There’s plenty of excitement at the announcement overnight that a new planet is potentially waiting to be found at the extremes of our solar system.
The possible ninth planet is thought to be quite substantial with a mass around ten times that of Earth and a radius that’s two-to-four times bigger than Earth’s. This characterises it as a Neptune-like object.
What’s truly remarkable about Planet Nine, as it has been dubbed, is its very long orbit. It is estimated to take between 10,000 to 20,000 years to orbit our sun, on an elliptical orbit that stretches way beyond the Kuiper Belt.
The Kuiper Belt is a ring of icy objects (which includes Pluto) that circles the sun beyond the orbit of Neptune. Neptune orbits about 30 times further from the sun than the Earth and astronomers refer to Neptune’s distance from the sun as being 30 astronomical units (au) (where one au is the Earth-sun distance). Pluto follows an elliptical orbit that brings it as close as 29.7au from the sun, then out to almost 50au at its most distant point.
Planet Nine’s proposed elliptical orbit takes it from 200au at its closest to the sun (or perihelion) and between 500au to 1,200au at its furthest (aphelion). When it comes in close, it should be bright enough for high-spec backyard telescopes to pick it up.
But unfortunately, most of the time the planet will be much more distant and that represents a greater challenge. It will require the world’s largest telescopes, such as the 10m diameter Keck telescopes and Japan’s 8.2m Subaru telescope (both located on Mauna Kea in Hawaii) to have a hope of seeing it.
Ghostly pull of gravity
The planet has yet to be seen. So why is it thought to be out there? And how can we know so much about it? Planet Nine is the best fit to explain the orbits of six distant objects.
What’s odd about these six objects is that they have peculiar but remarkably similar orbits. These objects have been nudged off kilter and yet they are all shepherded together in the same region of space.
The first of these objects to be discovered was Sedna. It was observed in 2003, as it approached perihelion. When its 11,400-year orbit was calculated, the discovery team realised that this object was orbiting in a kind of “no man’s land” (or more correctly “no person’s land”).
It was too distant to belong to the Kuiper Belt and not far enough away to be among the sphere of comets orbiting the sun in the Oort Cloud.
Sedna was also beyond the gravitational pull of Neptune, so something else, perhaps a large planet or possibly even a passing star (one of the sun’s many siblings perhaps), might have nudged it off course. What makes Planet Nine feasible is that it can explain the orbit of Sedna along with the other five objects.
At their closest approach to the sun, these six objects sit within the plane of the solar system. Planet Nine would have an orbit that is anti-aligned to the six objects and provides the gravitational tug needed to keep those planets in check.
And there’s more. What makes good science is when a proposed model explains something above and beyond its original intention. Simulations of Planet Nine predict that there should also be objects in the Kuiper Belt that have orbits perpendicularly inclined to the plane of the solar system.
Turns out, these objects exist. Five such objects have been known about since 2002, although their orbits have been unexplained until now.
Haven’t we seen this before?
If Planet Nine does exist, it’s not the first time that a planet in our solar system has been discovered theoretically before being directly observed. In 1845, deviations in the orbit of Uranus, suggested there might be an eighth planet to the solar system and in 1846, Neptune was observed exactly where it was predicted to be.
There have also been predictions that haven’t stood the test of time. Back in the 1980s, scientists proposed that the sun might be a binary, with a dim undiscovered companion moving along on elongated orbit. Every 23 million years (or so), this star named Nemesis would pass through the solar system causing a deluge of comets to impact Earth and produce mass extinctions.
More recently, around the turn of the millennium, astronomers noticed an asymmetry in the distribution of new comets coming in from the Oort Cloud. In theory, comets should come evenly from all directions, but there was a slight excess distributed around a great circle on the sky. One of the explanations was that there could be a Jupiter-mass planet in the Oort cloud, known as Tyche.
In 2014, NASA’s Wide-Field Infrared Survey Explorer (WISE) examined the entire sky across infrared wavelengths. It was the perfect telescope to detect Nemesis or Tyche, but failed to find any evidence of either.
Will we find Planet Nine?
Scientists are sceptical by nature. It’s exciting to have a model that predicts the existence of Planet Nine but this prediction must also be tested. Astronomers have begun searching through astronomical surveys, such as the WISE survey, the Catalina Sky Survey, and the Pan STARRS surveys in the hope of making a sighting.
So far, nothing has been seen. The conclusion, as described in a blog by astronomer, Mike Brown (who proposed Planet Nine along with colleague Konstantin Batygin) is that Planet Nine, if it exists, is likely in the hardest place to find.
It seems to currently be at its furthest point from the sun, at least 500au away; it’s probably fainter than 22nd magnitude (that’s 1,500 times fainter than Pluto); and very possibly it’s aligned with the plane of the Milky Way Galaxy (meaning that Planet Nine may currently be hidden against the background stars of our Galaxy).
Regardless, the hunt is on and there just may be a great discovery out there, waiting to happen.
Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria and Jonti Horner, Vice Chancellor’s Senior Research Fellow, University of Southern Queensland
This year is shaping up to be another exciting one for space after a bonanza of discoveries and celestial events in 2015.
One of my hoped-for highlights of 2016, NASA’s Insight mission, has unfortunately been scrubbed due to a serious leak in last stage testing. But here are three of my particular favourite space missions to watch out for as well as some key night sky events to try and experience throughout 2016.
The first stage is a joint Trace Gas Orbiter (TGO) and Schiaparelli lander which will launch in March (to arrive by October). The technologies demonstrated by Schiaparelli will then be used for a rover to land in any interesting sites identified by TGO as the next stage in ExoMars.
The orbiter will spend five years attempting to “sniff” out those gases in the Martian atmosphere such as methane that break down over time, with any trace amounts indicating a process of creation on Mars. Whether biological (that is, expelled by microbial life) or geological in nature will be investigated throughout the ExoMars programme.
A key story for 2016 will be the investigation of Jupiter by NASA’s Juno mission. As the enormous gravity of Jupiter pulls the spacecraft to ever higher speeds, ultimately travelling at more than 70km per second, Juno will become the fastest craft in human history.
It will fire its rockets to slow down and then enter one of the most challenging orbits ever attempted, skimming as low as just 5,000km above the cloud tops, ducking below the intense – and damaging – radiation belts of Jupiter to study the gas giant as never before. To put that in perspective, if Jupiter were a soccer ball, Juno would be skimming less than a centimetre off the surface.
The aim is to see if there’s water in the atmosphere (revealing the conditions from which the gas giants formed), to study the gas giant’s magnetic and gravitational field and the nature of the interior.
Thousands of kilometres of clouds crush the core to extraordinary pressures that might form a planet-sized diamond (as Arthur C. Clarke once wrote) or more likely a core of superconducting metallic hydrogen that powers the enormous magnetic field of the planet.
It will be deorbited in Februrary 2018 after 37 death-defying orbits threading through the incredibly dangerous radiation belts.
The latest telescope on Earth could hardly look more different to those that use light (be it visible or radio waves) but LIGO is searching the skies for colliding black holes, with the telltale signals as ripples in the very fabric of spacetime itself. These gravitational waves are Einstein’s final prediction and are yet to be verified.
As a gravitational wave passes through you, you’d be stretched one way becoming thinner and then as the wave continues through you are squashed and fattened. Since this doesn’t visibly appear to happen we can guess that the stretching and squashing is tiny. The expected change is less than the thickness of an atom in a ruler a million kilometres long.
To measure this incomprehensibly tiny change we use lasers (technically an interferometer bouncing two lasers back and forth) in different locations on Earth to triangulate the position to a few degrees on the sky (the width of a few full moons).
Discovering these waves will allow us to see the universe with an entirely new sense, as distinct from hearing to seeing. In 2016, humanity will gaze with entirely new eyes into the cosmos.
There are also some fantastic sights in the sky to watch out for in 2016.
Southeast Asia and Africa will get to enjoy the more visually impressive solar eclipses, with Australia, Europe and the United States missing out (although everyone can enjoy the stunning meteor showers).
These are selected from a more exhaustive list of all the motions of the planets and other celestial highlights. If not mentioned, all times and viewing directions are from an Australian perspective.
January 20 to February 20
All five planets visible to naked eye – Mercury, Venus, Mars, Jupiter and Saturn – will appear in morning sky. This is the first time since 2005 and should be something we can all manage to see without telescopes.
Jupiter at opposition
This the best time to see the gas giant as it forms a direct line with the sun – Earth – Jupiter. Similar to a full moon, Jupiter will be entirely illuminated by the sun making it appear brighter than any other time this year.
With binoculars you should easily discern the four largest (Galilean) moons sitting in a line either side of the planet.
Total solar eclipse
The total solar eclipse will be visible from central Indonesia and some Pacific Islands. Neighbouring regions, such as Northern Australia and Southeast Asia, will see a partial eclipse but protective eyewear should be used at all times. Check the NASA predicted track.
Eta Aquarids meteor shower
May 6 to 7
Eta Aquarids is a particularly good meteor shower with up to 60 meteors per hour at its peak in the southern hemisphere (the northern hemisphere might see half this). This meteor shower is from the Earth running through the dust tail of Halley’s comet.
The new moon will mean even more of the faintest shooting stars are visible. Look towards the constellation Aquarius after midnight.
Transit of Mercury across the sun
Mercury will pass between the Earth and the sun, with the dark disk of the planet visible across the face of the sun. There will not be another transit of Mercury until 2019 and then the next one will be in 2039.
This can only be seen with specialised protective eyewear and a telescope, including a pinhole camera. Unfortunately, this will not be visible from Australia but will be seen in most of the rest of the world, in particular the eastern United States and eastern South America.
As the third of four full moons in this season, it is known as a blue moon. This is a relatively rare occurrence, hence the term “once in a blue moon”.
Each season you could expect three full moons but the lunar cycle is every 29.53 days meaning on average every 2.7 years you can squeeze in a fourth full moon in a season, nothing to do with the colour changing!
Mars at opposition
The sun and Mars sit directly opposite one another as seen from the Earth ensuring the planet is fully illuminated by the sun.
Mars will be a clear red point of light in the night sky. Using an eight- to ten-inch telescope you can see darker regions amid the orange/rust coloured planet.
Saturn at opposition
Saturn is in a direct line between the Earth and sun meaning that it rises in the east just as the sun sets in the west.
Saturn will be a bright diamond coloured point of light, at its brightest for the entire year. A medium-sized telescope will be needed to see the famous rings.
Perseids meteor shower
August 12 to 13
With up to 60 meteors per hours the Perseids is a reliably good meteor shower as the Earth ploughs through the debris of the comet Swift-Tuttle.
There is only a minimal amount of moonshine (a waxing gibbous moon setting just after midnight) ensuring that the majority of shooting stars will be seen after midnight.
The shooting stars will radiate from the constellation Perseus.
Conjunction of Venus and Jupiter
Two of the brightest planets in the night sky will appear to move towards each other throughout August. They reach their closest point (a conjunction) just after sunset in the western sky (just seven arcminutes apart, or less than the nail of your little finger held at arm’s length) on August 27.
Annular solar eclipse
The moon is a little further from the Earth than the March 9 eclipse meaning that it will not completely cover the sun, revealing a burning bright ring (or annulus) around the moon.
The eclipse path will pass through Congo, Tanzania and Madagascar before ending in the Indian Ocean. A partial eclipse will be visible in the neighbouring African nations. Check the NASA predicted track.
First of this year’s three supermoons, when the moon is closest to the Earth in its orbit. This means it is bigger in the sky and hence brighter when fully illuminated by the sun (a full moon) on the opposite side of us from the sun.
Second of three supermoons for 2016.
Geminids meteor shower
December 13 to 14
The Geminids are usually the best meteor shower of the year with up to 120 meteors per hour, but unfortunately there is nearly a full moon this year (a supermoon no less) that will outshine all but the brightest shooting stars.
All other meteor showers this year are from the Earth ploughing through a debris tail from a comet, but the Geminids are unique in being from an asteroid (3200 Phaethon). The radiant is in the constellation Gemini (the Twins).
Third and final supermoon of 2016. What a great way to end the year, but a shame about the Geminids meteor shower.
The following link is to an article reporting on the possible discovery of a new planet in the solar system.