A simulation of how Earth’s sky might look if several of the solar system’s other planets orbited our home from around about the same distance out as the Moon does… wait though until Jupiter comes onto the scene.

Following the recent argument over what constitutes a planet – which famously saw Pluto downgraded in status a few years ago – debate is now raging over whether groups of stars should be considered galaxies when they may in-fact be star clusters instead.

Star clusters and galaxies both contain stars bound together by gravity, but while the members of a star cluster are thought to form simultaneously from a collapsing ball of gas, galaxies have richer histories. In the most popular cosmological model, they form along swathes of dark matter and contain enough gas to form many generations of stars. Yet the distinction is not always clear-cut. Take Omega Centauri, a round swarm of stars that orbits the Milky Way and is visible to the naked eye. It has long been classified as a star cluster, but there is now evidence that it contains multiple generations of stars, suggesting it is actually the remnant of a galaxy.

Mike Brown, who in January 2005 discovered what was at first hailed as the solar system’s tenth planet – initially dubbed Xena but later named Eris – writes how the discovery of the now dwarf planet led to Pluto, long regarded as the ninth planet, likewise being downgraded in stature, in his new book, How I Killed Pluto and Why it Had it Coming.

I hobbled back from the rocky beach up to the house. I woke Diane and told her that when the press called tomorrow I was going to have to tell them why the new proposed definition of planet was no good and why, in the end, it made sense all along for there to be just eight planets. I told her that I was going to have to kill Pluto and that Xena would go down as necessary and important collateral damage. All along, Diane had been more practical than I was. “Just let it be a planet,” she would say. “Try not to worry about it so much,” she had told me all year. “Relax” was her usual advice.

Following recent observations, doubt has been cast over the size of Eris, a dwarf planet orbiting the Sun at a (massive) distance of up to 14,510,993,390 kilometres, and previously thought to be bigger than Pluto, a finding that eventually resulted in Pluto being classed as a dwarf planet in 2006, after being regarded as a planet since its discovery in 1930.

Occultation measurements are by their nature very accurate, and although more precision is needed before we get a definitive answer, Eris is certainly a lot smaller than it was thought to be. “Almost certainly Eris has a radius smaller than 1,170 km [727 miles],” Bruno Sicardy, of the Paris Observatory, said in an email to Sky and Telescope Magazine. Pluto has a radius of approximately 1,172 kilometers (728 miles). Pluto therefore has an average density of 2.03 grams/cm³ and Eris has an average density of 2.5 grams/cm³.

While the new figures don’t show a great deal of variance in the size of either object, they could re-open the discussion over whether Pluto should continue to be regarded as a dwarf planet.

Reports circulating last week that the first ever photograph of an exoplant – a planet orbiting a star other than the Sun – had been taken, have turned out to be inaccurate, though the actual photo in question is notable for another reason.

I want people to understand that this discovery is being touted as the first direct image of a planet around another star. It isn’t. Nor is it the first direct image of a planet orbiting a sun-like star. What this is is the first direct image of a planet orbiting a sun-like star taken using a ground-based telescope. While that may sound overly picky, it’s actually a significant achievement, and worth noting.

The Kepler spacecraft, a NASA space observatory, identified more than 750 possible exoplanets – many of them less than half the size of Jupiter – over a six week period, which is more than has been found in the last fifteen years.

In the spring of 2009 the Kepler Mission conducted high precision photometry on nearly 156,000 stars to detect the frequency and characteristics of small exoplanets. Kepler studied an area in the constellation Cygnus, looking for the small changes in light that would signal a planet passing in front of its star.

While it is expected about half of these “candidate planets” will turn out to be other objects of some sort, given the relatively small area of the sky that Kepler scanned, it is likely the galaxy is teeming with such bodies.

SolarBeat serves up music and information… for example Earth makes about 250 orbits of the Sun in the time it takes Pluto to complete one revolution.

An explanation as to why the Earth, and the other (smaller) inner planets of the solar system, have not been consumed by the Sun.

It all made sense, except for one tiny problem: this same model also suggested that a little world like Earth shouldn’t exist at all; it (or more precisely, the Moon-size proto-planets that eventually assembled into the Earth) should have spiraled into the Sun more than 4 billion years ago. A star might not gobble a Jupiter whole when it moved close enough, but it could surely swallow a canapé like the proto-Earth.

P H E N O M E N A L, an online scale model of the Solar System, which is said to be half a mile in width.

Unlike most models, which are compressed for viewing convenience, the planets here are also shown at their true-to-scale average distances from the Sun. That makes this page rather large – on an ordinary 72 dpi monitor it’s just over half a mile wide, making it possibly one of the largest pages on the web.

To spare some scrolling here are shortcuts to Earth, and outer most planet Pluto (yes, I still regard Pluto as an actual planet).

A summary of the how the two Voyager space probes have expanded our knowledge of the solar system’s outer planets.

How then to summarise Voyager? As Stone observes “I felt that we were all in the tradition of Galileo. He was the first to see the moons of Jupiter and the first to apply an instrument to increase our ability to observe the universe. Voyager is just the latest tool which we as a civilization have managed to devise, and the tool was so powerful that we saw things that nobody had seen before and that nobody had imagined that we could see.”