18 mins read 25 Jun 2020
USQ plays role in finding cosmic exoplanet laboratory
Australian researchers have helped NASA to discover a new planet orbiting a very young star which can provide answers about the evolution of planetary systems. Vanessa had a chat with Prof. Jonti Horner about this exciting discovery.
In 1988, the world was watching movies like Cocktail and Crocodile Dundee, listening to music like Faith by George Michael and Australian’s celebrated the Bi-Centennial year of Cook’s landing. It was the year that Home and Away started, the Winter Games were held in Calgary, and the Soviet Shuttle Buran took its maiden and only flight.
In the same year, some 32 light-years away - a small, relatively cool red star emitted light that would travel in Earth’s direction, arriving at an observatory in Southern Queensland in 2020, revealing a hidden treasure that will reshape our understanding of planetary evolution.
In an international collaborative effort involving Australian researchers, a new planet has been discovered orbiting a very young star. The star AU Microscopii (AU Mic for short) is orbited by the planet AU Mic b, whilst also remaining surrounded by a disk of debris that is leftover from the star’s formation. This system has helped astronomers, including those from the University of Southern Queensland (USQ), draw conclusions about theories concerning the formation and evolution of planetary systems.
Australian Institutions Involved
The researchers involved in this collaborative project come from various institutions from around the world, including Australia. One such astrophysicist is Professor Jonti Horner from USQ, who was excited about the university’s contribution to the project. “We’re proud that the University of Southern Queensland is an important member of a global team hunting for exoplanets,” he said.
“It’s thrilling to be part of this discovery - to find a new planet is always amazing, but to find one in a system like this is even better. We’re seeing a really early snapshot into the formation of a new planetary system - and the way everything is lined up just right around AU Mic is an astronomer’s dream come true!”
USQ operates the exoplanet observatory MINERVA-Australis, which is located at the Mt. Kent Observatory near Toowoomba in southeast Queensland. MINERVA (Miniature Exoplanet Radial Velocity Array) is primarily used to support observations from NASA’s TESS (Transiting Exoplanet Survey Satellite) spacecraft - currently in highly stable orbit that maximises sky coverage with unobstructed views of the cosmos, yet remaining in a low-radiation, thermally benign environment.
TESS was launched on 18 April 2018 as a NASA Astrophysics Explorer mission led and operated by MIT (Massachusetts Institute of Technology) and managed by NASA’s Goddard Space Flight Centre. TESS monitors large areas of the sky for 27 days at a time looking for signs of exoplanets.
The MINERVA-Australis observatory so far consists of five PlaneWave CDK700 telescopes, with a sixth telescope yet to be installed, and is funded by USQ, the Australian Research Council, and several international partners.
Those telescopes move as one, swinging from one interesting target to the next, and gathering data to look for the telltale ‘wobble’ of a star that gives away the presence of its unseen planets. To do this, light from the telescopes is fed through fibre-optic cables to a spectrograph - which breaks that light down into its component colours, revealing the dark lines that are the fingerprint of the star’s chemical composition. By studying how those lines shift with time, the team can learn a great deal about alien worlds - all as part of a global effort to learn more about planets orbiting distant stars.
“From Mount Kent in Queensland’s Darling Downs, our team of astronomers are working with institutes around the world to confirm the existence of planets and learn more about them,” commented Professor Horner.
MINERVA-Australis was one of a number of observatories worldwide that were used to confirm the presence of AU Mic b, which was originally detected by the TESS spacecraft.
The AU Microscopii System
The star AU Microscopii is a temperamental stellar infant, known for the vast flares it sometimes emits that cause its brightness to vary dramatically, and is located within the southern constellation Microscopium. The constellation itself was given the name Microscopium around the 18th century by the French astronomer Nicolas de Lacaille - who also named a further 13 of the total of all 88 modern constellations. He named it after the microscope in a departure from previous traditions of naming constellations after animals or mythology, which saw 13 of his named constellations honour instruments from the Age of Enlightenment.
The star AU Mic is a cool red dwarf which is around 23 million years old, a mere infant when compared to our Sun (a middle-aged star that just celebrated its 4.567 billionth birthday!). It’s radius is roughly 60% the size of our stellar parent and, due to its low surface area and cooler temperature, it only emits about 9% of the light that our Sun outputs. As a result, despite being relatively close by, it is much too faint to see with the unaided eye. It is so young that it has not settled into its prime - and so astronomers consider it to be a ‘pre-main sequence star’ - a star that is still in the final stages of formation.
Located approximately 32 light-years away (making it our second closest pre-main sequence star), AU Mic has been studied for decades at almost all wavelengths (from radio waves through to x-rays), with particularly bright flares being first identified in 1973. The star itself exhibits fluctuations in brightness with a periodicity of about 4.8 days, with a variance in the brightness of approximately 0.3 magnitudes.
Science Check: Stellar Life Cycles
Stars vary in nature over the course of their lifetimes. The lifespan of a star can be divided into three broad phases - the pre-main sequence, main sequence, and post-sequence periods, and due to the enormous sample population of stars in the Universe - scientists can study the variety and range of stellar objects at different points in their evolution.
Stars form from large reserves of gas and materials, triggered by gravitational disturbances which cause localised density increases (‘clumps’) to form in the vast clouds of gas and dust that make up stellar nurseries. These ‘clumps’ begin to draw in more and more material, as their gravity overcomes the overall motion of dust and gas in the cloud, and they begin to collapse under their own gravity. As they do so, these regions continue to accumulate matter, building up not only the mass of the stellar object but also providing materials for other objects - like planets - to evolve in these systems.
The pre-main-sequence stage of stars are known as Protostars. As more and more material falls inwards towards a young protostar, that material collapses into a disk around the young star. Such disks are known as ‘protoplanetary disks’, and are where young planets are born. Eventually, the young star becomes luminous enough to blow out the gas from such a disk - leaving behind a broad disk of debris (rocks, and ice) around it - known, unsurprisingly, as a ‘debris disk’.
A regular star, much like our Sun, starts the main sequence stage of its life when enough mass and gravity create high temperature and pressure conditions to trigger the nuclear fusion of hydrogen into helium in the stellar object’s core. This thermonuclear process generates energy which eventually finds its way out, radiating into free space. The sunlight and heat we feel on our faces when we are outside is the resultant photons that have travelled across space - after being generated in our Sun’s core, between ten and a hundred thousand years ago.
However, this process doesn’t take place in protostars, which are instead fuelled by gravitational contraction and the fusion of deuterium during the early stages of their evolution. These young stars have not yet initiated core hydrogen burning, and so have not yet reached the main sequence stage of their life.
Which Stars are Good For Planets?
Red dwarf stars are the most common types of star in our galaxy. They are small in size and mass and are relatively dim and cool. Some of the nearest Red Dwarf stars to Earth are truly puny - barely large enough to undergo fusion in their interiors. Being so small, they are also very cool (as stars go), with surface temperatures as low as 2,000 Kelvin - temperatures which are far exceeded by some human activities on Earth. This low temperature is attributed to the stellar object’s low mass/gravity, which in turn drives a slower thermonuclear rate - converting hydrogen protons into helium at a lazy pace. An interesting outcome of this is that this slow burn would mean that Red Dwarfs have enough fuel in their reserve stocks to outlast the present age of the Universe!
Whilst Red Dwarfs themselves are difficult to observe due to their low brightness, this inversely makes them a suitable target for planet-hunting. Their relatively small size compared with orbiting planets also makes detection easier through a number of detection methods, such as transits and detecting Doppler shifts in the radial velocity of the host star caused by orbiting planets.
AU Mic is a pre-main sequence red dwarf, providing scientists with an opportunity to study a precursor of these well-known stars at this evolutionary stage, just as AU Mic prepares to join the main sequence, and enter the prime of its life.
“One of the most fascinating things about AU Mic is that it is surrounded by a spectacular debris disk. That disk is edge-on and is material left over from (and probably still participating in) the planet formation process. The disk being edge-on was particularly exciting when it was discovered, as it suggested that any planets in the system would have the potential to transit - a prediction we can now confirm with this latest research!”
The Importance of This Discovery
The discovery of a planet orbiting AU Mic has generated excitement amongst the astronomical community because of the system’s young age. Due to the bright debris disk that has been observed around the star, as well as a newly formed planet, this system can help astronomers better understand the formation and evolution of planetary systems. AU Mic presents a relatively nearby ‘laboratory’ to study these concepts now, and for decades to come. This is key in studying young planets and stars as it is rare to see these early stages of a system in action.
“One of the big questions astronomers still face in understanding exoplanets is how giant planets can end up so close to their host stars. A number of different mechanisms have been proposed - each of which would result in dramatically different systems.”
“In AU Mic, we have the perfect laboratory - a system that is still very young, but that hosts a ‘hot Neptune’, and a debris disk - and one where we know exactly how everything is oriented. In future years, this discovery will allow us to drill deep into how exoplanets form and evolve - and there is the very real possibility that other planets might lurk in the system, still awaiting discovery.”
Science Check: Debris Disks and Planetary Systems
Pre-main sequence stars are often still surrounded by a debris disk, a rotating disk of dust and debris. Even though the gas is gone, planets can continue to form in disks like this, through the accumulation of this material via gravity and collisions (a process known as accretion). Because pre-main-sequence stars can still have this disk, scientists can study planetary evolution forming around these stars and their effect on the system.
Debris disks can also be found around mature and evolved stars (there’s even a known case of a debris disk surrounding a neutron star), and are observed across a number of wavelengths. Debris and dust from the disks absorb radiation from the central stellar object, which causes that dust and debris to heat up. The dust and debris then re-radiates this excess heat at longer wavelengths (e.g. infrared) which can be observed to define the detail within the system.
Such multi-wavelength observations have led, over the years, to the discovery of a wide variety of debris and planetary disks featuring rings, gaps, clumps, and even the detection of multiple ring systems asymmetrically tilted from each other.
The formation of a circumstellar disk is a natural part of the formation of a star - with that material spiralling in under the influence of gravity tending to collapse down into a disk around the central object. It is in that disk that planets grow, through the process of accretion described above, shrouded from their young star by the thick gas and dust of the disk.
Once the star at the centre of the disk ‘turns on’, a combination of its radiation and stellar wind rapidly clears the gas from the disk, leaving behind planets, and a vast population of ‘planetesimals’ - rocky and icy bodies ranging in size from metres to thousands of kilometres across.
It is the collisional grinding of those objects that creates the dust we observe at infrared wavelengths - and that dust is continually replenished by collisions whilst the radiation and wind flowing out from the star attempts to blow it away, into interstellar space. So, in many ways, the infrared signature of a debris disk is a sign of planetesimals grinding themselves to dust - a natural byproduct of the planet formation process.
The last three decades have revealed an extra part of this planet formation story. Observations of the Solar system’s small bodies (particularly a group of asteroids called the Jovian Trojans, and the dwarf planet Pluto and its siblings the Plutinos) have revealed that the giant planets Jupiter, Saturn, Uranus, and Neptune must have migrated significant distances from where they formed to where they are today.
At the same time, the discovery of exoplanets orbiting perilously close to their host stars has shown that such migration is a common part of the life of planetary systems. Such planets (known as ‘hot Jupiters’, and ‘hot Neptunes’) can not have formed on their current orbits, and must instead have formed farther from their host stars, before spiraling inwards to end up on orbits that skim the surfaces of those stars.
The migration of giant planets dramatically affects the protoplanetary and debris disks in their home systems. They carve gaps in the dust and debris as they move, whilst also often pushing small objects around, causing them to ‘pile-up’ in certain regions (just like the Jovian Trojans in the Solar system). The result? Young planets create clear gaps and rings in the disks we observe around distant stars!
A New Planet!
Because of its bright debris disk, AU Mic was always going to be a promising target for exoplanet astronomers to study. Given that planets and disks form together, the fact that AU Mic’s debris disk is edge on to our line of sight suggests that any planets around the star should also be edge on (as the planets and disk would have formed together). In other words, if there are planets there, the TESS spacecraft should have a good chance of eventually seeing one of them transit the star, causing it to briefly dim, and then brighten again.
And with today’s announcement, that prediction has been spectacularly borne out. Using data from TESS, and observations from telescopes around the planet, astronomers have announced the discovery of AU Mic b, coexisting with the debris disk orbiting the young star AU Mic. This planet is roughly the size of Neptune, with an orbital period of 8.46 days. It orbits AU Mic at a distance of less than 10% of the distance that the Earth is from the Sun, and has a mass about 18% that of Jupiter.
AU Mic b was first observed by TESS during the first 27 days of its mission, back in 2018. When TESS observes what may be a planet candidate, researchers are then required to confirm the observation using their own systems, such as MINERVA-Australis.
TESS and MINERVA both observed AU Mic b through the transit technique - where the planet happens to cross in front of its star during a transit event (with reference to our line of sight), causing a distinct dip in the star’s brightness. If we see this change in brightness, it means that we are likely observing a planet orbiting a star.
But whilst TESS could just measure the brightness of AU Mic, MINERVA (and other telescopes around the world) could also measure the wobble of AU Mic, back and forth along our line of sight, resulting from the gravitational pull of the planet. This has allowed astronomers to measure both the mass of the planet (from the wobble) and its size (from the transit) - meaning that we already have a very good idea of the nature of this new alien world.
Observing AU Mic b hasn’t been without its challenges. AU Mic is a young star that is known to exhibit flares and variations in its brightness, driven by large starspots. This variability has made it difficult to detect the signature change in brightness that comes from a transit event. This perseverance however has paid off, as we are now able to study and learn from AU Mic b.
Despite these challenges, the new discovery has already led to a number of spin-off projects, whose results are being released to the world in a series of companion papers, uploaded to the arXiv to appear at the same time as the discovery is announced.
MINERVA-Australis has played a key role in those measurements, with the team at USQ leading a paper describing the measurement of the tilt of the planet’s orbit around its star, using a quirky method called the ‘Rossiter-McLaughlin’ technique.
The kicker of that new work? The planet’s orbit is aligned with the star’s equator, and with the disk of debris around it. Everything in essentially the same plane - just like at home in the Solar system.
Observing AU Mic b coexisting with a debris disk gives scientists the opportunity to test current predictions of planetary models. For instance, these observations of the AU Mic system already confirm that gaseous planet formation and primordial disk migration takes place in less than 20 million years.
The fact that the planet and disk orbit in the same plane suggests that the planet migrated as a result of interactions with the disk - rather than having been scattered by other planets (which could kick it onto a much more tilted path). It is hypothesised that the planet may well have companions, orbiting at a greater distance from AU Mic.
Beyond them, those planets have left a ring of debris at around 35 au (astronomical units) from the star - a feature remarkably reminiscent of the Solar system’s Edgeworth-Kuiper belt, beyond the orbit of Neptune, having cleared the inner disk of gas and dust. It is hoped that the AU Mic system will provide more answers like these in the decades to come.
Despite the discovery of exoplanets becoming common, the AU Mic system is a novel reservoir of knowledge about young planetary systems. This discovery also emphasises the importance of international collaboration between scientists in order to continue expanding our knowledge of the universe.
In the year 2020, people are still watching shows like Home and Away, celebrating the launch of the new SpaceX human-launch vehicle, and experiencing the detrimental effects of the COVID-19 outbreak.
Light from AU Mic, and the confirmed planet AU Mic b, which is leaving the system now won’t arrive until the year 2052. Given the huge technological advances made over the last 30 years, including the enormous expansion of exoplanetary knowledge by scientists - one can’t help but wonder what AU Mic b will look like to us by then, and what new secrets it will have revealed.
The exciting part for scientists like Jonti and the MINERVA-Australis team, is they’ve only just begun.
Read the paper in the journal Nature