A concept for a multi-generation ship being designed by the TU Delft Starship Team (DSTART), with support from the ESA. Credit and Copyright: Nils Faber & Angelo Vermeulen
Humanity has long dreamed about sending humans to other planets, even before crewed spaceflight became a reality. And with the discovery of thousands of exoplanets in recent decades, particularly those that orbit within neighboring star systems (like Proxima b), that dream seems closer than ever to becoming a reality. But of course, a lot of technical challenges need to be overcome before we can hope to mount such a mission.
In addition, a lot of questions need to be answered. For example, what kind of ship should we send to Proxima b or other nearby exoplanets? And how many people would we need to place aboard that ship? The latter question was the subject of a recent paper written by a team of French researchers who calculated the minimal number of people that would be needed in order to ensure that a healthy multi-generational crew could make the journey to Proxima b.
The Alpha Centauri system, shown in the optical and X-ray wavelenghts (using Chandra data). Credit: chandra.harvard.edu
At distance of just 4.367 light years, the triple star system of Alpha Centauri (Alpha Centauri A+B and Proxima Centauri) is the closest star system to our own. In 2016, researchers from the European Southern Observatory announced the discovery of Proxima b, a rocky planet located within the star’s habitable zone and the closest exoplanet to our Solar System. However, whether or not Alpha Centauri has any potentially habitable planets remains a mystery.
Between 2012 and 2015, three possible candidates were announced in this system, but follow-up studies cast doubt on their existence. Looking to resolve this mystery, Tom Ayres – a senior research associate and Fellow at the University of Colorado Boulder’s Center for Astrophysics and Space Astronomy – conducted a study of Alpha Centauri based on over a decade’s worth of observations, with encouraging results!
The results of this study were presented at the 232rd meeting of the American Astronomical Society, which took place in Denver, Colorado, from June 3rd to June 7th. The study was based on ten years worth of monitoring of Alpha Centauri, which was provided the Chandra X-ray Observatory. This data indicated that any planets that orbit Alpha Centauri A and B are not likely to be bombarded by large amounts of X-ray radiation.
The two brightest stars of the Centaurus constellation – (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker
This is good news as far as Alpha Centauri’s potential habitability goes since X-rays and related Space Weather effects are harmful to unprotected life. Not only can high doses of radiation be lethal to living creatures, they can also strip away planetary atmospheres. According to data provided by the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter, this is precisely what happened to Mars between 4.2 and 3.7 billion years ago.
As Tom Ayres explained in a recent Chandra press release:
“Because it is relatively close, the Alpha Centauri system is seen by many as the best candidate to explore for signs of life. The question is, will we find planets in an environment conducive to life as we know it?”
The stars in the Alpha Centauri system (A and B) are quite similar to our Sun and orbit relatively close to each other. Alpha Centauri A, a G2 V (yellow dwarf) star, is the most Sun-like of the two, being 1.1 times the mass and 1.519 times the luminosity of the Sun. Alpha Centauri B is somewhat smaller and cooler, at 0.907 times the Sun’s mass and 0.445 times its visual luminosity.
As such, the odds that the system could support an Earth-like planet are pretty good, especially around Alpha Centauri A. According to the Chandra data, the prospects for life (based on X-ray bombardment) are actually better for any planet orbiting Alpha Centauri A than for the Sun, and Alpha Centauri B is only slightly worse. This is certainly good news for those who are hoping that a potentially habitable exoplanet is found in close proximity to the Solar System.
The respective habitable zones around Alpha Centauri A and B. Credit: Planetary Habitability Laboratory
When the existence of Proxima b was first announced, there was naturally much excitement. Not only did this planet orbit within it’s star’s habitable zone, but it was the closest known exoplanet to Earth. Subsequent studies, however, revealed that Proxima Centauri is variable and unstable by nature, which makes it unlikely that Proxima b could maintain an atmosphere or life on its surface. As Ayers explained:
“This is very good news for Alpha Cen AB in terms of the ability of possible life on any of their planets to survive radiation bouts from the stars. Chandra shows us that life should have a fighting chance on planets around either of these stars.”
Meanwhile, astronomers continue to search for exoplanets around Alpha Centauri A and B, but without success. The problem with this system is the orbit of the pair, which has drawn the two bright stars close together in the sky over the past decade. To help determine if Alpha Centauri was hospitable to life, astronomers began conducting a long-term observation campaign with Chandra in 2005.
As the only X-ray observatory capable of resolving Alpha Centauri A and B during its current close orbital approach, Chandra observed these two main stars every six months for the past thirteen years. These long-term measurements captured a full cycle of increases and decreases in X-ray activity, in much the same way that the Sun has an 11-year sunspot cycle.
What these observations showed was that any planet orbiting within the habitable zone of A would receive (on average) a lower dose of X-rays compared to similar planets around the Sun. For planets orbiting withing the habitable zone of B, the X-ray dose they received would be about five times higher. Meanwhile, planets orbiting within Proxima Centauri’s habitable zone would get an average of 500 times more X-rays, and 50,000 times more during a big flare.
In addition to providing encouraging hints about Alpha Centauri’s possible habitability, the X-ray observations provided by Chandra could also go a long way towards informing astronomers about our Sun’s X-ray activity. Understanding this is key to learning more about space weather and the threat they can pose to human infrastructure, as well as other technologically-advanced civilizations.
In the meantime, astronomers continue to search for exoplanets around Alpha Centauri A and B. Knowing that they have a good chance of supporting life will certainly make any future exploration of this system (like Project Starshot) all the more lucrative!
Some of the study’s results also appeared in the January issue in the Research Notes of the American Astronomical Society, titled “Alpha Centauri Beyond the Crossroads“. And be sure to enjoy this video about Alpha Centauri’s potential habitability, courtesy of the Chandra X-ray Observatory:
The new DARKNESS camera developed by an international team of researchers will allow astronomers to directly study nearby exoplanets. Credit: Stanford/SRL
The hunt for planets beyond our Solar System has led to the discovery of thousands of candidates in the past few decades. Most of these have been gas giants that range in size from being Super-Jupiters to Neptune-sized planets. However, several have also been determined to be “Earth-like” in nature, meaning that they are rocky and orbit within their stars’ respective habitable zones.
Unfortunately, determining what conditions might be like on their surfaces is difficult, since astronomers are unable to study these planets directly. Luckily, an international team led by UC Santa Barbara physicist Benjamin Mazin has developed a new instrument known as DARKNESS. This superconducting camera, which is the world’s largest and most sophisticated, will allow astronomers to detect planets around nearby stars.
The team’s study which details their instrument, titled “DARKNESS: A Microwave Kinetic Inductance Detector Integral Field Spectrograph for High-contrast Astronomy“, recently appeared in the Publications of the Astronomy Society of the Pacific. The team was led by Benjamin Mazin, the Worster Chair in Experimental Physics at UCSB, and also includes members from NASA’s Jet Propulsion Laboratory, the California Institute of Technology, the Fermi National Accelerator Laboratory, and multiple universities.
The DARKNESS instrument is the worlds most advanced camera and will enable the detection of planets around the nearest stars. Credit: UCSB
Essentially, it is extremely difficult for scientists to study exoplanets directly because of the interference caused by their stars. As Mazin explained in a recent UCSB press release, “Taking a picture of an exoplanet is extremely challenging because the star is much brighter than the planet, and the planet is very close to the star.” As such, astronomers are often unable to analyze the light being reflected off of a planet’s atmosphere to determine its composition.
These studies would help place additional constraints on whether or not a planet is potentially habitable. At present, scientists are forced to determine if a planet could support life based on its size, mass, and distance from its star. In addition, studies have been conducted that have determined whether or not water exists on a planet’s surface based on how its atmosphere loses hydrogen to space.
The DARK-speckle Near-infrared Energy-resolved Superconducting Spectrophotometer (aka. DARKNESS), the first 10,000-pixel integral field spectrograph, seeks to correct this. In conjunction with a large telescope and adaptive optics, it uses Microwave Kinetic Inductance Detectors to quickly measure the light coming from a distant star, then sends a signal back to a rubber mirror that can form into a new shape 2,000 times a second.
MKIDs allow astronomers to determine the energy and arrival time of individual photons, which is important when it comes to distinguishing a planet from scattered or refracted light. This process also eliminates read noise and dark current – the primary sources of error in other instruments – and cleans up the atmospheric distortion by suppressing the starlight.
UCSB physicist Ben Mazin, who led the development of the DARKNESS camera. Credit: Sonia Fernandez
Mazin and his colleagues have been exploring MKIDs technology for years through the Mazin Lab, which is part of the UCSB’s Department of Physics. As Mazin explained:
“This technology will lower the contrast floor so that we can detect fainter planets. We hope to approach the photon noise limit, which will give us contrast ratios close to 10-8, allowing us to see planets 100 million times fainter than the star. At those contrast levels, we can see some planets in reflected light, which opens up a whole new domain of planets to explore. The really exciting thing is that this is a technology pathfinder for the next generation of telescopes.”
DARKNESS is now operational on the 200-inch Hale Telescope at the Palomar Observatory near San Diego, California, where it is part of the PALM-3000 extreme adaptive optics system and the Stellar Double Coronagraph. During the past year and a half, the team has conducted four runs with the DARKNESS camera to test its contrast ratio and make sure it is working properly.
In May, the team will return to gather more data on nearby planets and demonstrate their progress. If all goes well, DARKNESS will become the first of many cameras designed to image planets around nearby M-type (red dwarf) stars, where many rocky planets have been discovered in recent years. The most notable example is Proxima b, which orbits the nearest star system to our own (Proxima Centauri, roughly 4.25 light years away).
The Palomar Observatory, where the DARKNESS camera is currently installed. Credit: IPTF/Palomar Observatory
“Our hope is that one day we will be able to build an instrument for the Thirty Meter Telescope planned for Mauna Kea on the island of Hawaii or La Palma,” Mazin said. “With that, we’ll be able to take pictures of planets in the habitable zones of nearby low mass stars and look for life in their atmospheres. That’s the long-term goal and this is an important step toward that.”
In addition to the study of nearby rocky planets, this technology will also allow astronomers to study pulsars in greater detail and determine the redshift of billions of galaxies, allowing for more accurate measurements of how fast the Universe is expanding. This, in turn, will allow for more detailed studies of how our Universe has evolved over time and the role played by Dark Energy.
These and other technologies, such as NASA’s proposed Starshade spacecraft and Stanford’s mDot occulter, will revolutionize exoplanet studies in the coming years. Paired with next-generation telescopes – such as the James Webb Space Telescope and the Transiting Exoplanet Survey Satellite (TESS), which recently launched – astronomers will not only be able to discover more in the way exoplanets, but will be able to characterize them like never before.
In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle
Since the beginning of the Space Age, humans have relied on chemical rockets to get into space. While this method is certainly effective, it is also very expensive and requires a considerable amount of resources. As we look to more efficient means of getting out into space, one has to wonder if similarly-advanced species on other planets (where conditions would be different) would rely on similar methods.
Harvard Professor Abraham Loeb and Michael Hippke, an independent researcher affiliated with the Sonneberg Observatory, both addressed this question in two recently–released papers. Whereas Prof. Loeb looks at the challenges extra-terrestrials would face launching rockets from Proxima b, Hippke considers whether aliens living on a Super-Earth would be able to get into space.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
For the sake of his study, Loeb considered how we humans are fortunate enough to live on a planet that is well-suited for space launches. Essentially, if a rocket is to escape from the Earth’s surface and reach space, it needs to achieve an escape velocity of 11.186 km/s (40,270 km/h; 25,020 mph). Similarly, the escape velocity needed to get away from the location of the Earth around the Sun is about 42 km/s (151,200 km/h; 93,951 mph).
As Prof. Loeb told Universe Today via email:
“Chemical propulsion requires a fuel mass that grows exponentially with terminal speed. By a fortunate coincidence the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets. But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.”
As Loeb indicates in his essay, the escape speed scales as the square root of the stellar mass over the distance from the star, which implies that the escape speed from the habitable zone scales inversely with stellar mass to the power of one quarter. For planets like Earth, orbiting within the habitable zone of a G-type (yellow dwarf) star like our Sun, this works out quite while.
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: Pale Red Dot
Unfortunately, this does not work well for terrestrial planets that orbit lower-mass M-type (red dwarf) stars. These stars are the most common type in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone. In addition, recent exoplanet surveys have discovered a plethora of rocky planets orbiting red dwarf stars systems, with some scientists venturing that they are the most likely place to find potentially-habitable rocky planets.
Using the nearest star to our own as an example (Proxima Centauri), Loeb explains how a rocket using chemical propellant would have a much harder time achieving escape velocity from a planet located within it’s habitable zone.
“The nearest star to the Sun, Proxima Centauri, is an example for a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, it was discovered that this star has an Earth-size planet, Proxima b, in its habitable zone, which is 20 times closer than the separation of the Earth from the Sun. At that location, the escape speed is 50% larger than from the orbit of the Earth around the Sun. A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets.”
Hippke’s paper, on the other hand, begins by considering that Earth may in fact not be the most habitable type of planet in our Universe. For instance, planets that are more massive than Earth would have higher surface gravity, which means they would be able to hold onto a thicker atmosphere, which would provide greater shielding against harmful cosmic rays and solar radiation.
Artists impression of a Super-Earth, a class of planet that has many times the mass of Earth, but less than a Uranus or Neptune-sized planet. Credit: NASA/Ames/JPL-Caltech
In addition, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and shallower oceans – an ideal situation where biodiversity is concerned. However, when it comes to rocket launches, increased surface gravity would also mean a higher escape velocity. As Hippke indicated in his study:
“Rockets suffer from the Tsiolkovsky (1903) equation : if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive.”
For comparison, Hippke uses Kepler-20 b, a Super-Earth located 950 light years away that is 1.6 times Earth’s radius and 9.7 times it mass. Whereas escape velocity from Earth is roughly 11 km/s, a rocket attempting to leave a Super-Earth similar to Kepler-20 b would need to achieve an escape velocity of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times as much fuel as a rocket on Earth to get into orbit.
To put it into perspective, Hippke considers specific payloads being launched from Earth. “To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships,” he writes. “For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400,000 t.”
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
While Hippke’s analysis concludes that chemical rockets would still allow for escape velocities on Super-Earths up to 10 Earth masses, the amount of propellant needed makes this method impractical. As Hippke pointed out, this could have a serious effect on an alien civilization’s development.
“I am surprised to see how close we as humans are to end up on a planet which is still reasonably lightweight to conduct space flight,” he said. “Other civilizations, if they exist, might not be as lucky. On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope. This should alter their way of development in certain ways we can now analyze in more detail.”
Both of these papers present some clear implications when it comes to the search for extra-terrestrial intelligence (SETI). For starters, it means that civilizations on planets that orbit red dwarf stars or Super-Earths are less likely to be space-faring, which would make detecting them more difficult. It also indicates that when it comes to the kinds of propulsion humanity is familiar with, we may be in the minority.
“This above results imply that chemical propulsion has a limited utility, so it would make sense to search for signals associated with lightsails or nuclear engines, especially near dwarf stars,” said Loeb. “But there are also interesting implications for the future of our own civilization.”
Artist’s concept of a bimodal nuclear rocket making the journey to the Moon, Mars, and other destinations in the Solar System. Credit: NASA
“One consequence of the paper is for space colonization and SETI,” added Hippke. “Civs from Super-Earths are much less likely to explore the stars. Instead, they would be (to some extent) “arrested” on their home planet, and e.g. make more use of lasers or radio telescopes for interstellar communication instead of sending probes or spaceships.”
However, both Loeb and Hippke also note that extra-terrestrial civilizations could address these challenges by adopting other methods of propulsion. In the end, chemical propulsion may be something that few technologically-advanced species would adopt because it is simply not practical for them. As Loeb explained:
“An advanced extraterrestrial civilization could use other propulsion methods, such as nuclear engines or lightsails which are not constrained by the same limitations as chemical propulsion and can reach speeds as high as a tenth of the speed of light. Our civilization is currently developing these alternative propulsion technologies but these efforts are still at their infancy.”
One such example is Breakthrough Starshot, which is currently being developed by the Breakthrough Prize Foundation (of which Loeb is the chair of the Advisory Committee). This initiative aims to use a laser-driven lightsail to accelerate a nanocraft up to speeds of 20% the speed of light, which will allow it to travel to Proxima Centauri in just 20 years time.
Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
Hippke similarly considers nuclear rockets as a viable possibility, since increased surface gravity would also mean that space elevators would be impractical. Loeb also indicated that the limitations imposed by planets around low mass stars could have repercussions for when humans try to colonize the known Universe:
“When the sun will heat up enough to boil all water off the face of the Earth, we could relocate to a new home by then. Some of the most desirable destinations would be systems of multiple planets around low mass stars, such as the nearby dwarf star TRAPPIST-1 which weighs 9% of a solar mass and hosts seven Earth-size planets. Once we get to the habitable zone of TRAPPIST-1, however, there would be no rush to escape. Such stars burn hydrogen so slowly that they could keep us warm for ten trillion years, about a thousand times longer than the lifetime of the sun.”
But in the meantime, we can rest easy in the knowledge that we live on a habitable planet around a yellow dwarf star, which affords us not only life, but the ability to get out into space and explore. As always, when it comes to searching for signs of extra-terrestrial life in our Universe, we humans are forced to take the “low hanging fruit approach”.
Basically, the only planet we know of that supports life is Earth, and the only means of space exploration we know how to look for are the ones we ourselves have tried and tested. As a result, we are somewhat limited when it comes to looking for biosignatures (i.e. planets with liquid water, oxygen and nitrogen atmospheres, etc.) or technosignatures (i.e. radio transmissions, chemical rockets, etc.).
As our understanding of what conditions life can emerge under increases, and our own technology advances, we’ll have more to be on the lookout for. And hopefully, despite the additional challenges it may be facing, extra-terrestrial life will be looking for us!
Artist's impression of a flaring red dwarf star, orbited by an exoplanet. Credit: NASA, ESA, and G. Bacon (STScI)
Since its discovery was announced in August of 2016, Proxima b has been an endless source of wonder and the target of many scientific studies. In addition to being the closest extra-solar planet to our Solar System, this terrestrial planet also orbits within Proxima Centauri’s circumstellar habitable zone (aka. “Goldilocks Zone”). As a result, scientists have naturally sought to determine if this planet could actually be home to extra-terrestial life.
Many of these studies have been focused on whether or not Proxima b could retain an atmosphere and liquid water on its surface in light of the fact that it orbits an M-type (red dwarf) star. Unfortunately, many of these studies have revealed that this is not likely due to flare activity. According to a new study by an international team of scientists, Proxima Centauri released a superflare that was so powerful, it would have been lethal to any life as we know it.
The study, titled “The First Naked-Eye Superflare Detected from Proxima Centauri“, recently appeared online. The team was led by Howard Ward, a PhD candidate in physics and astronomy at the UNC Chapel Hill, with additional members from the NASA Goddard Space Flight Center, the University of Washington, the University of Colorado, the University of Barcelona and the School of Earth and Space Exploration at Arizona State University.
Artist impression of a red dwarf star like Proxima Centauri, the nearest star to our sun. New analysis of ALMA observations reveal that Proxima Centauri emitted a powerful flare that would have created inhospitable conditions for planets in that system. Credit: NRAO/AUI/NSF; D. Berry
As they indicate in their study, solar flare activity would be one of the greatest potential threats to planetary habitability in a system like Proxima Centauri. As they explain:
“[W]hile ozone in an Earth-like planet’s atmosphere can shield the planet from the intense UV flux associated with a single superflare, the atmospheric ozone recovery time after a superflare is on the order of years. A sufficiently high flare rate can therefore permanently prevent the formation of a protective ozone layer, leading to UV radiation levels on the surface which are beyond what some of the hardiest-known organisms can survive.”
In addition stellar flares, quiescent X-ray emissions and UV flux from a red dwarf star can would be capable of stripping planetary atmospheres over the course of several billion years. And while multiple studies have been conducted that have explored low- and moderate-energy flare events on Proxima, only one high-energy event has even been observed.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
As the team indicates in their study, the March 2016 superflare was the first to be observered from Proxima Centauri, and was rather powerful:
“In March 2016 the Evryscope detected the first-known Proxima superflare. The superflare had a bolometric energy of 10^33.5 erg, ~10× larger than any previously-detected flare from Proxima, and 30×larger than any optically measured Proxima flare. The event briefly increased Proxima’s visible-light emission by a factor of 38× averaged over the Evryscope’s 2-minute cadence, or ~68× at the cadence of the human eye. Although no M-dwarfs are usually visible to the naked-eye, Proxima briefly became a magnitude-6.8 star during this superflare, visible to dark-site naked-eye observers.”
The superflare coincided with the three-month Pale Red Dot campaign, which was responsible for first revealing the existence of Proxima b. While monitoring the star with the HARPS spectrograph – which is part of the 3.6 m telescope at the ESO’s La Silla Observatory in Chile – the campaign team also obtaining spectra on March 18th, 08:59 UT (just 27 minutes after the flare peaked at 08:32 UT).
The team also noted that over the last two years, the Evryscope has recorded 23 other large Proxima flares, ranging in energy from 10^30.6 erg to 10^32.4 erg. Coupled with rates of a single superflare detection, they predict that at least five superflares occur each year. They then combined this data with the high-resolution HARPS spectroscopy to constrain the superflare’s UV spectrum and any associated coronal mass ejections.
The Red Dots project is successor to the Pale Red Dot project, which discovered Proxima b last summer. Credit: ESO
The team then used the HARPS spectra and the Evryscope flare rates to create a model to determine what effects this star would have on a nitrogen-oxygen atmosphere. This included how long the planet’s protective ozone layer would be able to withstand the blasts, and what effect regular exposure to radiation would have on terrestrial organisms.
“[T]he repeated flaring is sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years. We estimate complete depletion occurs within several hundred kyr. The UV light produced by the Evryscope superflare therefore reached the surface with ~100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would struggle to survive in the areas of Proxima b exposed to these flares.”
Essentially, this and other studies have concluded that any planets orbiting Proxima Centauri would not be habitable for very long, and likely became lifeless balls of rock a long time ago. But beyond our closest neighboring star system, this study also has implications for other M-type star systems. As they explain, red dwarf stars are the most common in our galaxy – roughly 75% of the population – and two-thirds of these stars experience active flare activity.
As such, measuring the impact that superflares have on these worlds will be a necessary component to determining whether or not exoplanets found by future missions are habitable. Looking ahead, the team hopes to use the Evryscope to examine other star systems, particularly those that are targets for the upcoming Transiting Exoplanet Survey Satellite (TESS) mission.
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
“Beyond Proxima, Evryscope has already performed similar long-term high-cadence monitoring of every other Southern TESS planet-search target, and will therefore be able to measure the habitability impact of stellar activity for all Southern planetsearch-target M-dwarfs,” they write. “In conjunction with coronal-mass-ejection searches from long- wavelength radio arrays like the [Long Wavelength Array], the Evryscope will constrain the long-term atmospheric effects of this extreme stellar activity.”
For those who hoped that humanity might find evidence of extra-terrestrial life in their lifetimes, this latest study is certainly a letdown. It’s also disappointing considering that in addition to being the most common type of star in the Universe, some research indicates that red dwarf stars may be the most likely place to find terrestrial planets. However, even if two-thirds of these stars are active, that still leaves us with billions of possibilities.
It is also important to note that these studies help ensure that we can determine which exoplanets are potentially habitable with greater accuracy. In the end, that will be the most important factor when it comes time to decide which of these systems we might try to explore directly. And if this news has got you down, just remember the worlds of the immortal Carl Sagan:
“The universe is a pretty big place. If it’s just us, seems like an awful waste of space.”
Artist impression of a red dwarf star like Proxima Centauri, the nearest star to our sun. New analysis of ALMA observations reveal that Proxima Centauri emitted a powerful flare that would have created inhospitable conditions for planets in that system. Credit: NRAO/AUI/NSF; D. Berry
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.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
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.”
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
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:
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
Welcome back to our series on Exoplanet-Hunting methods! Today, we look at another widely-used and popular method of exoplanet detection, known as the Radial Velocity (aka. Doppler Spectroscopy) Method.
The hunt for extra-solar planets sure has heated up in the past decade or so! Thanks to improvements made in instrumentation and methodology, the number of exoplanets discovered (as of December 1st, 2017) has reached 3,710 planets in 2,780 star systems, with 621 system boasting multiple planets. Unfortunately, due to the limits astronomers are forced to contend with, the vast majority have been discovered using indirect methods.
When it comes to these indirect methods, one of the most popular and effective is the Radial Velocity Method – also known as Doppler Spectroscopy. This method relies on observing the spectra stars for signs of “wobble”, where the star is found to be moving towards and away from Earth. This movement is caused by the presence of planets, which exert a gravitational influence on their respective sun.
Artist's impression of a hypothetical planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. Credit: ESO
At a distance of 4.37 light-years from Earth, Alpha Centauri is the nearest star system to our own. For generations, scientists and speculative thinkers have pondered whether it might have a planetary system like our own Sun, and whether or not life may also exist there. Unfortunately, recent efforts to locate extra-solar planets in this star system have failed, with potential detections later shown to be the result of artifacts in the data.
In response to these failed efforts, several more ambitious projects are being developed to find exoplanets around Alpha Centauri. These include direct-imaging space telescopes like Project Blue and the interstellar mission known as Breakthrough Starshot. But according to a new study led by researchers from Yale University, existing data can be used to determine the probability of planets in this system (and even which kind).
The study which detailed their findings recently appeared in The Astronomical Journal under the title “Planet Detectability in the Alpha Centauri System“. The study was led by Lily Zhao, a graduate student from Yale University and a fellow with the National Science Foundation (NSF), and was co-authored by Debora Fischer, John Brewer and Matt Giguere of Yale and Bárbara Rojas-Ayala of the Universidad Andrés Bello in Chile.
Artist’s impression of what the surface might look like on a planet orbiting Alpha Centauri system. Credit: Michael S. Helfenbein
For the sake of their study, Zhao and her team considered why efforts to locate planets within the the closest star system to our own have so far failed. This is surprising when one considers how, statistically speaking, Alpha Centauri is very likely to have a system if its own. As Prof. Fischer indicated in a recent Yale News press release:
“The universe has told us the most common types of planets are small planets, and our study shows these are exactly the ones that are most likely to be orbiting Alpha Centauri A and B… Because Alpha Centauri is so close, it is our first stop outside our solar system. There’s almost certain to be small, rocky planets around Alpha Centauri A and B.”
In addition to being a professor of astronomy at Yale University, Debora Fischer is also one of the leaders of the Yale Exoplanets Group. As an expert in her field, Fischer has devoted decades of her life to researching exoplanets and searching for Earth analogues beyond our Solar System. With partial funding provided by NASA and the National Science Foundation, the team relied on existing data collected by some of the latest exoplanet-hunting instruments.
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
Using ten years of data collected by these instruments, Zhao and her colleagues then set up a grid system for the Alpha Centauri system. Rather than looking for signs of planets that did exist, they used the data to rule out what types of planets could not exist there. As Zhao told Universe Today via email:
“This study was special in that it used existing data of the Alpha Centauri system not to find planets, but to characterize what planets could not exist. By doing so, it returned more information about the system as a whole and provides guidance for future observations of this uniquely charismatic system.
In addition, the team analyzed the chemical composition of the stars in the Alpha Centauri system to learn more about the kinds of material that would be available to form planets. Based on the different values obtained by observations campaigns conducted by different telescopes on Alpha Centauri’s three stars (Alpha, Beta and Proxima), they were able to place constraints on what kinds of planets could exist there.
“We found that existing data rules out planets in the habitable zone above 53 Earth masses for alpha Centauri A, 8.4 Earth masses for Alpha Centauri B, and 0.47 Earth masses for Proxima Centauri,” said Zhao. “As for the chemical compositions, we found that the ratios of Carbon/Oxygen and Magnesium/Silicon for Alpha Centauri A and B are quite similar to that of the Sun.”
Artist’s impression of how the surface of a planet orbiting a red dwarf star may appear. Credit: M. Weiss/CfA
Basically, the results of their study effectively ruled out the possibility of any Jupiter-sized gas giants in the Alpha Centauri system. For Alpha Centauri A, they further found that planets that were less than 50 Earth masses could exist, while Alpha Centauri B might have planets smaller than 8 Earth masses. For Proxima Centauri, which we know to have at least one Earth-like planet, they determined that there might more that are less than half of Earth’s mass.
In addition to offering hope for exoplanet-hunters, this study carries with it some rather interesting implications for planetary habitability. Basically, the presence of rocky planets in the system is encouraging; but with no gas giants, a key ingredient in ensuring that planets remain habitable could be missing.
“[N]ot only could there still be habitable, Earth-mass planets around our closest stellar neighbors, but there also aren’t any gas giants that could endanger the survival of these potentially habitable, rocky planets,” said Zhao. “Furthermore, if these planets do exist, they are likely to have similar compositions to our very own Earth given the similarity in Alpha Cen A/B and our beloved Sun.”
At present, there are no instruments that have been able to confirm the existence of any exoplanets in Alpha Centauri. But as Zhao indicated, her and her teammates are optimistic that future surveys will have the necessary sensitivity to do it:
“[T]his very month has seen the commissioning of several next-generation instruments promising the precision necessary to discover these possible planets in the near future, and this analysis has shown that it is for sure worth it to keep looking!”
The ESO’s Paranal Observatory, located in the Atacama Desert of Chile. Credit: ESO
“These instruments are promising a precision of down to 10-30 cm/s and should be able to detect many more smaller, and further away planets – such as habitable planets around the Centauri stars,” said Zhao. “The field of view of these two instruments are slightly different (ESPRESSO has the southern hemisphere, where Alpha Centauri is, while EXPRES covers the northern hemisphere, for instance where the Kepler and many of the K2 fields are).”
With new instruments at their disposal, and methods like the one Zhao and her team developed, the closest star system to Earth is sure to become a veritable treasure trove for astronomers and exoplanet-hunters in the coming years. And anything we find there will surely become targets for direct studies by groups like Project Blue and Breakthrough Starshot. If ET resides next door, we’re sure to hear about it soon!
This artist’s impression shows how the newly discovered belts of dust around the closest star to the Solar System, Proxima Centauri, may look. Credit: ESO/M. Kornmesser
Proxima Centauri, in addition to being the closest star system to our own, is also the home of the closest exoplanet to Earth. The existence of this planet, Proxima b, was first announced in August of 2016 and then confirmed later that month. The news was met with a great deal of excitement, and a fair of skepticism, as numerous studies followed t were dedicated to determining if this planet could in fact be habitable.
Another important question has been whether or not Proxima Centauri could have any more objects orbiting it. According to a recent study by an international team of astronomers, Proxima Centauri is also home to a belt of cold dust and debris that is similar to the Main Asteroid Belt and Kuiper Belt in our Solar System. The existence of this dusty belt could indicate the presence of more planets in this star system.
View of the Atacama Large Millimeter/submillimeter Array (ALMA) site in the Atacama Desert of northern Chile. Credit: A. Marinkovic/X-Cam/ALMA (ESO/NAOJ/NRAO)
For their study, the team relied on data obtained by the Atacama Large Millimeter/submillimter Array (ALMA) at the ALMA Observatory in Chile. These observations revealed the glow of a cold dust belt that is roughly 1 to 4 AUs from Proxima Centauri – one to four times the distance between the Earth and the Sun. This puts it significantly further out than Proxima b, which orbits its sun at a distance of 0.0485 AU (~5% of Earth’s distance from the Sun).
Dust belts are essentially the leftover material that did not form into larger bodies withing a star system. The particles of rock and ice in these belts vary in size from being smaller than a millimeter across to asteroids that are many kilometers in diameter. Based on their observations, the team estimated that the belt in Proxima Centauri has a total mass that is about one-hundredth the mass of Earth.
The team also estimated that this belt experiences temperatures of about 43 K (-230°C; -382 °F), making it as cold as the Kuiper Belt. As Dr. Anglada explained the significance of these findings in a recent ESO press release:
“The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it’s the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our Sun.”
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO
The ALMA data also provided indications that Proxima Centauri might also have another belt located about ten times further out. In other words, Proxima Centauri may have two belts, just like our Solar System. If confirmed, this could indicate that this neighboring star also has a system of planets that fall within and between belts of unconsolidated material, which in turn is leftover from the early days of planet formation. As Dr. Anglada explained:
“This result suggests that Proxima Centauri may have a multiple planet system with a rich history of interactions that resulted in the formation of a dust belt. Further study may also provide information that might point to the locations of as yet unidentified additional planets.”
The very cold environment of this outer belt could also have some interesting implications, since its parent star is much dimmer than our own. Pedro Amado, who also hails from the Astrophysical Institute of Andalusia, was similarly enthusiastic about these findings. As he indicated, they are just the beginning of what is sure to be a long process of discovery about this system.
“These first results show that ALMA can detect dust structures orbiting around Proxima,” he said. “Further observations will give us a more detailed picture of Proxima’s planetary system. In combination with the study of protoplanetary discs around young stars, many of the details of the processes that led to the formation of the Earth and the Solar System about 4600 million years ago will be unveiled. What we are seeing now is just the appetiser compared to what is coming!”
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
This study is also likely to be of interest to those planning on conducting direct observations of the Alpha Centauri system, such as Project Blue. In the coming years, they hope to deploy a space telescope that will observe Alpha Centauri directly to study any exoplanets it may have. With a slight adjustment, this telescope could also take a gander at Proxima Centauri and aid in the hunt for a system of planets there.
And then there’s Breakthrough Starshot, the first proposed interstellar voyage which hopes to send a laser sail-driven nanocraft to Alpha Centauri in the coming decades. Recently, the scientists behind Starshot discussed the possibility of extending the mission to include a stopover in Proxima Centauri. Before such a mission can take place, the planners need to know what kind of dusty environment awaits it.
And of course, future studies will benefit from the deployment of next-generation instruments, like the James Webb Space Telescope (scheduled for launch in 2019) and the ESO’s Extremely Large Telescope (ELT) – which is expected to collect its first light in 2024.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
Back in of August of 2016, the existence of an Earth-like planet right next door to our Solar System was confirmed. To make matters even more exciting, it was confirmed that this planet orbits within its star’s habitable zone too. Since that time, astronomers and exoplanet-hunters have been busy trying to determine all they can about this rocky planet, known as Proxima b. Foremost on everyone’s mind has been just how likely it is to be habitable.
However, numerous studies have emerged since that time that indicate that Proxima b, given the fact that it orbits an M-type (red dwarf), would have a hard time supporting life. This was certainly the conclusion reached in a new study led by researchers from NASA’s Goddard Space Flight Center. As they showed, a planet like Proxima b would not be able to retain an Earth-like atmosphere for very long.
Red dwarf stars are the most common in the Universe, accounting for an estimated 70% of stars in our galaxy alone. As such, astronomers are naturally interested in knowing just how likely they are at supporting habitable planets. And given the distance between our Solar System and Proxima Centauri – 4.246 light years – Proxima b is considered ideal for studying the habitability of red dwarf star systems.
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: Pale Red Dot
On top of all that, the fact that Proxima b is believed to be similar in size and composition to Earth makes it an especially appealing target for research. The study was led by Dr. Katherine Garcia-Sage of NASA’s Goddard Space Flight Center and the Catholic University of America in Washington, DC. As she told Universe Today via email:
“So far, not many Earth-sized exoplanets have been found orbiting in the temperate zone of their star. That doesn’t mean they don’t exist – larger planets are found more often because they are easier to detect – but Proxima b is of interest because it’s not only Earth-sized and at the right distance from its star, but it’s also orbiting the closest star to our Solar System.”
For the sake of determining if Proxima b could be habitable, the research team sought to address the chief concerns facing rocky planets that orbit red dwarf stars. These include the planet’s distance from its stars, the variability of red dwarfs, and the presence (or absence) of magnetic fields. Distance is of particular importance since habitable zones (aka. temperate zones) around red dwarfs are much closer and tighter.
“Red dwarfs are cooler than our own Sun, so the temperate zone is closer to the star than Earth is to the Sun,” said Dr. Garcia-Sage. “But these stars may be very magnetically active, and being so close to a magnetically active star means that these planets are in a very different space environment than what the Earth experiences. At those distances from the star, the ultraviolet and x-ray radiation may be quite large. The stellar wind may be stronger. There could be stellar flares and energetic particles from the star that ionize and heat the upper atmosphere.”
At one time, Mars had a magnetic field similar to Earth, which prevented its atmosphere from being stripped away. Credit: NASA
In addition, red dwarf stars are known for being unstable and variable in nature when compared to our Sun. As such, planets orbiting in close proximity would have to contend with flare ups and intense solar wind, which could gradually strip away their atmospheres. This raises another important aspect of exoplanet habitability research, which is the presence of magnetic fields.
To put it simply, Earth’s atmosphere is protected by a magnetic field that is driven by a dynamo effect in its outer core. This “magnetosphere” has prevented solar wind from stripping our atmosphere away, thus giving life a chance to emerge and evolve. In contrast, Mars lost its magnetosphere roughly 4.2 billion years ago, which led to its atmosphere being depleted and its surface becoming the cold, desiccated place it is today.
To test Proxima b’s potential habitability and capacity to retain liquid surface water, the team therefore assumed the presence of an Earth-like atmosphere and a magnetic field around the planet. They then accounted for the enhanced radiation coming from Proxima b. This was provided by the Harvard Smithsonian Center for Astrophysics (CfA), where researchers determined the ultraviolet and x-ray spectrum of Proxima Centauri for this project.
From all of this, they constructed models that began to calculate the rate of atmospheric loss, using Earth’s atmosphere as a template. As Dr. Garcia-Sage explained:
“At Earth, the upper atmosphere is ionized and heated by ultraviolet and x-ray radiation from the Sun. Some of these ions and electrons escape from the upper atmosphere at the north and south poles. We have a model that calculates how fast the upper atmosphere is lost through these processes (it’s not very fast at Earth)… We then used that radiation as the input for our model and calculated a range of possible escape rates for Proxima Centauri b, based on varying levels of magnetic activity.”
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
What they found was not very encouraging. In essence, Proxima b would not be able to retain an Earth-like atmosphere when subjected to Proxima Centauri’s intense radiation, even with the presence of a magnetic field. This means that unless Proxima b has had a very different kind of atmospheric history than Earth, it is most likely a lifeless ball of rock.
However, as Dr. Garcia-Sage put it, there are other factors to consider which their study simply can’t account for:
“We found that atmospheric losses are much stronger than they are at Earth, and the for high levels of magnetic activity that we expect at Proxima b, the escape rate was fast enough that an entire Earth-like atmosphere could be lost to space. That doesn’t take into account other things like volcanic activity or impacts with comets that might be able to replenish the atmosphere, but it does mean that when we’re trying to understand what processes shaped the atmosphere of Proxima b, we have to take into account the magnetic activity of the star. And understanding the atmosphere is an important part of understanding whether liquid water could exist on the surface of the planet and whether life could have evolved.”
So it’s not all bad news, but it doesn’t inspire a lot of confidence either. Unless Proxima b is a volcanically-active planet and subject to a lot of cometary impacts, it is not likely be temperate, water-bearing world. Most likely, its climate will be analogous to Mars – cold, dry, and with water existing mostly in the form of ice. And as for indigenous life emerging there, that’s not too likely either.
These and other recent studies have painted a rather bleak picture about the habitability of red dwarf star systems. Given that these are the most common types of stars in the known Universe, the statistical likelihood of finding a habitable planet beyond our Solar System appears to be dropping. Not exactly good news at all for those hoping that life will be found out there within their lifetimes!
But it is important to remember that what we can say definitely at this point about extra-solar planets is limited. In the coming years and decades, next-generation missions – like the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS) – are sure to paint a more detailed picture. In the meantime, there’s still plenty of stars in the Universe, even if most of them are extremely far away!
