In August of 2016, astronomers from the European Southern Observatory (ESO) announced the discovery of an exoplanet in the neighboring system of Proxima Centauri. The news was greeted with consider excitement, as this was the closest rocky planet to our Solar System that also orbited within its star’s habitable zone. Since then, multiple studies have been conducted to determine if this planet could actually support life.
Unfortunately, most of the research so far has indicated that the likelihood of habitability are not good. Between Proxima Centauri’s variability and the planet being tidally-locked with its star, life would have a hard time surviving there. However, using lifeforms from early Earth as an example, a new study conducted by researchers from the Carl Sagan Institute (CSI) has shows how life could have a fighting chance on Proxima b after all.
In the course of searching for extra-solar planets, some very interesting finds have been made. Some of them have even occurred within our own galactic neighborhood. Just two years ago, astronomers from the Red Dots and CARMENES campaigns announced the discovery of Proxima b, a rocky planet that orbits within the habitable zone of our nearest stellar neighbor – Proxima Centauri.
This rocky world, which may be habitable, remains the closest exoplanet ever discovered to our Solar System. A few days ago (on Nov. 14th), Red Dots and CARMENES announced another find: a rocky planet orbiting Barnard’s star, which is just 6 light years from Earth. This planet, Barnard’s Star b, is now the second closest exoplanet to our Solar System, and the closest planet to orbit a single star.
In of August of 2016, astronomers from the European Southern Observatory (ESO) confirmed the existence of an Earth-like planet around Proxima Centauri – the closest star to our Solar System. In addition, they confirmed that this planet (Proxima b) orbited within its star’s habitable zone. Since that time, multiple studies have been conducted to determine if Proxima b could in fact be habitable.
Unfortunately, most of this research has not been very encouraging. For instance, many studies have indicated that Proxima b’s sun experiences too much flare activity for the planet to sustain an atmosphere and liquid water on its surface. However, in a new NASA-led study, a team of scientists has investigated various climate scenarios that indicate that Proxima b could still have enough water to support life.
Humanity has long dreamed about sending humans to other planets, even before crewed spaceflight became a reality. And with the discovery of thousands 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.
However, such missions are still a long way off and/or do not involve crewed spaceflight (which is the case with Starshot). As such, Dr. Marin and Dr. Beluffi also took into account missions that will be launching in the coming years like NASA’s Parker Solar Probe. This probe will reach record-breaking orbital velocities of up to 724,205 km/h, which works out to about 200 km/s-¹ (or 0.067% the speed of light).
As Dr. Marin told Universe Today via email:
“This purely and entirely rely on the technology available at the time of the mission. If we would create a spacecraft right now, we could only reach about 200 km/s, which translates into 6300 years of travel. Of course technology is getting better with time and by the time a real interstellar project will be created, we can expect to have improved the duration by one order of magnitude, i.e. 630 years. This is speculative as technology as yet to be invented.”
With their baseline for speed and travel time established – 200 km/s-¹ and 6300 years – Dr. Marin and Dr. Beluffi then set out to determine the minimum number of people needed to ensure that a healthy crew arrived at Proxima b. To do this, the pair conducted a series of Monte Carlo simulations using a new code created by Dr. Marin himself. This mathematical technique takes into account chance events in decision making to produce distributions of possible outcomes.
“We are using a new numerical software that I have created,” said Dr. Marin. “It is named HERITAGE, see the first paper of the series. It is a stochastic Monte Carlo code that accounts for all possible outcomes of space simulations by testing every randomized scenario for procreation, life and death. By looping the simulation thousands of times, we get statistical values that are representative of a real space travel for a multi-generational crew. The code accounts for as many biological factors as possible and is currently being developed to include more and more physics.”
These biological factors include things like the number of women vs. men, their respective ages, life expectancy, fertility rates, birth rates, and how long the crew would have to reproduce. It also took into account some extreme possibilities, which included accidents, disasters, catastrophic events, and the number of crew members likely to be effected by them.
They then averaged the results of these simulations over 100 interstellar journeys based on these various factors and different values to determine the size of the minimum crew. In the end, Dr. Marin and Dr. Beluffi concluded that under conservative conditions, a minimum of 98 crew members would be needed to sustain a multi-generational voyage to the nearest star system with a potentially-habitable exoplanet.
Any less than that, and the likelihood of success would drop off considerably. For instance, with an initial crew of 32, their simulations indicated that the chances for success would reach 0%, largely because such a small community would make inbreeding inevitable. While this crew might eventually arrive at Proxima b, they would not be a genetically healthy crew, and therefore not a very good way to start a colony! As Dr. Marin explained:
“Our simulations allows us to predict with great precision the minimum size of the initial crew that will leave for centuries-long space travels. By allowing the crew to evolve under a list of adaptive social engineering principles (namely, yearly evaluations of the vessel population, offspring restrictions and breeding constraints), we show in this paper that it is possible to create and maintain a healthy population virtually indefinitely.”
While the technology and resources needed to make an interstellar voyage is still generations away, studies of this kind could be of profound significance for those missions – if and when they occur. Knowing in advance the likelihood that such a mission will succeed, and what will increase that likelihood to the point that success is virtually guaranteed, will also increase the likelihood that such missions are mounted.
This study and the one that preceded it are also significant in that they are the first to take into account key biological factors (like procreation) and how they will affect a multi-generational crew. As Dr. Marin concluded:
“Our project aims to provide realistic simulations of multi-generational space ships in order to prepare future space exploration, in a multidisciplinary project that utilizes the expertise of physicists, astronomers, anthropologists, rocket engineers, sociologists and many others. HERITAGE is the first ever dedicated Monte Carlo code to compute the probabilistic evolution of a kin-based crew aboard an interstellar ship, which allows one to explore whether a crew of a proposed size could survive for multiple generations without any artificial stocks of additional genetic material. Determining the minimum size of the crew is an essential step in the preparation of any multi-generational mission, affecting the resources and budget required for such an endeavor but also with implications for sociological, ethical and political factors. Furthermore, these elements are essential in examining the creation of any self-sustaining colony – not only humans establishing planetary settlements, but also with more immediate impacts: for example, managing the genetic health of endangered species or resource allocation in restrictive environments.”
Dr. Marin was also quoted recently in an article in The Conversation about the goals of his and Dr. Beluffi’s project, which is all about determining what is needed to ensure the health and safety of future interstellar voyagers. As he said in the article:
“Of the 3757 exoplanets that have been detected, the closest Earth-like planet lies at 40 trillion kilometers from us. At 1% of the speed of light, which is far superior to the highest velocities achieved by state-of-the-art spacecraft, it would still take 422 years for ships to reach their destination. One of the immediate consequences of this is that interstellar voyages cannot be achieved within a human lifespan. It requires a long-duration space mission, which necessitates finding a solution whereby the crew survive hundreds of years in deep space. This is the goal of our project: to establish the minimum size of a self-sustaining, long duration space mission, in terms of both hardware and population. By doing so, we intend to obtain scientifically-accurate estimates of the requirements for multi-generational interstellar travel, unlocking the future of human space exploration, migration and habitation.”
In the coming decades, next-generation telescopes are expected to discover thousands more exoplanets. But more importantly, these high-resolution instruments are also expected to reveal things about exoplanets that will allow us to characterize them. These will include spectra from their atmospheres that will let scientists know with greater certainty if they are actually habitable.
With more candidates to choose from, we will be all the more prepared for the day when interstellar voyages can be launched. When that time comes, our scientists will be armed with the necessary information for ensuring that the people that arrive will be hail, hearty, and prepared to tackle the challenges of exploring a new world!
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.
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.
“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.
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 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.
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.
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).
“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.
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.
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.
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.
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.”
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.”
“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.
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!
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.
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.
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 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.
“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.”
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:
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.
Essentially, the Radial Velocity Method consists not of looking for signs of planets themselves, but in observing a star for signs of movement. This is deduced by using a spectometer to measure the way in which the star’s spectral lines are displaced due to the Doppler Effect – i.e. how light from the star is shifted towards the red or blue end of the spectrum (redshift/blueshift).
These shifts are indications that the star is moving away from (redshift) or towards (blueshift) Earth. Based on the star’s velocity, astronomers can determine the presence of a planet or system of planets. The speed at which a star moves around its center of mass, which is much smaller than that of a planet, is nevertheless measurable using today’s spectrometers.
Until 2012, this method was the most effective means of detecting exoplanets, but has since come to be replaced by the Transit Photometry. Nevertheless, it remains a highly effective method and is often relied upon in conjunction with the Transit Method to confirm the existence of exoplanets and place constraints on their size and mass.
The Radial Velocity method was the first successful means of exoplanet detection, and has had a high success rate for identifying exoplanets in both nearby (Proxima b and TRAPPIST-1‘s seven planets) and distant star systems (COROT-7c). One of the main advantages is that it allows for the eccentricity of the planet’s orbit to be measured directly.
The radial velocity signal is distance-independent, but requires a high signal-to-noise-ratio spectra to achieve a high degree of precision. As such, it is generally used to look for low-mass planets around stars that are within 160 light-years from Earth, but can still detect gas giants up to a few thousand light years away.
The radial velocity technique is able to detect planets around low-mass stars, such as M-type (red dwarf) stars. This is due to the fact that low mass stars are more affected by the gravitational tug of planets and because such stars generally rotate more slowly (leading to more clear spectral lines). This makes the Radial Velocity Method highly useful for two reasons.
For one, M-type stars are the most common in the Universe, accounting for 70% of stars in spiral galaxies and 90% of stars in elliptical galaxies. Second, recent studies have indicated that low-mass, M-type stars are the most likely place to find terrestrial (i.e. rocky) planets. The Radial Velocity Method is therefore well-suited for the study of Earth-like planets that orbit closely to red dwarf suns (within their respective habitable zones).
Another major advantage is the way the Radial Velocity Method is able to place accurate constraints on a planet’s mass. Although the radial velocity of a star can only yield estimates a planet’s minimum mass, distinguishing the planet’s own spectral lines from those of the the star can yield measurements of the planet’s radial velocity.
This allows astronomers to determine the inclination of the planet’s orbit, which enables the measurement of the planet’s actual mass. This technique also rules out false positives and provides data about the composition of the planet. The main issue is that such detection is possible only if the planet orbits around a relatively bright star and if the planet reflects or emits a lot of light.
As of December 2017, 662 of all exoplanet discoveries (both candidates and those that have been confirmed) were detected using the Radial Velocity Method alone – almost 30% of the total.
That being said, the Radial Velocity Method also has some notable drawbacks. For starters, it is not possible to observe hundreds or even thousands of stars simultaneously with a single telescope – as is done with Transit Photometry. In addition, sometimes Doppler spectrography can produces false signals, especially in multi-planet and multi-star systems.
This is often due to the presence of magnetic fields and certain types of stellar activity, but can also arise from a lack of sufficient data since stars are not generally observed continuously. However, these limitations can be mitigated by pairing radial velocity measurements with another method, the most popular and effective of which is Transit Photometry.
While distinguishing between the spectral lines of a star and a planet can allow for better constraints to be placed on a planet’s mass, this is generally only possible if the planet orbits around a relatively bright star and the planet reflects or emits a lot of light. In addition, planet’s that have highly inclined orbits (relative to the observer’s line of sight) produce smaller visible wobbles, and are therefore harder to detect.
In the end, the Radial Velocity Method is most effective when paired with Transit Photometry, specifically for the sake of confirming detections made with the latter method. When both methods are used in combination, the existence of a planet can not only be confirmed, but accurate estimates of its radius and true mass can be made.
Exoplanet-hunting surveys that rely on the Radial Velocity Method are expected to benefit greatly form the deployment of the James Webb Space Telescope (JWST), which is scheduled for 2019. Once operational, this mission will obtain Doppler measurements of stars using its advanced suite of infrared instruments to determine the presence of exoplanet candidates. Some of these will then be confirmed using the Transiting Exoplanet Survey Satellite (TESS) – which will deploy in 2018.
Thanks to improvements in technology and methodology, exoplanet discovery has grown by leaps and bounds in recent years. With thousands of exoplanets confirmed, the focus has gradually shifted towards the characterizing of these planets to learn more about their atmospheres and conditions on their surface. In the coming decades, thanks in part to the deployment of new missions, some very profound discoveries are expected to be made!