In 2017, an international team of astronomers announced a momentous discovery. Based on years of observations, they found that the TRAPPIST-1 system (an M-type red dwarf located 40 light-years from Earth) contained no less than seven rocky planets! Equally exciting was the fact that three of these planets were found within the star’s Habitable Zone (HZ), and that the system itself has had 8 billion years to develop the chemistry for life.
At the same time, the fact that these planets orbit tightly around a red dwarf star has given rise to doubts that these three planets could maintain an atmosphere or liquid water for very long. According to new research by an international team of astronomers, it all comes down to the composition of the debris disk that the planets formed from and whether or not comets were around to distribute water afterward.
We tend to think of our Earthly circumstances as normal. A watery, temperate world orbiting a stable yellow star. A place where life has persisted for nearly 4 billion years. It’s almost inevitable that when we think of other places where life could thrive, we use our own experience as a benchmark.
NASA’s TESS (Transiting Exoplanet Survey Satellite) has found its first Earth-sized planet located in the habitable zone of its host star. The find was confirmed with the Spitzer Space Telescope. This planet is one of only a few Earth-sized worlds ever found in a habitable zone.
In 2018, scientists announced the discovery of a extrasolar planet orbiting Barnard’s star, an M-type (red dwarf) that is just 6 light years away. Using the Radial Velocity method, the research team responsible for the discovery determined that this exoplanet (Barnard’s Star b) was at least 3.2 times as massive as Earth and experienced average surface temperatures of about -170 °C (-274 °F) – making it both a “Super-Earth” and “ice planet”.
Based on these findings, it was a foregone conclusion that Barnard b would be hostile to life as we know it. But according to new study by a team of researchers from Villanova University and the Institute of Space Studies of Catalonia (IEEC), it is possible – assuming the planet has a hot iron/nickel core and experiences enhanced geothermal activity – that this giant iceball of a planet could actually support life.
In recent years, the number of extra-solar planets discovered around nearby M-type (red dwarf stars) has grown considerably. In many cases, these confirmed planets have been “Earth-like“, meaning that they are terrestrial (aka. rocky) and comparable in size to Earth. These finds have been especially exciting since red dwarf stars are the most common in the Universe – accounting for 85% of stars in the Milky Way alone.
Unfortunately, numerous studies have been conducted of late that indicate that these planets may not have the necessary conditions to support life. The latest comes from Harvard University, where postdoctoral researcher Manasvi Lingam and Professor Abraham Loeb demonstrate that planets around M-type stars may not get enough radiation from their stars for photosynthesis to occur.
New research from the Hubble Space Telescope and the ESO’s Very Large Telescope is dampening some of the enthusiasm in the search for life. Observations by both ‘scopes suggest that the raw materials necessary for life may be rare in solar systems centered around red dwarfs.
And if the raw materials aren’t there, it may mean that many of the exoplanets we’ve found in the habitable zones of other stars just aren’t habitable after-all.
The most common type of star in the galaxy is the red dwarf star. None of these small, dim stars can be seen from Earth with the naked eye, but they can emit flares far more powerful than anything our Sun emits. Two astronomers using the Hubble space telescope saw a red dwarf star give off a powerful type of flare called a superflare. That’s bad news for any planets in these stars’ so-called habitable zones.
Red dwarfs make up about 75% of the stars in the Milky Way, so they probably host many exoplanets. In fact, scientists think most of the planets that are in habitable zones are orbiting red dwarfs. But the more astronomers observe these stars, the more they’re becoming aware of just how chaotic and energetic it can be in their neighbourhoods. That means we might have to re-think what habitable zone means.
“When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, ‘Whoa.'” – Parke Loyd, Arizona State University.
In February of 2017, the world was astounded to learn that astronomers – using data from the TRAPPIST telescope in Chile and the Spitzer Space Telescope – had identified a system of seven rocky exoplanets in the TRAPPIST-1 system. As if this wasn’t encouraging enough for exoplanet-enthusiasts, it was also indicated that three of the seven planets orbited within the stars’ circumstellar habitable zone (aka. “Goldilocks Zone”).
Since that time, this system has been the focus of considerable research and follow-up surveys to determine whether or not any of its planets could be habitable. Intrinsic to these studies has been the question whether or not the planets have liquid water on their surfaces. But according to a new study by a team of American astronomers, the TRAPPIST planets may actually have too much water to support life.
Since it’s discovery was announced in August of 2016, Proxima b has been an endless source of wonder and the target of many scientific studies. As the closest extra-solar planet to our Solar System – and a terrestrial planet that orbits within Proxima Centauri’s circumstellar habitable zone (aka. “Goldilocks Zone”) – scientists have naturally wondered whether or not this planet could be habitable.
Unfortunately, many of these studies have emphasized the challenges that life on Proxima b would likely face, not the least of which is harmful radiation from its star. According to a recent study, a team of astronomers used the ALMA Observatory to detect a large flare emanating from Proxima Centauri. This latest findings, more than anything, raises questions about how habitable its exoplanet could be.
For the sake of their study, the team used data obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between January 21st to April 25th, 2017. This data revealed that the star underwent a significant flaring event on March 24th, where it reached a peak that was 1000 times brighter than the star’s quiescent emission for a period of ten seconds.
Astronomers have known for a long time that when compared to stars like our Sun, M-type stars are variable and unstable. While they are the smallest, coolest, and dimmest stars in our Universe, they tend to flare up at a far greater rate. In this case, the flare detected by the team was ten times larger than our Sun’s brightest flares at similar wavelengths.
Along with a smaller preceding flare, the entire event lasted fewer than two minutes of the 10 hours that ALMA was observing the star between January and March of last year. While it was already known that Proxima Centauri, like all M-type stars, experiences regular flare activity, this one appeared to be a rare event. However, stars like Proxima Centauri are also known to experienced regular, although smaller, X-ray flares.
All of this adds up to a bad case for habitability. As MacGregor explained in a recent NRAO press statement:
“It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”
MacGregor and her colleagues also considered the possibility that Proxima Centauri is circled by several disks of dust. This was suggested by a previous study (also based on ALMA data) that indicated that the light output of both the star and flare together pointed towards the existence of debris belts around the star. However, after examining the ALMA data as a function of observing time, they were able to eliminate this as a possibility.
As Alycia J. Weinberger, also a researcher with the Carnegie Institution for Science and a co-author on the paper, explained:
“There is now no reason to think that there is a substantial amount of dust around Proxima Cen. Nor is there any information yet that indicates the star has a rich planetary system like ours.”
To date, studies that have looked at possible conditions on Proxima b have come to different conclusions as to whether or not it could retain an atmosphere or liquid water on its surface. While some have found room for “transient habitability” or evidence of liquid water, others have expressed doubt based on the long-term effects that radiation and flares from its star would have on a tidally-locked planet.
In the future, the deployment of next-generation instruments like the James Webb Space Telescope are expected to provide more detailed information on this system. With precise measurements of this star and its planet, the question of whether or not life can (and does) exist in this system may finally be answered.
And be sure to enjoy this animation of Proxima Centauri in motion, courtesy of NRAO outreach:
Ever since the Kepler space telescope began discovering thousands of exoplanets in our galaxy, astronomers have been eagerly awaiting the day when next-generation missions are deployed. These include the much-anticipated James Webb Space Telescope, which is scheduled to take to space in 2019, but also the many ground-based observatories that are currently being constructed.
The Transit Method (aka. Transit Photometry) consists of monitoring stars for periodic dips in brightness. These dips are caused by planets passing in front of the star (aka. transiting) relative to the observer. In the past, detecting planets around M-type stars using this method has been challenging since red dwarfs are the smallest and dimmest class of star in the known Universe and emit the majority of their light in the near-infrared band.
However, these stars have also proven to be treasure trove when it comes to rocky, Earth-like exoplanets. In recent years, rocky planets have been discovered around star’s like Proxima Centauri and Ross 128, while TRAPPIST-1 had a system of seven rocky planets. In addition, there have been studies that have indicated that potentially-habitable, rocky planets could be very common around red dwarf stars.
Unlike other facilities, the ExTrA project is well-suited to conduct surveys for planets around red dwrfs because of its location on the outskirts of the Atacama Desert in Chile. As Xavier Bonfils, the project’s lead researcher, explained:
“La Silla was selected as the home of the telescopes because of the site’s excellent atmospheric conditions. The kind of light we are observing – near-infrared – is very easily absorbed by Earth’s atmosphere, so we required the driest and darkest conditions possible. La Silla is a perfect match to our specifications.”
In addition, the ExTrA facility will rely on a novel approach that involves combining optical photometry with spectroscopic information. This consists of its three telescopes collecting light from a target star and four companion stars for comparison. This light is then fed through optical fibers into a multi-object spectrograph in order to analyze it in many different wavelengths.
This approach increases the level of achievable precision and helps mitigate the disruptive effect of Earth’s atmosphere, as well as the potential for error introduced by instruments and detectors. Beyond the goal of simply finding planets transiting in front of their red dwarf stars, the ExTrA telescopes will also study the planets it finds in order to determine their compositions and their atmospheres.
In short, it will help determine whether or not these planets could truly be habitable. As Jose-Manuel Almenara, a member of the ExTrA team, explained:
“With ExTrA, we can also address some fundamental questions about planets in our galaxy. We hope to explore how common these planets are, the behaviour of multi-planet systems, and the sorts of environments that lead to their formation,”
The potential to search for extra-solar planets around red dwarf stars is an immense opportunity for astronomers. Not only are they the most common star in the Universe, accounting for 70% of stars in our galaxy alone, they are also very long-lived. Whereas stars like our Sun have a lifespan of about 10 billion years, red dwarfs are capable of remaining in their main sequence phase for up to 10 trillion years.
For these reasons, there are those who think that M-type stars are our best bet for finding habitable planets in the long run. At the same time, there are unresolved questions about whether or not planets that orbit red dwarf stars can stay habitable for long, owing to their variability and tendency to flare up. But with ExTrA and other next-generation instruments entering into service, astronomers may be able to address these burning questions.
“With the next generation of telescopes, such as ESO’s Extremely Large Telescope, we may be able to study the atmospheres of exoplanets found by ExTra to try to assess the viability of these worlds to support life as we know it. The study of exoplanets is bringing what was once science fiction into the world of science fact.”