This artist's impression shows the temperate planet Ross 128 b, with its red dwarf parent star in the background. Photo / ESO
ROSS 128b: It's the latest best address in intergalactic real estate.
It's Earth sized.
It sits in a "Goldilocks Zone" where it's neither too hot, nor too cold, for liquid water.
The star it orbits is gentle and calm.
It's just 11 light-years away. And getting closer.
Now it's the focus of attention for the European Southern Observatory, which is set to scour scattered photons of light coming from its direction for clues as to the makeup of its atmosphere.
A team at the ESO's High Accuracy Radial Velocity Planet Searcher (HARPS) facility at the La Silla Observatory in Chile published their discovery overnight in the journal Astronomy & Astrophysics.
In the hierarchy of habitable worlds, this one seems to score high.
But ANU astrobiologist and planetary scientist Dr Charles Lineweaver says: "This is good stuff. Very good stuff. But don't go buying a ticket to take yourself there yet ..."
Earlier this year three promising worlds were identified in the TRAPPIST-1 system, about 39 light years from our own Sun. It's believed all seven planets in the red dwarf system could hold water.
Last year, a possible planet was identified in the habitable zone of Proxima b - our Sun's (soon to be ousted) nearest neighbour. But that star is somewhat cantankerous.
In 2015, excitement centred upon Kepbler 452b. This planet orbits a yellow star like our own, has a year of 385 days - but is 60 per cent bigger than Earth and 1200 light years away.
But now ROSS 128b is the address to be.
And it's soon to become next door: Astronomers calculate it will become our closest neighbour in just 79,000 years as its solar system is rapidly closing on our own.
"I wonder if that will give us enough time to figure out interstellar transport," quips Lineweaver, "If we stay alive long enough, that is."
By being so close, it may already possible to take a peek into its backyard.
Ross 128b spins around its red dwarf star once every 9.9 days at a distance just one 20th that of Earth. But its star is much, much cooler than our own.
"Despite this proximity, Ross 128b receives only 1.38 times more irradiation than the Earth," the ESO statement says. "As a result, Ross 128b's equilibrium temperature is estimated to lie between -60 and 20°C."
But one alien aspect to ROSS 128b is that it always keeps one face permanently pointed towards its star's fiery surface. The other side is in perpetual darkness.
This changes everything.
Its weather and terrain will be utterly unlike our own. And this poses unique challenges for the evolution of life, which would likely cling to the perpetual "twilight zone' between the heat of day and the chill of night.
This is one of the reasons why astronomers are as yet uncertain as to exactly how habitable the world may be.
"While the scientists involved in this discovery consider Ross 128b to be a temperate planet, uncertainty remains as to whether the planet lies inside, outside, or on the cusp of the habitable zone, where liquid water may exist on a planet's surface," they say.
But Lineweaver is dubious: he says he's not much of a believer in the black-and-white nature of what is called a habitable zone.
Lineweaver says the public is split on the search for "Earth 2.0".
"Some say we've got to find a second Earth because we're quickly screwing this one up," he says.
Ultimately, he says, we just don't yet know what "habitable" means anyway.
"Life is something that seems to need liquid water. But it probably also needs other things we don't know about," Lineweaver says.
"We know so little about habitability, it's difficult to say what it is. What does it take to get life started? We just don't know.
"If M-class red dwarfs are good host stars for life, why aren't we around one?"
Red dwarf stars are the most common stars in the known universe.
But they're also among the coolest and smallest - making them ideal to examine for the telltale flickers and wobbles that give away the presence of orbiting planets.
Study lead author Xavier Bonfils (Institut de Planétologie et d'Astrophysique de Grenoble - University Grenoble-Alpes/CNRS, Grenoble, France) says their HARPS telescope program was nicknamed "The shortcut to happiness" as it is easier to detect small cool siblings of Earth around these stars, than around stars more similar to the Sun.
But most red dwarf stars are unstable.
"Many red dwarf stars, including Proxima Centauri, are subject to flares that occasionally bathe their orbiting planets in deadly ultraviolet and X-ray radiation," the statement reads. "However, it seems that Ross 128 is a much quieter star, and so its planets may be the closest known comfortable abode for possible life."
But Lineweaver says that while Ross 128 is stable now, that wasn't the case in its youth.
M-class stars (red dwarfs) are very luminous when born, decreasing rapidly over their first billion years by a factor of 100.
"What this means is any close planets were being blasted for their first billion years - which has huge implications as to whether or not any water survived on their surface," he says.
"This could completely eliminate the possibility of life developing later on. Not much thought has gone into that yet."
Red dwarf stars are getting all the attention because they're easier to analyse via current techniques: Their planets orbit quickly - mostly a matter of days.
Using similar techniques to detect a planet similar to Earth orbiting a yellow sun could take up to a century, he says.
"It's not that planets don't exist around stars like our Sun. We're just not in a position to say that for certain. But we are in a position today that, statistically, we're pretty sure they exist."
"Astronomers are now detecting more and more temperate exoplanets, and the next stage will be to study their atmospheres, composition and chemistry in more detail," the ESO researchers say.
ROSS 128b could offer an ideal opportunity to do just that.
"Biomarkers" such as oxygen in an exoplanet's atmosphere leave "imprints" on the light which passes through, or reflects off, an atmosphere. Finding these will be a "huge next step" in our understanding of just how habitable the universe may be, the ESO says.
But our current technology only allows us to extract such data from a limited selection of stars.
The ESO's Extremely Large Telescope (ELT) is in a prime position to do just that with the likes of Ross 128b.
It's undergoing an upgrade to further refine its observations of Earth-sized exoplanets.
"In particular, NIRPS, the infra-red arm of HARPS, will boost our efficiency in observing red dwarfs, which emit most of their radiation in the infra-red. And then, the ELT will provide the opportunity to observe and characterise a large fraction of these planets," Bonfils says.
The process of extracting these "fingerprints" is time consuming, says Lineweaver, with enough photons needing to be captured to map out a comprehensive spectrum defining.
"Even then we won't have an unambiguous 'hey this is it, this is life' - because we don't know what that is," he says.
But the race is on to figure that out.
"The precision of these instruments is incredible, the amount of effort is incredible," Lineweaver says.
But when it comes to the sensitivity to separate the light of distant stars into detailed "absorption line" biomarkers - we're not there yet.
"That's the task of the next generation of instruments that are being thought about now," he says.
"This is a whole new science that is only just beginning. A whole bunch of scientists are piling into the field, which is why we're making so much progress."
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