Posts Tagged ‘IAU’

It was announced last week that the Canada-France-Hawaii Telescope (CFHT) had discovered a new dwarf planet beyond the orbit of Neptune. The planet, provisionally named 2015 RR245 by the International Astronomical Union (IAU), has had its orbit measured over several months and from this it has been determined to have a highly elliptical orbit which brings it to within 34 AUs from the Sun, but takes it out as far as 120 AUs. By comparison, Neptune’s orbit is far closer to circular and at about 30 AUs (varying between 29.8 and 30.3 AUs).

From its current distance and brightness, its size has been estimated to be about 700km in diameter (Pluto, in comparison, has a diameter of 2374 km). This is based on an assumed albedo (reflectivity), so if it is more reflective than assumed it could be smaller, if it is less reflective it could be larger.


The Canada-France-Hawaii Telescope has discovered a new dwarf planet beyond the orbit of Neptune.

2015 RR245 was discovered as part of the Outer Solar System Origins Survey (OSSOS), and was spotted in February 2016 on an image taken in September 2015. We thus have images of it spanning nearly 10 months of its orbit, enough to get a decent idea of its orbital parameters.


If 2015 RR245 is as large as 700km in diameter then it will be amongst the largest dwarf planets known.

As of yet, the IAU has not admitted 2015 RR245 to the select club of official dwarf planets. At the moment, there are only five dwarf planets in this ‘official list’; namely Pluto, Eris, Ceres (in the asteroid belt), Makemake and Haumea. Since the creation of the dwarf planet category in 2006, these five are the only objects which have been classified as such. Haumea was the latest object to be added to the list, in September 2008. We shall have to see whether 2015 RR245 makes the list at some point in the future.


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NASA’s New Horizons space probe has been on its way to Pluto since its launch in January 2006, and NASA has recently announced that it has started its “approach phase”. As part of testing its instruments, and in order to make any final corrections before it flies past Pluto in July, the probe is already taking images even though it is still some 200 million km from Pluto (which is more than the distance from the Earth to the Sun).

Pluto, discovered by Clyde Tombaugh in 1930, was listed as one of the nine planets in our Solar System. But, in 2006, the International Astronomical Union (IAU) demoted it to the status of “dwarf planet”; a status it shares with a few other objects like Ceres (the largest object in the asteroid belt between Mars and Jupiter) and Eris, which is in the Kuiper belt. In fact, it was the discovery of objects in the Kuiper belt in the 1990s which led to Pluto being demoted to a dwarf planet status, as it was realised that it just happened to be the largest of the known Kuiper belt objects, and the first to be discovered.

Although we have sent space probes to all the other “classical” planets, and in fact to some asteroids, this will be the first time a space probe has visited Pluto. Because of this, we hope to learn a lot more about Pluto as the probe flies past it. Because of the speed of the probe, and the relatively small mass of Pluto, the probe will not be able to go into orbit about Pluto, but instead will pass about 14,000 km from its surface (roughly the same as the diameter of the Earth). As it whizzes by the various instruments will make their measurements, and they will only get one chance at it.

NASA's New Horizons space  probe has been travelling towards a fly-by of Pluto since 2006.

NASA’s New Horizons space probe has been travelling towards a July 2015 fly-by of Pluto since January 2006.

It was discovered in 1978 that Pluto had a satellite, Charon, the discovery being made from ground-based telescope images of Pluto. Since then, four other satellites have been discovered; Nix and Hydra were discovered in 2006, Kerberos in 2011 and Styx in 2012. So, as things stand, we now know of five moons orbiting Pluto, although New Horizons may discover more. We also hope to learn more about Pluto and Charon’s surface features, their composition, and whether Charon has an atmosphere. I will write future blogs on this as NASA releases these results.

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