Newfound exoplanet may be closest candidate for alien life
Meteorologist/Science Writer
Wednesday, November 15, 2017, 2:51 PM - Astronomers using the ESO’s unique planet-hunting instrument, HARPS, have discovered a nearby alien world which could be our best chance of finding extrasolar alien life.
Ross 128 b, located roughly 11 light years away, orbiting a small, cool red dwarf star, is now the second closest "temperate" exoplanet that astronomers have found. The closest is Proxima Centauri b, which orbits the tiny red dwarf star just next door to us, 4.3 light years away.
At roughly 35 per cent more massive than Earth, Ross 128 b is estimated to be from 0.8 to 1.5 times the size of Earth, and it orbits its star once every 9.9 Earth days.
Ross 128 b's orbit around its star, compared to Mercury's orbit around the Sun. Proxima Centauri b has a similar orbit around its star. Credit: Scott Sutherland
Both Proxima Centauri b and Ross 128 b orbit inside their host star's "Goldilocks Zone" or "habitable zone", the band of space around the star where temperatures are just right - not too hot or too cold - for a planet to have liquid water on its surface. This makes them prime candidates for planets beyond our solar system that could host alien life.
Although, as of this discovery, newfound Ross 128 b now counts as the second closest "Goldlocks Zone" exoplanet to us, it may have a better chance of actually hosting life, due to its star.
The stars Proxima Centauri and Ross 128 are both red dwarfs, and thus are much smaller and much cooler than our Sun. That means for a planet to orbit inside the temperate, habitable zone of a red dwarf, it must be much closer to the star than Earth is to the Sun. In fact, both Proxima Centauri b and Ross 128 b orbit their stars roughly 20 times closer than Earth orbits the Sun. Proxima Centauri b orbits its star in just over 11 days, while Ross 128 b's orbit is just under 10 days.
A major problem that arises for planets orbiting so close to red dwarf stars is that, red dwarfs tend to blast out numerous and powerful solar flares, which would bathe any potentially life-bearing planet with deadly, high energy UV radiation and X-rays. This could easily turn an Earth-like planet into something more Mars-like - stripped of most of its atmosphere and apparently lifeless. Proxima Centauri is one of these active dwarf stars, so there would have to be some other factors in place (such as having a strong planetary magnetic field) to keep Proxima Centauri b hospitable to life as we know it.
An active red dwarf star, blasting out powerful stellar flares and throwing off immense stellar eruptions, bombarding its close-in planet. Credit: David A. Aguilar (Harvard-Smithsonian Center for Astrophysics)
Ross 128, however, is a relatively quiet red dwarf star, which would make Ross 128 b possibly more hospitable.
"Maybe when it was younger, the planet experienced the same irradiation as Proxima b. But today it would probably not experience as much irradiation," astronomer Xavier Bonfils, who is the lead author of the study describing this discovery, told CBC News. "Even if the atmosphere evaporated a few billion years ago, volcanism could have built another atmosphere and it would remain today."
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Based on this study, which gathered observations of the Ross 128 system for over a period of 10 years, the planet does orbit somewhere inside its star's habitable zone, but it's uncertain exactly where in the habitable zone it is. Due to this uncertainty, the planet could have an equilibrium temperature of anywhere between 20oC and -60oC. For comparison, Earth's equilibrium temperature (its surface temperature if it did not have a heat-trapping atmosphere, and was only heated by the light from the Sun) would be -18oC.
Since the planet doesn't transit its star, from our perspective here on Earth, Ross 128 b was discovered using the radial-velocity method, which measures how a star's position in space "wobbles", due to an orbiting planet pulling on it (as seen to the right).
An Earth-sized planet may not be visible from 11 light years away, however the wobble of the star can be detected by watching for a doppler shift of the light it is emitting (towards the blue if it is getting closer to us, or towards the red if it is getting farther away). A star that reveals a regular, periodic shift, back and forth between blue and red, is said to "wobble", and this indicates that there is something orbiting the star, producing a center of gravity that's slightly off from the centre of the star (the red cross in the animation). A large planet, like Jupiter, would produce a large, very noticable doppler shift in a red dwarf's spectrum, but an Earth-sized planet would only cause a very tiny shift by comparison.
Bonfils, who is an astronomer at the Institut de Planétologie et d'Astrophysique de Grenoble, in France, found Ross 128 b by using the best instrument available for astronomers on the ground - the High Accuracy Radial velocity Planet Searcher (HARPS), at the European Southern Observatory's La Silla Observatory, which is located high up on a mountain plateau in the northern Atacama Desert, in Chile.
Is it Earth-like?
Astronomers have detected many planets that are known to orbit in their star's habitable zone. Some of these are even Earth-sized, like Ross 128 b.
However, knowing this does not mean that the planet is actually "Earth-like", and thus habitable to life as we know it. There are many other factors that will have to be taken into consideration before we know if Ross 128 b actually hosts life.
Where, exactly, does the planet lie within the habitable zone? Too close and it may be Venus-like. Too far and it may be Mars-like (or if it's larger, with a thicker atmosphere, perhaps a snowball planet, like the fictional world of Hoth).
Does it have an atmosphere? If so, what is it composed of? Is it a nitrogen-oxygen atmosphere like ours? Oxygen would result in an ozone layer, which would protect the surface from damaging UV radiation. Is the greenhouse gas composition of the atmosphere just right to keep the temperature on the surface tolerable? Too little and the planet freezes. Too much and it bakes.
Does it have water on its surface? In combination with the above factors, this could be a boon or bane for the world. Both Venus and Mars are thought to have had oceans in their distant past, but this did not help either world to be habitable in the long-run.
Does the planet have a protective planetary magnetic field? Some planetary scientists have found that even tidally-locked worlds (planets that have one side always facing their star) can generate magnetic fields like Earth's, produced by tidal friction in their cores as they orbit their star. This would be helpful for protecting the surface from stellar flares, and from the stripping of its atmosphere by their star's stellar wind.
The Weird! Signal
Although there are plenty of considerations to address before we can say anything about aliens on Ross 128 b, astronomers with the Arecebo Radio Observatory did detect something weird there - literally, they named it The Weird! Signal.
Earlier in 2017, Ross 128 was one of eight stars included in a survey conducted by Arecibo, led by Prof. Abel Méndez, Director of the Planetary Habitability Laboratory of the University of Puerto Rico at Arecibo. The survey listened in on these specific red dwarf stars, looking for radio emissions that would tell the astronomers about flare activity, and possibly reveal the precense of planets orbiting the stars (at the time of the suvey, they did not know about the existence of Ross 128 b).
On May 12, 2017, the telescope picked up a 10 minute-long signal from the direction of Ross 128, which Méndez and his team labled "Weird!", similar to how astronomer Jerry Ehman labelled the "Wow! Signal", picked up by Ohio State University's Big Ear radio telescope on August 15, 1977.
A weird radio signal from the direction of Ross 128 was labelled as such by Arecibo astronomers. Credit: Planetary Habitability Laboratory at University of Puerto Rico at Arecibo
Other observations, by SETI and Breatkthrough Listen, failed to pick up the same weird signal, and Arecibo's own analysis came up with three (non-alien) possibilities - stellar activity from Ross 128, local radio interference, or radio signals from Earth-orbiting satellites.
Although each of these explanation had its own problems, which prevented a definitive answer, Méndez and his team believe that the best explanation is the last of the three - radio signals from one or more geosynchronous satellites. It doesn't explain away all of the weirdness of the signal, but it is the best explanation available.
As disappointing as that may be, it certainly doesn't rule out alien life entirely. Telescopes such as James Webb Space Telescope (JWST), slated for launch in 2019, and the ESO's Extremely Large Telescope (ELT), planned to start imaging in 2024, will be able to better explore the Ross 128 system, and perhaps answer some of our questions about the planet and its potential for life.
Sources: ESO | PHL @ UPR at Arecibo | CBC News
