris, and now that we see it has a heart, we imagine it breaking.

Pluto is a lonely wanderer, its elliptical flying well above and below the main plane of the solar system and within the orbit of Neptune. Tuesday morning, the historic images from NASA’s New Horizons space probe revealed its pockmarked face, wrinkled with valleys and mountains, and blanketed in sheets of ice, supporting the character we have imagined over the past 85 years. No less an idiosyncratic body than Jupiter, Saturn, or our own life-supporting Earth, Pluto’s face, now lifted out of obfuscation, reminds us of the magic of other worlds and reignites the spark of possibility.


Clyde W. Tombaugh. Wikipedia Commons.

Not only have we become closer to this famous (and infamous) planetoid during the nine-year journey of the New Horizons, we have also succeeded in slinging a piece of engineering three billion miles at speeds greater than 30,000 miles per hour, maintained control through the veil of a four-hour lag time, and captured high-resolution photos. The images of Pluto represent the scientific precision of which we are now capable in 2015. Dr. Carolyn Sumners, Vice President of Astronomy and Physics at the Houston Museum of Natural Science, explains this feat in terms of scale using objects familiar to Houstonians.

“If the sun were the Astrodome, the Earth is a ball the diameter of a tall person in Pasadena, and Pluto is a ball about a foot in diameter out close to Amarillo. We aimed, and we hit it,” she said, adding that space is a very empty place.


HMNS hosted a flyby watch party in the Burke Baker Planetarium Tuesday, with streaming images from NASA.

To further complicate the logistics, engineers had to compensate for the movement of Pluto, looking into the distant future for the point of interception. They also factored in a slingshot around Jupiter, using its intense gravity to accelerate the probe to speeds much faster than it was launched.

“There’s a lot of very complicated, solid geometry and physics that went into the mission. That, to me, is the most impressive,” Sumners said.

Due to the four-hour delay, engineers had to send a command and cross their fingers for eight hours every time a button was pushed. Four hours out, four hours back. They could only trust in their math and hope that the spectrograph and camera were aimed at Pluto at the right time.

“We got our science right,” Sumners said. “That we can do something this technologically advanced that far away is amazing.”

Starry Night Express to Pluto banner

HMNS hosted a Pluto flyby watch party in the Burke Baker Planetarium, with guests including Fox News and KHOU. Beloved astrophysicist Neil deGrasse Tyson made a cameo appearance over the Internet from his seat at the American Museum of Natural History. Visitors watched via live video stream as New Horizons came within 7,767 miles of Pluto at 6:49 a.m. CT. Houston Public Media highlights the significance of the event to our city and to the United States.

As the data from New Horizons continues to stream in over the next few months and areas of Pluto become more visible, scientists will begin the process of identifying and naming features of the planet’s surface. A day after the flyby, with the first close-up images sent by New Horizons, NASA has discovered ice mountains 11,000 feet high, comparable in height to the Rockies, and now we know it snows. Imagine that.

mountains on pluto

Ice mountains on Pluto.

At HMNS, the Planetarium has altered its Starry Night Express programming with a new major focus on incoming images and data from New Horizons. Guests can now get updates on the latest information about our famous dwarf planet during Starry Night Express: To Pluto! Visitors to the museum’s George Observatory in Brazos Bend State Park on Saturday nights can see several planets with their own eyes and maybe even Pluto through the Gueymard Research Telescope if the night is perfect.

HMNS will host a lecture by Dr. Paul Schenk of the Lunar and Planetary Institute on the exploration of Ceres and Pluto Tuesday, August 11 at 6:30 p.m. Schenk, who is currently assisting NASA with Pluto research and was a participant in the Dawn mission to Vesta in 2011, will review the unprecedented explorations of the inner and outer solar system and share the top questions scientists hope to answer with the data they gather. Tickets $18, members $12.

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:

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?