Astronomers have only been able to obtain evidence for their existence in recent decades by studying black holes accreting (attracting) gas from nearby stars and finding fast-moving stars in the vicinity of black holes.
But since 2015 an exciting third way to detect black holes has become available: gravitational waves from merging black holes.
From one extreme…
Stellar mass black holes can have masses between a few to a few tens of solar masses – the mass of our Sun. They are thought to form at the end of the lives of massive stars. When these stars run out of gas from which to produce energy, they leave behind massive remnants that can only collapse into black holes.
So far, astronomers have discovered a dozen stellar mass black hole candidates in the Milky Way, most of which accrete matter from nearby companion stars.
They also detected gravitational waves from several merging stellar mass black hole pairs in distant galaxies.
It’s estimated that our Milky Way alone should contain about 100 million stellar mass black holes, most of which do not have close companions from which they can accrete matter, and which therefore stay invisible.
… to the other
At the other end of the mass scale are what astronomers call supermassive black holes. These are about a million to a few billion times more massive than our Sun.
The Milky Way, for example, contains a black hole of about 4 million solar masses, called Sagittarius A* (Sgr A*), in its centre. Astronomers can study this black hole by looking at the motion of stars that are close to Sgr A* and are flung through space by the huge gravitational attraction of the black hole.
Although astronomers have gained a good understanding of the distribution and masses of supermassive black holes in galaxies in the nearby universe, they still do not know where supermassive black holes come from.
Observations show that some supermassive black holes already existed and were actively accreting gas from their surroundings when the universe was just a few hundred million years old.
In 2011 a team of astronomers said they had found evidence of a supermassive black hole that existed only 770 million years after the Big Bang. Then, last month, another team of astronomers revealed what they think could be evidence of a supermassive black hole from when the universe was only 690 million years old.
This creates a problem for theories that assume that supermassive black holes grew out of the stellar-mass black holes left behind by the first generation of stars in the early universe.
There is not enough time for these black holes to have grown to reach the huge masses that we can see in observations of the first galaxies.
The middle ground for black holes
An alternative theory is that supermassive black holes form from so-called intermediate-mass black holes. These hypothetical black holes could have masses from a few hundred to a few hundred thousand solar masses.
Starting more massive, supermassive black holes would need less time to grow to their present sizes. They could also accrete mass more efficiently since the maximum amount of mass that a black hole can accrete is directly proportional to its size.
Intermediate mass black holes could form out of the collapse of very massive stars that might have existed in the very early universe.
Nowadays stars form with an upper mass limit of at most a few 100 solar masses. Conditions in the very early universe might have been more favourable towards building more massive stars and might have allowed the formation of stars of a few thousand or maybe even up to a million times the mass of our sun.
The hunt is on
Astronomers are currently searching for intermediate mass black holes and there are a few potential candidates. Like their more massive cousins they could reveal their existence by accreting material from nearby stars or by the fast motion of nearby stars.
A prime place to look for intermediate mass black holes could be globular clusters, dense clusters of a few hundred thousand of stars to a few million of stars.
Like supermassive black holes, globular clusters are old and are among the first objects which have formed in the universe.
Astronomers – including at the University of Queensland – recently found evidence that such an intermediate mass black hole with about 2,200 times the mass of our Sun could exist at the centre of the globular cluster 47 Tucanae.
They did this by studying the acceleration of pulsars (compact remnants of dead stars that formed with about 20 times the mass of our Sun) in the globular cluster.
If more of these can be found, they might provide the missing link between stellar mass and supermassive black holes and could shed light on how supermassive black holes have formed.
A Blood Moon, a trip to the Moon and back for two explorers, a space station crashing to Earth and the launch of a new mission to find planets around other stars: these are just some of the exciting things to watch in space in 2018.
Elon Musk’s Space X also plans to launch one of the new Falcon Heavy rockets, the largest since the manned Moon landings.
After a failure of two reaction wheels (the things that help it point) in 2013, a new mission, K2, was conceived. It was able to keep stable by using a combination of short thruster firings and using the Sun to steer it like a sail.
Where one missions ends, a new one begins. The Transiting Exoplanet Survey Satellite (TESS), is set to be launched between March and June, aboard a SpaceX Falcon 9 rocket. If the stars align, we might even have overlap between these two exoplanet-discovering machines.
Rockets, rockets and more rockets
The privatisation of space continued this year with the US-based Rocket Labs having its first successful launch, from a site across the Tasman in New Zealand.
Asteroids are not forgotten in all of this space exploration. Japan’s Hayabusa-2 is set to arrive at asteroid 162173 Ryugu. It’s a new version of Hayabusa, which surveyed the asteroid 25143 Itokawa and took samples before returning back to Earth, landing near Woomera, South Australia in 2010.
If you were around in 1979 and happened to be in Western Australia, you might have a unique souvenir – part of the US space station Skylab, which re-entered and crashed outside Esperance, WA.
If you’ve seen the 2013 movie Gravity (and a spoiler alert for those who haven’t!) you might remember the final scene in which Sandra Bullock’s character returns home by hijacking Tiangong-1, the Chinese space station. She returns safely, but the same can’t be said for Tiangong-1.
You can track its progress but in short, somewhere between +43 and -43 latitude (or half the Earth), it will re-enter and break apart. Currently, the likely potential (land) areas are around Central and South America, Northern Africa and the Mediterranean, and indeed Western Australia.
Like Skylab, there are likely to be large pieces that survive re-entry. Hopefully you are lucky to be in a position to see it with your eyes, but not so close that it lands on your house, as it’s unlikely to be covered by your insurance policy.
So that’s a summary of some of the things we’re expecting to happen this year. But as with all science, I’m just as excited for those discoveries that we do not know about that will happen in 2018.
A total lunar eclipse will occur on Wednesday, January 31, and Australia is in the perfect position to see it. But it’s also being called many other lunar things, from a Blood Moon to a Blue Moon and a Super Moon.
So what is really going to happen on the night?
This is the first time in three years that we have the chance to see a total lunar eclipse from Australia, and the Moon will spend just over three hours passing through Earth’s shadow.
The great thing about lunar eclipses is that they are lovely to watch and no special equipment is needed to see the events unfold.
From light to dark
At first we’ll see the Full Moon begin to darken. For Wednesday’s lunar eclipse the shadow will approach from the bottom-right, leaving the top part of the Moon in sunlight.
It takes an hour before the Earth’s shadow crosses the Moon entirely and once the Moon is completely engulfed the period known as totality begins.
Totality brings its own surprise. The Earth’s shadow is not completely black, but has a reddish hue. This has led many cultures, including some Indigenous Australian communities, to describe a lunar eclipse as a Blood Moon.
Sunlight still manages to reach the Moon but it must first pass through Earth’s atmosphere. This both reddens the light (by scattering away the shorter wavelengths or blue light) and also bends the path of the light, directing it into the shadow.
This week’s lunar eclipse is a fairly deep one and totality will last just over an hour. Thereafter, the Moon will begin to emerge from the shadows, and it will be another hour before we see the brilliance of the Full Moon once more.
How I can see it?
The eclipse can be seen by the entire night side of the globe and everyone will experience the event at precisely the same moment. What affects the eclipse timings are local time zones.
For Western Australia, the eclipse occurs in the early evening, within an hour after sunset. The Moon will be low to the eastern horizon at the start of the eclipse but will move higher in the sky and towards the northeast as the eclipse progresses.
For the rest of Australia, the eclipse occurs two to three hours after sunset. The eclipse will begin with the Moon in the northeast and climbing towards the north.
It seems these days that it’s not enough to be treated to a beautiful natural phenomenon like a total lunar eclipse. Instead, I’ve been hearing a lot of hype surrounding this eclipse and the numerous names applied.
It’s true that lunar eclipses can only occur around the time of Full Moon. That’s when the Sun is on one side of the Earth, while the Moon is located on Earth’s opposite side.
Most of the time the Full Moon sits above or below Earth’s shadow and the Moon remains flooded with sunlight. But twice a year, the three bodies fall into line so that Earth casts its shadow on the Moon.
As well as being a Full Moon, eclipses can also be described as a Blood Moon because of the Moon’s reddish appearance, as mentioned previously.
But the descriptions of Super Moon and Blue Moon may not be quite what they seem.
Look to the sky … it’s a Super Moon!
I’ve written before about the Super Moon sensation and it’s a term that has only taken off in the past seven years.
Back in March 2011, NASA published an article describing a “super full moon”. The precise time of Full Moon that month occurred 59 minutes before perigee, that is, the Moon’s closest approach to Earth as it travels along its elliptical orbit.
As quoted in the article:
The full Moon of March 19th  occurs less than one hour from perigee – a near-perfect coincidence that happens only every 18 years or so.
It must have seemed a worthwhile curiosity to report on at the time.
Seven years later and the Super Moon craze is now a bit out of hand, with some claiming three Super Moons a year depending on the chosen definition.
As a Super Moon this lunar eclipse is definitely on the outer limits, with the Full Moon occurring 27 hours after perigee and at a distance of more than 360,000km (calculated in the usual way from the centre of Earth to the centre of the Moon).
Considering that it’s also quite difficult to tell the difference in both size and brightness between a regular Full Moon and a Super Moon, this one is really pushing the limits of credibility.
Once in a Blue Moon
According to Philip Hiscock, a folklorist at the Memorial University, USA (now retired), the classic saying “once in a blue moon” is more than 400 years old. It originated as something so absurd it could never actually happen, similar to saying “when pigs fly”.
Intense volcanic activity or smoky forest fires can fill Earth’s atmosphere with dust particles that are slightly larger than usual. As a result, red light is scattered away, giving everything a blue tinge, including the Moon (normally the atmosphere scatters blue light, hence why the sky is blue).
But when it comes to this lunar eclipse, it’s not the colour of the Moon but a quirk of our timekeeping that is in play.
What a difference a day makes
A Full Moon occurs every 29.5 days, but our months are longer (excluding February). This mismatch of timing means that every couple of years there comes a month with two Full Moons.
In recent times, a Blue Moon has referred to the second full moon of a calendar month. For most of the world, this lunar eclipse is occurring during a Blue Moon, except for Australia’s eastern states of New South Wales, Victoria, Tasmania and the Australian Capital Territory.
Those states follow daylight saving, which pushes the Full Moon into the following day and out of the month of January (the actual time of Full Moon is 12:26am AEDT, February 1). This leaves January with only one Full Moon for those states and territory.
But there’s more. This modern definition of Blue Moon arose only 30 years ago.
The original definition is as follows: if four Full Moons occur between an equinox and a solstice (for example, in the three months between a spring equinox and a summer solstice) then the third Full Moon should be called a Blue Moon.
This ensured that the proper names of the Full Moons (common in North America, such as the Harvest Moon) were correct relative to the equinoxes and solstices.
But regardless of the exact flavour of this lunar eclipse, what’s certainly true is that we are part of a grand universe, and Wednesday night is the perfect reminder of that.
We are bathed in starlight. During the day we see the Sun, light reflected off the surface of the Earth and blue sunlight scattered by the air. At night we see the stars, as well as sunlight reflected off the Moon and the planets.
But there are more ways of seeing the universe. Beyond visible light there are gamma rays, X-rays, ultraviolet light, infrared light, and radio waves. They provide us with new ways of appreciating the universe.
Have you looked at the Moon during the daytime? You will see part of the Moon bathed in sunlight and the Earth’s blue sky in front of the Moon.
Now put on your X-ray specs, courtesy of the ROSAT satellite, and you will see something intriguing.
The Sun emits X-rays, so you can see the daytime side of the Moon easily enough. But the night time side of the Moon is silhouetted against the X-ray sky. The X-ray sky is behind the Moon!
Just what is the X-ray sky? Well, X-rays are more energetic than visible light photons, so X-rays often come from the hottest and most violent celestial objects. Much of the X-ray sky is produced by active galactic nuclei, which are powered by matter falling towards black holes.
In X-rays, the Moon is silhouetted against many millions of celestial sources, powered by black holes, scattered across billions of light years of space.
If you’re in the southern sky and away from light pollution (including the Moon), then you can see the Small Magellanic Cloud. This is a companion galaxy to our own Milky Way. With the unaided eye it looks like a diffuse cloud, but what we are actually seeing is the combined light of millions of distant stars.
The hydrogen gas is cold enough that the atoms hang onto their electrons (unlike ionised hydrogen). It can also cool further and collapse (under the force of gravity) to produce clouds of molecular hydrogen gas and eventually new stars.
Radio waves thus allow us to see the fuel for star formation, and the Small Magellanic Cloud is indeed producing new stars right now.
Feeling the heat in the microwave
If the universe were infinitely large and infinitely old, then presumably every direction would eventually lead the surface of a star. This would lead to a rather bright night sky. The German astronomer Heinrich Olbers, among others, recognised this “paradox” centuries ago.
When we look up at the night sky, we can see the stars, planets and Milky Way. But most of the night sky is black, and this tells us something important.
But lets take a look at the universe in microwave light. The Planck satellite reveals glowing gas and dust in the Milky Way. Beyond that, in every direction, there is light! Where does it come from?
At microwave wavelengths we can observe the afterglow of the Big Bang. This afterglow was produced 380,000 years after the Big Bang, when the universe had a temperature of roughly 2,700℃.
But the afterglow we see now doesn’t look like a 2,700℃ ball of gas. Instead, we see a glow equivalent to -270℃. Why? Because we live in an expanding universe. The light we observe now from the Big Bang’s afterglow has been stretched from visible light into lower-energy microwave light, resulting in the colder observed temperature.
But you get a less familiar view of Jupiter when you switch to radio waves. A radio telescope reveals the dull warm glow of the planet itself. But what really stands out are radio waves coming from above the planet.
On Earth we use particle accelerators to produce such radiation. But in Jupiter’s powerful magnetic field it occurs naturally (and copiously).
The synchrotron produced by Jupiter is so powerful that you can detect it on Earth – not just with multimillion-dollar radio telescopes, but with equipment that can be bought for several hundred dollars. You don’t need to be a professional astronomer to expand your view of the universe beyond visible light.
For each shower, we give the forecast activity period and the predicted time of maximum. We also provide charts showing you where to look, and give the peak rates that could be seen under perfect conditions (known as the maximum Zenithal Hourly Rate, or ZHR).
The actual rate you see will always be lower than this value – but the higher a shower’s radiant in the sky and the darker the conditions, the closer the observed rate will get to this ideal value.
The Lyrids hold the record for the shower with the longest recorded history, having been observed since at least 687BC.
That longevity is linked to the orbit of the Lyrid’s parent comet, discovered in 1861 by A. E. Thatcher. Comet Thatcher moves on a highly inclined, eccentric orbit, swinging through the inner Solar system every 415 years or so. Its most recent approach to Earth was in 1861.
Compared with many other comets, Thatcher’s orbit is relatively stable, as the only planet with which it can experience close encounters is Earth. This means the meteors it sheds continue to follow roughly the same orbit.
Over the millennia, that shed debris has spread all around the comet’s vast orbit, meaning that for thousands of years, every time Earth intersects Comet Thatcher’s orbit, the Lyrids have been seen, as regular as clockwork.
These days, the Lyrids are usually a moderately active shower, producing somewhere between 10 and 20 fast, bright meteors per hour at their peak. Occasionally, though, the Lyrids have thrown up a surprise, with rates climbing far higher for a period of several hours.
The best of those outbursts seem to occur every 60 years or so, with the most recent occurring in 1982 when observed rates reached or exceeded 90 per hour.
No such outburst is predicted for 2018, but even in quiet years, the Lyrids are still a fun shower to observe.
They are best seen from northern latitudes, but their radiant is far enough south for observers throughout Australia to observe them in the hours before dawn.
For observers at mid-northern latitudes, the Lyrid radiant reaches suitable altitude by about 11pm local time. Viewers in the southern hemisphere have to wait until the early hours of the morning before reasonable rates can be observed.
The forecast time of maximum this year favours observers in Australia and east Asia but the timing of maximum has been known to vary somewhat, so observers around the globe will likely be keeping their eyes peeled, just in case!
Active: July 17 – August 24
Maximum: August 12, 8pm UT – August 13, 8am UT = from August 12, 9pm BST (UK) = 10pm CEST (Europe) = 6pm EDT (East Coast, US) = 3pm PDT (West Coast, US) for 12 hours
For observers in the northern hemisphere, the Perseids are a spectacular summer highlight. At their peak, rates often reach or exceed 100 meteors per hour, and they are famed for their frequent spectacular fireballs.
The Perseids are probably the best known and most widely observed of all modern meteor showers. They are remarkably consistent, with peak rates usually visible for a couple of evenings, and fall in the middle of the northern hemisphere summer holiday season. The warm nights and frequent clear skies at that time of year make the shower a real favourite!
Like the Lyrids, the Perseids have a long and storied history, having been observed for at least 2,000 years. Their parent comet, 109P/Swift-Tuttle, is a behemoth, with the largest nucleus of the known periodic comets – some 26km in diameter.
It has likely moved on its current orbit for tens of thousands of years, all the time laying down the debris that gives us our annual Perseid extravaganza. It will next swing past Earth in 2126 when it will be a spectacular naked eye object.
This year the forecast maximum for the Perseids favours observers in Europe, although given the length of peak activity, any location in the northern hemisphere has the potential to see a spectacular show on the night of August 12.
But don’t despair if it’s cloudy that night, as the Perseids have a relatively broad period of peak activity, meaning that good rates can be seen for a few days either side of their peak.
In 2018, the peak of the Perseid shower coincides with the New Moon, and so is totally unaffected by moonlight, which makes this an ideal year to observe the shower.
The further north you are, the earlier the shower’s radiant will be visible. But reasonable rates can typically be seen any time after about 10pm, local time. The later in the night you observe, the better the rates will be, as the radiant climbs higher into the sky.
It is not uncommon for enthusiastic observers to watch the shower until dawn on the night of maximum, seeing several hundred meteors in a single night.
Active: October 6-10
Maximum: October 9, 12:10am UT = 1:10am BST (UK) = 2:10am CEST (Europe)
The Draconids are a fascinating meteor shower, although in most years, somewhat underwhelming. Unlike the previous showers, the Draconids are a relatively young meteor shower that can vary dramatically from one year to the next.
That comet was the first to be visited by a spacecraft, and has frequent close encounters with Jupiter, which continually nudges its orbit around. These encounters also perturb the meteor stream the comet is laying down, sometimes enhancing rates at Earth and sometimes diminishing them.
In the early 20th century, it was realised that Comet Giacobini-Zinner’s orbit comes close enough to Earth that we might be able to see meteors as we plough through the debris it leaves behind.
This led to the first predictions of Draconid activity. Sure enough, in 1920, the great meteor observer W. F. Denning confirmed the existence of the shower, with a mere five meteors observed between October 6 and October 9.
In 1933 and 1946, the Draconids produced two of the greatest meteor displays of the 20th century – great storms, with peak rates of several thousand meteors per hour. In those years, Earth crossed the comet’s orbit just a month or two after the comet passed through perihelion (closest approach to the Sun), and Earth ploughed through dense material in the comet’s wake.
After 1946, the Draconids went quiet, all but vanishing from our skies. Jupiter had swung the comet onto a less favourable orbit. Only a few Draconids were seen in 1972, then again in 1985 and 1998.
The late 1990s saw a renaissance in our ability to predict and understand meteor showers, born of enhanced activity exhibited by the Leonid meteor shower. Using the techniques developed to study the Leonids, astronomers predicted enhanced activity from the Draconids in 2011, and the predicted outburst duly occurred, with rates of around 300 meteors per hour being observed.
This year comet Giacobini-Zinner once again passes through perihelion and swings close to Earth’s orbit. The chances are good that the shower will be active – albeit unlikely to produce a spectacular storm.
Modelling suggests that rates of 20 to 50 faint meteors per hour might be seen around 12:14am UT on October 9. Other models suggest that rates will peak about 45 minutes earlier, with lower rates of 15 to 20.
The Draconid radiant is circumpolar (that is, it never sets) for locations north of 44°N, and is highest in the sky before midnight. This year, the Moon is new at the time of the forecast peak, which is ideally timed for observers in Europe.
If skies are clear that evening, it is well worth heading out at around 11:30pm BST on October 8 (12:30am CEST on October 9) and spending a couple of hours staring north, just in case the Draconids put on another spectacular show.
Active: September 10 – December 10
Maxima: October 10 (Southern Taurids); November 12 (Northern Taurids)
Of all the year’s meteor showers, the one that dumps the greatest amount of dust into Earth’s atmosphere are the Taurids. The inner Solar system contains a vast swathe of debris known as the Taurid stream. It is so spread out that Earth spends a quarter of the year passing through it.
In June, that debris spawns the Daytime Taurid meteor shower, which (as the name suggests) occurs during daylight hours, and is only really known thanks to radio observations.
After leaving the stream for a little while, Earth penetrates it again at the start of September, and activity continues right through until December. Hourly rates fluctuate up and down, with several distinct peaks and troughs through October and November.
The Taurid stream is complex – with at least two main components, known as the northern and southern branches. Typically, the Southern Taurids are active a little earlier in the year and reach their peak about a month before the northern branch.
The Taurids are slow meteors and feature plenty of bright fireballs. So even though their rates are low, they are well worth looking out for, particularly when other showers are also active, such as the Draconids, the Orionids and the Leonids.
Put together, these showers make the northern autumn or the southern spring a great time to get out and look for natural fireworks.
Twice a year, Earth runs through the stream of debris littered around the orbit of Comet 1P/Halley. Throughout the month of October this gives rise to the Orionid meteor shower.
The Orionids are a fairly reliable meteor shower with a long, broad maximum. Typically, peak rates can last for almost a week, centred on the nominal maximum date. Throughout that week, Orionid rates can fluctuate markedly, leading to a number of distinct maxima and minima.
Orionid meteors are fast – much faster than the Taurids that are active at the same of year. Like the Taurids, they are often bright, the result of the high speed at which the meteoroids hit Earth’s atmosphere.
The Orionid radiant rises in the late evening and is only really high enough in the sky for reasonable rates to be seen after midnight. As a result, the best rates are usually observed in the hours before dawn.
This works well this year, as the Moon will be in its waxing gibbous phase, setting some time after midnight and leaving the sky dark, allowing us to watch for pieces of the most famous comet of them all.
Active: December 4-17
Maximum: December 14, 12:30pm UT = Australia: December 14, 8pm AWST (WA) = 10:30pm (QLD) = 11:30pm AEST (NSW/ACT/Vic/Tas) = United States: December 14, 7:30am (EST) = 5:30am (PST) = 2:30am (Hawaii)
As the year comes to a close, we reach the most reliable and spectacular of the annual meteor showers – the Geminids. Unlike the Perseids and the Lyrids, which have graced our skies for thousands of years, the Geminids are a relatively new phenomenon.
They were first observed just 150 years ago, and through the first part of the 20th century were a relatively minor shower. But since then rates have improved decade-on-decade, to the point where they are now the best of the annual showers, bar none.
The reason for their rapid evolution is that their orbit (and that of their parent body, the asteroid Phaethon) is shifting rapidly over time, precessing around the Sun (wobbling like a slow spinning top). As it does so, the centre of Phaethon’s orbit, and the centre of the Geminid stream, are moving ever closer to Earth.
For northern locations, the radiant rises shortly after sunset, and good rates can be seen from mid-evening onwards. For observers in the southern hemisphere, the radiant rises later, so good rates are delayed until later at night (as detailed in our 2015 report on the shower).
Although the time of maximum this year seems to favour observers in the Americas and Australia, peak rates from the Geminids usually last around 24 hours, and so good rates should be visible around the globe.
This year the maximum falls a day before the Moon reaches first quarter so the best rates are visible (after midnight, local time) when the Moon will have set and moonlight will not interfere.
Given that rates from the Geminids continue to climb, the estimated ZHR of 120 is likely to be somewhat conservative. Rates in excess of 130, and even as high as 200 per hour, have been seen in recent years.
Geminids are medium-speed meteors and are often bright. The individual meteors also seem to last just that bit longer than other showers, a fact likely related to their parent object’s rocky nature.
Wherever you are on the planet, the Geminids are a fantastic way to bring the year to an end, and we will hopefully be treated to a magnificent display this year.
Two new exoplanets have been discovered thanks to NASA’s collaboration with Google’s artificial intelligence (AI). One of those in today’s announcement is an eighth planet – Kepler-90i – found orbiting the Sun-like star Kepler-90. This makes it the first system discovered with an equal number of planets to our own Solar system.
A mere road trip away, at 2,545 light-years from Earth, Kepler-90i orbits its host star every 14.4 Earth days, with a sizzling surface temperature similar to Venus of 426°C.
The new exoplanets are added to the growing list of known worlds found orbiting other stars.
This new Solar system rival provides evidence that a similar process occurred within Kepler-90 that formed our very own planetary neighbourhood: small terrestrial worlds close to the host star, and larger gassy planets further away. But to say the system is a twin of our own Solar system is a stretch.
The entire Kepler-90 system of eight planets would easily fit within Earth’s orbit of the Sun. All eight planets, bar Kepler-90h, would be too hostile for life, lying outside the so-called habitable zone.
Evidence also suggests that planets within the Kepler-90 system started out farther apart, much like our own Solar system. Some form of migration occurred, dragging this system inwards, producing the orbits we see in Kepler-90 today.
Google’s collaboration with NASA’s space telescope Kepler mission has now opened up new and exciting opportunities into AI helping with scientific discoveries.
So how exactly did Google’s AI discover these planets? And what sort of future discoveries can this technology provide?
Training AI for exoplanet discoveries
Traditionally, software developers program computers to perform a particular task, from playing your favourite cat video, to determining exoplanetary signals from space based telescopes such as NASA’s Kepler Mission.
These programs are executed to serve a single purpose. Using code intended for cat videos to hunt exoplanets in light curves would lead to some very interesting, yet false, results.
Googles’s AI is programmed rather differently, using machine learning. In machine learning, AI is trained through artificial neural networks – somewhat replicating our brain’s biological neural networks – to perform tasks like reading this article. It then learns from its mistakes, becoming more efficient at its particular task.
Google’s DeepMind AI, AlphaGo, was trained previously to play Go, an extremely complex yet elegant Chinese board game. Last year, AlphaGo defeated Lee Sedol, the world’s best Go player, by four games to one. It simply trained itself by watching thousands of previously played games, then competing against itself.
In our exoplantary case, AI was trained to identify transiting exoplanets, sifting through 15,000 signals from the Kepler exoplanet catalogue. It learned what was and wasn’t a signal caused by an exoplanet eclipsing its host star. These 15,000 signals were previously vetted by NASA scientists prior to the AI’s training, guiding it to a 96% efficiency of detecting known exoplanets.
Researchers then directed their AI network to search in multiplanetary systems for weaker signals. This research culminated in today’s announcement of both Kepler-90i and another Earth-sized exoplanet, Kepler-80g, in a separate planetary system.
Hunting for more exoplanets using AI
Google’s AI has analysed only 10% of the 150,000 stars NASA’s Kepler Mission has been eyeing off across the Milky Way galaxy.
There’s potential then for sifting through Kepler’s entire catalogue and finding other exoplanetary worlds that have either been skimmed by scientist or haven’t been checked yet, due to Kepler’s rich data set. And that’s exactly what Google’s researchers are planning to do.
Machine learning neural networks have been assisting astronomers for a few years now. But the potential for AI to assist in exoplanetary discoveries will only increase within the next decade.
The Kepler mission has been running since 2009, with observations slowly coming to an end. Within the next 12 months, all of its on-board fuel will be fully depleted, ending what has been, one of the greatest scientific endeavours in modern times.
Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS) will be launching this coming March.
TESS is predicted to find 20,000 exoplanet candidates during its two-year mission. To put that into perspective, in the past 25 years, we’ve managed to discover just over 3,500.
This unprecedented inundation of exoplanetary data needs to either be confirmed by other transiting observations or other methods such as ground-based radial velocity measurements.
There just isn’t enough people-power to sift through all of this data. That’s why these machine learning networks are needed, so they can aid astronomers in sifting through big data sets, ultimately assisting in more exoplanetary discoveries. Which begs the question, who exactly gets credit for such a discovery?
The discovery of a planet with a highly elliptical orbit around an ancient star could help us understand more about how planetary systems form and evolve over time.
The new planet, HD76920b, is four times the mass of Jupiter, and can be found some 587 light years away in the southern constellation Volans, the Flying Fish. At its closest it skims the surface of its host star, HD76920. At its furthest, it orbits almost twice as far from its star as Earth does from the Sun.
Details of the planet and its discovery are published today. So how does this fit into the planet formation narrative, and are planets like it common in the cosmos?
The eight planets move in almost circular orbits, close to the same plane. The bulk of the debris also lies close to that plane, although on orbits that are somewhat more eccentric and inclined.
How did this system form? The idea was that it coalesced from a disk of material surrounding the embyronic Sun. The colder outer reaches were rich in ices, while the hotter inner regions contained just dust and gas.
Over millions of years, the tiny particles of dust and ice collided with one another, slowly building ever larger objects. In the icy depths of space, the giant planets grew rapidly. In the hot, rocky interior, growth was slower.
Eventually, the Sun blew away the gas and dust leaving a (relatively) orderly system – roughly co-planar planets, moving on near-circular orbits.
The exoplanet era
The first exoplanets, discovered in the 1990s, shattered this simple model of planet formation. We quickly learned that they are far more diverse than we could have possibly imagined.
As a result of these discoveries, astronomers developed two competing models for planet formation.
The first is core accretion, where planets form gradually, through collisions between grains of dust and ice. The theory has grown out of our old models of Solar system formation.
The competing theory is dynamical instability. Once again, the story begins with a disk of material around a youthful star. But that disk is more massive, and becomes unstable under its own self-gravity, causing clumps to grow. These clumps rapidly form planets, in thousands of years.
Both models can explain some, but not all, of the newly discovered planets. Depending on the initial conditions around the star, it seems that both processes can occur.
Each theory offers potential to explain eccentric worlds in somewhat different ways.
How do you get an eccentric planet?
In the dynamical instability model you can easily get several clumps forming and interacting, slinging one another around until their orbits are both tilted and eccentric.
Under the core accretion model things are a bit harder, as this method naturally creates co-planar, ordered planetary systems. But over time those systems can become unstable.
One possible outcome is for one planet to eject the others through a series of chaotic encounters. That would naturally leave it as a solitary body, following a highly elongated orbit.
But there is another option. Many stars in our galaxy are binary – they have stellar companions. The interactions between a planet and its host star’s sibling could readily stir it up and eventually eject it, or place it on an extreme orbit.
An eccentric planet
This brings us to our newly discovered world, HD76920b. A handful of similarly eccentric worlds have been found before, but HD76920b is unique. It orbits an ancient star, more than two billion years older than the Sun.
The orbit HD76920b is following is not tenable in the long-term. As it swings close to its host star, it will experience dramatic tides.
A gaseous planet, HD76920b will change shape as it swings past its star, stretched by its enormous gravity. Those tides will be far greater than any we experience on Earth.
That tidal interaction will act over time to circularise the planet’s orbit. The point of closest approach to the star will remain unchanged, but the most distant point will gradually be dragged closer in, driving the orbit towards circularity.
All of this suggests that HD76920b cannot have occupied its current orbit since its birth. If that were the case, the orbit would have circularised aeons ago.
Perhaps what we’re seeing is evidence of a planetary system gone rogue. A system that once contained several planets on circular (or near circular) orbits.
Over time, those planets nudged one another around, eventually hitting a chaotic architecture as their star evolved. The result – chaos – with most planets scattered and flung to the depths of space leaving just one – HD76920b.
The truth is, we just don’t know – yet. As is always the case in astronomy, more observations are needed to truly understand the life story of this peculiar planet.
One thing we do know is the story is coming to a fiery end. In the next few million years, the star will swell, devouring its final planet. Then, HD76920b will be no more.