Thoughts on the 9th Planet

The internet was abuzz last week with news of a possible 9th planet in our Solar System, as well as an opportunity to view all five planets visible to the naked eye in the sky at the same time for the first time in more than a decade. Coming on the heels of the BBC StargazingLIVE show, and ongoing coverage of Tim Peake's mission oboard the International Space Station) it feels like space and astronomy has never been more at the front of the UK public’s consciousness.

The possibility of the discovery of a new planet so close to us might seem to make a mockery of how much we claim to know about the vast expanse of space. You might ask how we can claim we understand the structure of galaxies in the distant Universe when we have potentially missed an entire planet in our own solar system for so long. But this planet, if it turns out to be real, will be very dark indeed. The predictions suggest it orbits the Sun, 20 times further out that Neptune (which in turn is 30 times further out than the Earth). At this distance the planet would take 10,000-20,000 years to orbit the Sun, moving incredibly slowly against the background stars. And the illumination from the Sun would be over 300,000 times less than it is here at Earth, making it both a very cold and dark place, as well as an incredibly hard thing to spot with a telescope. Astronomers all over the world will now be searching for this tiny speck of light, in an interesting parallel of several previous searches which have happened following earlier predictions of missing planets (these earlier searches led to the discover of Neptune, as well as Ceres and Pluto – two objects we now consider dwarf planets, but which we initially called planets). 

Of course the last time the number of planets in our Solar System was in the news we all had to come to terms with losing a planet. Pluto is still exactly where it always was, but in 2006 was reclassified as a dwarf planet following the discovery of potentially hundreds of Pluto like objects in the outer solar system. As an astronomer it never fails to surprise me how much this reclassification, which you might dismiss as an obscure technical discussion, captures the public imagination. This has recently been back in the news following the amazing pictures of Pluto sent back by the New Horizons Mission. While I’m often surprised at the interest this generates, I’m also pleased for the opportunity it gives to remind us all that science isn’t a fixed and static thing. We reclassify planets on the basis of new information, and we can still have the opportunity to discover massive new planets in our own backyard. 

The already iconic view of Pluto from the NASA New Horizon's Mission.

If you want to see all six planets visible to the naked eye at once (Mercury, Venus, Earth, Mars, Jupiter and Saturn) you can get up early on any clear morning over the next month or so. Venus and Jupiter will be the two brightest points of light you can find in the sky – Jupiter is to the West, and Venus to the East. In a rough line between them will be the noticeably orange tinged Saturn and Mars. Mercury is a challenge as it’ll be a small point of light fading into the dawn light as the Sun rises – of course Earth you can see all the time (just look down). 

A screenshot from the free planetarium software Stellarium showing all 5 visible planets together in the sky at 6.50am on Tue 26th Jan 2016

 

To see the International Space Station and wave at Tim Peake as he passes by, you can look for notifications of the next visible ISS passesonline. The next ones visible from where I live (in the UK) are Feb 2nd and 3rd in the evening. The ISS looks like a steadily moving constant point of light to the naked eye, and you see the reflected sunlight off its solar panels.