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Enceladus images "dazzling success"

...just two days after the closest flyby yet, and images of Enceladus' icy south pole terrain are back on Earth...

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Hubble celebrates 100,000 trips around the Earth

...Hubble is today celebrating its 100,000th orbit around the Earth with the release of a spectacular image of a fantasy-like landscape embellished with scenes of stellar birth and renewal...

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Computer simulations put Solar System in its place

...traditional theories of Solar System formation assume our neighbourhood to be pretty run of the mill, but in a new study using data from 300 exoplanets discovered orbiting other stars, our planetary haven turns out to be one of a kind...

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STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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The great Pluto debate continues

Posted: August 13, 2008

In 2006, the International Astronomical Union (IAU) reclassified Solar System objects such that Pluto became a dwarf planet, and in June of this year it was placed into the ‘plutoid’ sub-category (see our report Pluto assigned plutoid tag in new IAU classification for more details). Now astronomers have reopened the debate at this week’s Great Planet Debate: Science as a Process conference in Maryland.

Astronomers are, once again, debating the nature of Pluto and its place in our Solar System. Image: IAU.

This Thursday, Mark Sykes, the director of the Planetary Science Institute (PSI), and astronomer Neil deGrasse Tyson of the American Museum of Natural History and host of NOVA Science Now, will debate the IAU's actions in reclassifying Solar System objects into planets, dwarf planets and plutoids, as a part of the three day Great Planet Debate: Science as a Process conference at the Johns Hopkins Applied Physics Laboratory in Maryland. But despite the hype surrounding Pluto, and whether or not the little planet should be considered as part of the grown-up planetary gang or not, the scientists are more concerned about the effect the ongoing debate is having on the public’s perception and understanding of science.

"The IAU damaged the public perception of science by the high-profile spectacle of imposing, by vote, a controversial definition of a commonly used term," says Sykes. "Too often, science is presented as lists of facts to be learned from authority, instead of the dynamic open-ended process that it really is. The IAU reinforced this misconception of science".

"It's good for people to know that debate in science is the norm. "Science is dynamic. Science is argumentative. Science is continual testing and challenging. Science is not about something everyone has to memorize because some organisation has given it its blessing." Mark Sykes, PSI.

The debate is sparked by the IAU definition of a planet, which is summed up in three points as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. But it’s this last point that is causing a lot of confusion.

"The public and even many scientists are confused about what this means, and wonder if asteroids crossing the orbits of Earth and even Jupiter compromise their planetary status because these are objects that have not been cleared from their orbits," says Sykes. In order to clear its orbit, an object needs to be more and more massive the farther it is from the Sun.

"At the end of the day, scientific debate is a tool we use to increase our knowledge and understanding. It is also fun." Mark Sykes, PSI.

Sykes is a strong proponent of the 13 planet Solar System, seeing Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Eris, and recently discovered Makemake all as ‘proper’ planets, since they are all round, and all orbit the Sun. Moreover, Pluto and Charon would be considered a double planet because they both satisfy the criterion for being massive enough to be round, and therefore are fundamentally different from small, irregularly shaped asteroids and comets. However, the IAU have an eight-planet Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, with Ceres, Pluto, Eris and Makemake as dwarf planets, and the denomination of plutoid assigned to those dwarf planets orbiting the Sun beyond the orbit of Neptune.

The eight largest trans-neptunian objects in the Solar System, to date. Eris, Pluto and Makemake are currently classified as dwarf planets, and also fall into the sub-category of plutoids, since their orbits fall outside that of Neptune. The other bodies shown in this graphic await classification as dwarf planets (and plutoids), or will remain under the heading of 'trans-neptunian objects' or more simply 'Kuiper belt objects'. Image: NASA.

"The IAU definition, better worded, may be useful for identifying those objects that gravitationally dominate their vicinity, or perhaps those objects that managed to sweep up most of the mass of the primordial disc of the Solar System, but it does not help us understand which objects orbiting the Sun should have similar physical characteristics," continues Sykes. "And it is those physical characteristics that we go to study with spacecraft."

Some new understanding of the differences between our Solar System's planetary inhabitants and their defining characteristics will come from NASA’s New Horizons mission, which is due to arrive at the Pluto system in 2015, and the Dawn spacecraft, which will begin studying inner Solar System dwarf planet Ceres. No doubt the ‘what makes a planet a planet’ debate will begin all over again then.