6 mins read 26 Jun 2020

Aussie astronomers help detect two nearby ‘super-earths’

Australian astronomers, working as part of an international collaboration, have detected two ‘Super-Earths’ orbiting a nearby star.

Artist illustration of the Gliese 887 system with planets 887b and 887c. Credit: Mark Garlick.

Two new ‘Super-Earths’ have been detected orbiting a quiet, yet bright Red Dwarf star, providing scientists with the opportunity to study a nearby exoplanet system that could potentially harbour liquid water.  

The newly discovered planets, called Gliese 887b and Gliese 887c, could orbit at just the ‘right’ distance from their host star – a key parameter that drives the ‘habitable’ conditions that could potentially allow life (as we know it) to develop. The announcement of the multi-planetary system was published today in the journal Science.

The discovery comes right off the back of further exoplanet announcements made earlier this week – when the University of Southern Queensland scientists announced the confirmation of a young Neptune-sized planet orbiting the Red Dwarf star AU Microscopii.

“The exciting thing about these planets are that they orbit a star so close to the Sun, and so very bright,” UNSW-based planet hunter Prof Chris Tinney, who is a co-author on the paper, said.

“We now know of thousands of planets of Super-Earth-mass, or smaller."

"But most of those planets orbit distant and faint stars. Planets orbiting nearby stars are key for searches with future telescopes for both exoplanetary atmospheres, and eventually evidence for life.”

Astronomers from the University of Southern Queensland, UNSW Sydney and Macquarie University were all part of the international team known as “Red Dots”. The team used data from the Anglo-Australian Planet Search, using the 3.9m Anglo-Australian Telescope near Coonabarabran and combined this with data obtained from other large telescopes sitting high in the mountains atop Chile and Hawaii.

Star System: Gliese 887

Artist illustration of the Gliese 887 system. Credit: Getty.

The host star the exoplanets have been detected orbiting is known as Gliese 887, and is one of the closest star systems to Earth. Being a Red Dwarf star, it is much smaller than our Sun (roughly half the Sun’s radius), making it not as bright – and thus, harder for us to see with our naked eyes.

However, in terms of Red Dwarfs – Gliese 887 is bright enough (and close enough) to study the finer details of the object – like how it moves amongst the background more distant stars (a process known as ‘measuring parallax’) and how fast it is zipping around the galaxy (i.e. measuring its ‘proper motion’).

Like most Red Dwarfs, Gliese 887 is a cooler star with an atmospheric temperature around 3,600 Kelvin – this lower heat (with respect to other stars like our Sun) due to the more relaxed thermonuclear conversion of Hydrogen to Helium deep within its core. Red Dwarfs are known for this slow conversion – and contain enough fuel reserve to outlive the current present age of the Universe!

Fellow University of Southern Queensland researcher and co-author, Professor Rob Wittenmyer said the star Gliese 887 was also very stable.

“The brightness of Gliese 887 is really very constant,” he said.

“This will make it much easier to detect the atmospheres of these super-Earths using coming space-based facilities like the James Webb Space Telescope – the successor to the Hubble Telescope, which is due to be launched soon.”

The 'Goldilocks Zone'

Illustration of the ‘Goldilocks Zone’ relative to the type of star (y-axis) vs. the amount of starlight the planet will receive (analogous to its distance). Red dwarf stars are at the bottom. Credit: C. Harman.

The energy output of a star is dependent on its mass, with a simple correlation that higher mass stars have more gravity, creating more thermonuclear fusion in their core. The result of this is higher mass stars burn brighter but also consume their fuel reserves quicker.

Opposing this are lower mass stars – who have less mass and gravity, less fuel conversion, and less energy output. Our Sun for example resides somewhere between a low-mass Red Dwarf star, and the mind-bogglingly enormous Blue Supergiants.

An interesting result of the energy output of stars (derived by mass and gravity) is that there is a ‘zone’ – a lower and higher distance range – away from the star where liquid water can exist as a solid, liquid, and a gas. The Earth is in this region, known as the ‘Goldilocks Zone’ – where it is not too close nor too far from the Sun, for liquid water, ice, and snow and water vapour to exist.

The importance of the Goldilocks Zone is that it allows any planets in this region to have the right conditions for water to exist in these states – a key ingredient for life as we know it to evolve. Without water in these forms, life doesn’t seem to exist anywhere else in the Universe (as far as we can tell).

The exciting news about Gliese 887b and Gliese 887c is that they could potentially exist within the Goldilocks Zone of Gliese 887. In fact, the team estimates the surface temperature of the outer planet (Gliese 887c) to be around 70-degrees C – and whilst that might seem hot to our everyday experiences – scientists have found lifeforms on Earth, known as ‘Extremophiles’ which exist and thrive near these temperatures.

A more humble star

Artist impression of a triple-planet system orbiting a Red Dwarf star. Credit: ESO/M. Kornmesser.

Red Dwarf stars are known to be rather temperamental – exhibiting strong magnetic fields, and giant flares that lash and strip atmospheres – which would make the development of life almost impossible.

However, the team discovered a few more interesting facts about Gliese 887 that turn out to be good news for both the newly discovered planets, and for astronomers studying them.

“The red dwarf has little in the way of starspots of other magnetic activity,” University of Southern Queensland astrophysicist and co-author Professor Brad Carter said.

“If Gliese 887 were as active as many other Red Dwarf stars its stellar winds and activity would likely significantly erode any planetary atmospheres."

"So given the star is relatively inactive the newly discovered planets could be expected to retain their atmospheres”.

Expanding Exoplanet Numbers

Artist illustration of the TESS Spacecraft in orbit. Credit: NASA

In recent years the number of exoplanets has drastically increased, mostly due to new, space-based observatories finding them. Orbital observatories have an advantage of not ‘seeing’ through Earth’s atmosphere. As a result, more sensitive detections for potential candidates can be made through a number of different methods.

Space telescopes like TESS and KEPLER have revolutionised our understanding of planets outside our system – with scientists now agreeing that most star systems likely harbour one or more planets.

Detection methods include observing a ‘transit’ – where a planet moves in front of the host star (with reference to the telescope’s line of sight) and causes a slight dip in the total starlight output. Another method is observing the star itself and looking for little ‘wobbles’ that the star exhibits – like it is shifting back and forth – which indicates that a planet might be pulling on it as it orbits.

Both these methods have outlined some excellent results for planetary candidates – which are then followed up for detection confirmation by bigger, more powerful ground-based observatories.

“In the era of space-based exoplanet-hunting telescopes like NASA’s Kepler and TESS, this result shows that astronomy from the ground continues to play a crucial role in our understanding of planets in our local neighbourhood,” Dr Simon O’Toole, a co-author on the study from Macquarie University, said.

The paper is available through the journal, Science.