In the past thirty years, the number of planets discovered beyond our Solar System has grown exponentially. Unfortunately, due to the limitations of our technology, the vast majority of these exoplanets have been discovered by indirect means, often by detecting the transits of planets in front of their stars (the Transit Method) or by the gravitational influence they exert on their star (the Radial Velocity Method).
Very few have been imaged directly, where the planets have been observed in visible light or infrared wavelengths. One such planet is Beta Pictoris b, a young massive exoplanet that was first observed in 2008 by a team from the European Southern Observatory (ESO). Recently, the same team tracked this planet as it orbited its star, resulting in some stunning images and an equally impressive time-lapse video.
Rogue planets are a not-too-uncommon occurrence in our Universe. In fact, within our galaxy alone, it is estimated that there are billions of rogue planets, perhaps even more than there are stars. These objects are basically planet-mass objects that have been ejected from their respective star systems (where they formed), and now orbit the center of the Milky Way. But it is especially surprising to find one orbiting so close to our own Solar System!
In 2016, scientists detected what appeared to be either a brown dwarf or a star orbiting just 20 light years beyond our Solar System. However, using the National Science Foundation’s Karl G. Jansky Very Large Array(VLA), a team of astronomers recently concluded that it is right at the boundary between a massive planet and a brown dwarf. This, and other mysterious things about this object, represent a mystery and an opportunity to astronomers!
The study which describes their findings recently appeared the Astrophysical Journal under the title “The Strongest Magnetic Fields on the Coolest Brown Dwarfs.” The team was led by Melodie Kao – who led this study while a graduate student at Caltech, and is now a Hubble Postdoctoral Fellow at Arizona State University – and included members from Arizona State University, the University of Colorado Boulder, the California Institute of Technology, and the University of California San Diego.
To summarize, brown dwarfs are objects that are too massive to be considered planets, but not massive enough to become stars. Originally, such objects were not thought to emit radio waves, but in 2001, a team using the VLA discovered a brown dwarf that exhibited both strong radio emissions and magnetic activity. Ongoing observations also revealed that some brown dwarfs have strong auroras, similar to the gas giants in our Solar System.
This particular object, known as SIMP J01365663+0933473, was first discovered in 2016 by the Caltech team as one of five brown dwarfs. This survey was part of VLA study to gain new knowledge about magnetic fields and the mechanisms by which the coolest astronomical objects can produce strong radio emissions. Since brown dwarfs are incredibly difficult to measure, the object was initially though to be too old and too massive to be a brown dwarf.
However, last year, an independent team of scientists discovered that SIMP J01365663+0933473 was part of a very young group of stars whose age, size and mass indicated that it was likely to be a free-floating (aka. rogue) planet rather than a star. In short, the object was determined to be 200 million years old, 1.22 times the radius of Jupiter and 12.7 times its mass.
It was also estimated to have a surface temperature of about 825 °C (1500 °F) – compared to the Sun’s, which is 5,500 °C (9932 °F). Simultaneously, the Caltech team that originally detected its radio emission in 2016 observed it again in a new study at even higher radio frequencies. From this, they confirmed that its magnetic field was even stronger than first measured, roughly 200 times stronger than Jupiter’s.
As Dr. Kao explained in a recent NRAO press release, this all presents a rather mysterious find:
“This object is right at the boundary between a planet and a brown dwarf, or ‘failed star,’ and is giving us some surprises that can potentially help us understand magnetic processes on both stars and planets… When it was announced that SIMP J01365663+0933473 had a mass near the deuterium-burning limit, I had just finished analyzing its newest VLA data.”
In short, the VLA observations provided both the first radio detection and the first measurement of the magnetic field of a planetary-mass object beyond our Solar System. The presence of a such a strong magnetic field represents a huge challenge to astronomers’ understanding of the dynamo mechanisms that create magnetic fields in brown dwarfs, not to mention the mystery of what drives their auroras.
Ever since brown dwarfs were observed to have auroral activity, scientists have wondered what could be powering them. On Earth, as with Jupiter and the other Solar planets that experience them, aurorae are the result of solar wind interacting with a planet’s magnetic field. But in the case of brown dwarfs, which have no parent star, some other mechanism must be involved. As Kao explained:
“This particular object is exciting because studying its magnetic dynamo mechanisms can give us new insights on how the same type of mechanisms can operate in extrasolar planets — planets beyond our Solar System. We think these mechanisms can work not only in brown dwarfs, but also in both gas giant and terrestrial planets.”
Kao and her team think that one possibility is that this object has an orbiting planet or moon that is interacting with its magnetic field, similar to what happens between Jupiter and its moon Io. Given its proximity to our Solar System, scientists will have the opportunity to address this and other questions, and to learn a great deal about the mechanics that power gas giants and brown dwarfs.
Studying this object will also help astronomers place more accurate constraints on the dividing line between massive planets and brown dwards. And last, but not least, it also presents new opportunities as far exoplanet research is concerned. As Gregg Hallinan, who was Dr. Kao’s advisor and a co-author on the Caltech study, explained:
“Detecting SIMP J01365663+0933473 with the VLA through its auroral radio emission also means that we may have a new way of detecting exoplanets, including the elusive rogue ones not orbiting a parent star.”
Between finding planets that orbit distant stars to planetary-mass objects that orbit the center of the Milky Way, astronomers are making exciting discoveries that are pushing the boundaries of what we know about planetary formation and the different types that can exist. And with next-generation instruments coming online, they plan to learn a great deal more!
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!
When searching for extra-solar planets, astronomers most often rely on a number of indirect techniques. Of these, the Transit Method (aka. Transit Photometry) and the Radial Velocity Method (aka. Doppler Spectroscopy) are the two most effective and reliable (especially when used in combination). Unfortunately, direct imaging is rare since it is very difficult to spot a faint exoplanet amidst the glare of its host star.
However, improvements in radio interferometers and near-infrared imaging has allowed astronomers to image protoplanetary discs and infer the orbits of exoplanets. Using this method, an international team of astronomers recently captured images of a newly-forming planetary system. By studying the gaps and ring-like structures of this system, the team was able to hypothesize the possible size of an exoplanet.
In the past, rings of dust have been identified in many protoplanetary systems, and their origins and relation to planetary formation are the subject of much debate. On the one hand, they might be the result of dust piling up in certain regions, of gravitational instabilities, or even variations in the optical properties of the dust. Alternately, they could be the result of planets that have already developed, which cause the dust to dissipate as they pass through it.
As Dipierro and his colleagues explained in their study:
“The alternative scenario invokes discs that are dynamically active, in which planets have already formed or are in the act of formation. An embedded planet will excite density waves in the surrounding disc, that then deposit their angular momentum as they are dissipated. If the planet is massive enough, the exchange of angular momentum between the waves created by the planet and the disc results in the formation of a single or multiple gaps, whose morphological features are closely linked to the local disc conditions and the planet properties.”
For the sake of their study, the team used data from the Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle 2 observations – which began back in June of 2014. In so doing, they were able to image the dust around Elias 24 with a resolution of about 28 AU (i.e. 28 times the distance between the Earth and the Sun). What they found was evidence of gaps and rings that could be an indication of an orbiting planet.
From this, they constructed a model of the system that took into account the mass and location of this potential planet and how the distribution and density of dust would cause it to evolve. As they indicate in their study, their model reproduces the observations of the dust ring quite well, and predicted the presence of a Jupiter-like gas giant within forty-four thousand years:
“We find that the dust emission across the disc is consistent with the presence of an embedded planet with a mass of ?0.7?MJ at an orbital radius of ? 60?au… The surface brightness map of our disc model provides a reasonable match to the gap- and ring-like structures observed in Elias 24, with an average discrepancy of ?5?per?cent of the observed fluxes around the gap region.”
These results reinforce the conclusion that the gaps and rings that have been observed in a wide variety of young circumstellar discs indicate the presence of orbiting planets. As the team indicated, this is consistent with other observations of protoplanetary discs, and could help shed light on the process of planetary formation.
“The picture that is emerging from the recent high resolution and high sensitivity observations of protoplanetary discs is that gap and ring-like features are prevalent in a large range of discs with different masses and ages,” they conclude. “New high resolution and high fidelity ALMA images of dust thermal and CO line emission and high quality scattering data will be helpful to find further evidences of the mechanisms behind their formation.”
One of the toughest challenges when it comes to studying the formation and evolution of planets is the fact that astronomers have been traditionally unable to see the processes in action. But thanks to improvements in instruments and the ability to study extra-solar star systems, astronomers have been able to see system’s at different points in the formation process.
This in turn is helping us refine our theories of how the Solar System came to be, and may one day allow us to predict exactly what kinds of systems can form in young star systems.
When we think of other planetary systems, we tend to think that they will operate by the same basic rules as our own. In the Solar System, the planets orbit close to the equatorial plane of the Sun – meaning around its equator. The Sun’s rotational axis, the direction of its poles based to its rotation, is also the same as most of the planets’ (the exception being Uranus, which rotates on its side).
But if the study of extra-solar planets has taught us anything, it is that the Universe is full of possibilities. Consider the star known as GJ436, a red dwarf located about 33 light-years from Earth. For years, astronomers have known that this star has a planet that behaves very much like a comet. But according to a recent study led by astronomers from the University of Geneva (UNIGE), this planet also has a very peculiar orbit.
GJ436 has already been the source of much scientific interest, thanks in part to the discovery that its only confirmed exoplanet has a gaseous envelop similar a comet. This exoplanet, known as GJ436b, was first observed in 2004 using radial velocity measurements taken by the Keck Observatory. In 2007, GJ436b became the first Neptune-sized planet known to be orbiting very closely to its star (aka. a “Hot Neptune”).
And in 2015, GJ436 b made headlines again when scientists reported that its atmosphere was evaporating, resulting in a giant cloud around the planet and a long, trailing tale. This cloud was found to be the result of hydrogen in the planet’s atmosphere evaporating, thanks to the extreme radiation coming from its star. This never-before-seen phenomena essentially means that GJ436 b looks like a comet.
Another interesting fact about this planet is its orbital inclination, which astronomers have puzzled over for the past 10 years. Unlike the planets of the Solar System – whose orbits are largely circular – GJ436b follows a very eccentric, elliptical path. And as the research team indicated in their study, the planet also doesn’t orbit along the star’s equatorial plane, but passes almost above the its poles.
As Vincent Bourrier – a researcher at the Department of Astronomy of the UNIGE Faculty of Science, a member of the European Research Council project FOUR ACES, and the lead author of the study – explained in a UNIGE press release:
“This planet is under enormous tidal forces because it is incredibly close to its star, barely 3% of the Earth-Sun distance. The star is a red dwarf whose lifespan is very long, the tidal forces it induces should have since circularized the orbit of the planet, but this is not the case!”
This was an especially interesting find for many reasons. On the one hand, it is the first instance where a planet was found to have a polar orbit. On the other, studying how planets orbit around a star is a great way to learn more about how that system formed and evolved. For instance, if a planet has been disturbed by the passage of a nearby star, or is being influenced by the presence of other massive planets, that will be apparent from its orbit.
As Christophe Lovis, a UNIGE researcher and co-author of the study, explained:
“Even if we have already seen misaligned planetary orbits, we do not necessarily understand their origin, especially since here it is the first time we measure the architecture of a planetary system around a red dwarf.”
Hervé Beust, an astronomer from the University of Grenobles Alpes, was responsible for doing the orbital calculations on GJ436b. As he indicated, the likeliest explanation for GJ436b’s orbit is the existence of a more massive and more distant planet in the system. While this planet is not currently known, this could be the first indication that GJ436 is a multi-planet system.
“If that is true, then our calculations indicate that not only would the planet not move along a circle around the star, as we’ve known for 10 years, but it should also be on a highly inclined orbit,” he said. “That’s exactly what we just measured!”
Another interesting takeaway from this study was the prediction that the planet has not always orbited so closely to its star. Based on their calculations, the team hypothesizes that the GJ436b may have migrated over time to become a “evaporating planet” that it is today. Here too, the existence of an as-yet-undetected companion is believed to be the most likely cause.
As with all exoplanet studies, these findings have implications for our understanding of the Solar System as well. Looking ahead, the team hopes to conduct further studies of this system in the hopes of determining if there is an elusive planetary companion to be found. These surveys will likely benefit from the deployment of next-generation missions, particularly the James Webb Space Telescope (JWST).
As Bourier indicated, “Our next goal is to identify the mysterious planet that has upset this planetary system.” Locating it will be yet another indirect way in which astronomers discover exoplanets – determining the presence of other planets based on orbital inclination of already discovered ones. The orbital inclination method, perhaps?
Red dwarf stars have become a major focal point for exoplanet studies lately, and for good reason. For starters, M-type (red dwarf) stars are the most common type in our Universe, accounting for 75% of stars in the Milky Way alone. In addition, in the past decade, numerous terrestrial (i.e rocky) exoplanets have been discovered orbiting red dwarf stars, and within their circumstellar habitable zones (“Goldilocks Zones”) to boot.
This has naturally prompted several studies to determine whether or not rocky planets can retain their atmospheres. The latest study comes from NASA, using data obtained by the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter. Having studied Mars’ atmosphere for years to determine how and when it was stripped away, the MAVEN mission is well-suited when it comes to measuring the potential habitability of other planets.
Launched in November 18th, 2013, the MAVEN mission established orbit around Mars on September 22nd, 2014. The purpose of this mission has been to explore the Red Planet’s upper atmosphere, ionosphere and its interactions with the Sun and solar wind for the sake of determining how and when Mars’ atmosphere went from being thicker and warmer in the past (and thus able to support liquid water on the surface) to thin and tenuous today.
Since November of 2014, MAVEN has been measuring Mars’ atmospheric loss using its suite of scientific instruments. From the data it has obtained, scientists have surmised that the majority of the planet’s atmosphere was lost to space over time due to a combination of chemical and physical processes. And in the past three years, the Sun’s activity has increased and decreased, giving MAVEN the opportunity to observe how Mars’ atmospheric loss has risen and fallen accordingly.
Because of this, David Brain – a professor at the Laboratory for Atmospheric and Space Physics (LASP) at the CU Boulder is also a MAVEN co-investigator – and his colleagues began to think about how these insights could be applied to a hypothetical Mars-like planet orbiting around an red dwarf star. These planets include Proxima b (the closest exoplanet to our Solar System) and the seven planet system of TRAPPIST-1.
“The MAVEN mission tells us that Mars lost substantial amounts of its atmosphere over time, changing the planet’s habitability. We can use Mars, a planet that we know a lot about, as a laboratory for studying rocky planets outside our solar system, which we don’t know much about yet.”
To determine if this hypothetical planet could retain its atmosphere over time, the researchers performed some preliminary calculations that assumed that this planet would be positioned near the outer edge of the star’s habitable zone (as Mars is). Since red dwarf’s are dimmer than our Sun, the planet would have to orbit much closer to the star – even closer than Mercury does to our Sun – to be within this zone.
They also considered how a higher proportion of the light emanating from red dwarf stars is in the ultraviolet wavelength. Combined with a close orbit, this means that the hypothetical planet would be bombarded with about 5 times more UV radiation the real Mars gets. This would also mean that the processes responsible for atmospheric loss would be increased for this planet.
Based on data obtained by MAVEN, Brain and colleagues were able to estimate how this increase in radiation would affect Mars’ own atmospheric loss. Based on their calculations, they found that the planet’s atmosphere would lose 3 to 5 times as many charged particles through ion escape, while about 5 to 10 times more neutral particles would be lost through photochemical escape (where UV radiaion breaks apart molecules in the upper atmosphere).
Another form of atmospheric loss would also result, due to the fact that more UV radiation means that more charged particles would be created. This would result in a process called “sputtering”, where energetic particles are accelerated into the atmosphere and collide with other molecules, kicking some out into space and sending others crashing into neighboring particles.
Lastly, they considered how the hypothetical planet might experience about the same amount of thermal escape (aka. Jeans escape) as the real Mars. This process occurs only for lighter molecules such as hydrogen, which Mars loses at the top of its atmosphere through thermal escape. On the “exo-Mars”, however, thermal escape would increase only if the increase in UV radiation were to push more hydrogen into the upper atmosphere.
In conclusion, the researchers determined that orbiting at the edge of the habitable zone of a quiet M-type star (instead of our Sun) could shorten the habitable period for a Mars-like planet by a factor of about 5 to 20. For a more active M-type star, the habitable period could be cut by as much as 1,000 times. In addition, solar storm activity around a red dwarf, which is thousands of times more intense than with our Sun, would also be very limiting.
However, the study is based on how an exo-Mars would fair around and M-type star, which kind of stacks the odds against habitability in advance. When different planets are considered, which possess mitigating factors Mars does not, things become a bit more promising. For instance, a planet that is more geologically active than Mars would be able to replenish its atmosphere at a greater rate.
Other factors include increase mass, which would allow for the planet to hold onto more of its atmosphere, and the presence of a magnetic field to shield it from stellar wind. As Bruce Jakosky, MAVEN’s principal investigator at the University of Colorado (who was not associated with this study), remarked:
“Habitability is one of the biggest topics in astronomy, and these estimates demonstrate one way to leverage what we know about Mars and the Sun to help determine the factors that control whether planets in other systems might be suitable for life.”
In the coming years, astronomers and exoplanet researchers hope to learn more about the planets orbiting nearby red dwarf stars. These efforts are expected to be helped immensely thanks to the deployment of the James Webb Space Telescope, which will be able to conduct more detailed surveys of these star systems using its advanced infrared imaging capabilities.
These studies will allow scientists to place more accurate constraints on exoplanets that orbit red dwarf stars, which will allow for better estimates about their size, mass, and compositions – all of which are crucial to determining potential habitability.
Other panelists that took part in the presentations included Giada Arney and Katherine Garcia-Sage of NASA Goddard Space Flight Center and Stephen Kane of the University of California-Riverside. You can access the press conference materials by going to NASA Goddard Media Studios.
The hunt for exoplanets has turned up many fascinating case studies. For example, surveys have turned up many “Hot Jupiters”, gas giants that are similar in size to Jupiter but orbit very close to their suns. This particular type of exoplanet has been a source of interest to astronomers, mainly because their existence challenges conventional thinking about where gas giants can exist in a star system.
Hence why an international team led by researchers from the European Southern Observatory (ESO) used the Very Large Telescope (VLT) to get a better look at WASP-19b, a Hot Jupiter located 815 light-years from Earth. In the course of these observations, they noticed that the planet’s atmosphere contained traces of titanium oxide, making this the first time that this compound has been detected in the atmosphere of a gas giant.
Like all Hot Jupiters, WASP-19b has about the same mass as Jupiter and orbits very close to its sun. In fact, its orbital period is so short – just 19 hours – that temperatures in its atmosphere are estimated to reach as high as 2273 K (2000 °C; 3632 °F). That’s over four times as hot as Venus, where temperatures are hot enough to melt lead! In fact, temperatures on WASP-19b are hot enough to melt silicate minerals and platinum!
The study relied on the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument on the VLT, a multi-mode optical instrument capable of conducting imaging, spectroscopy and the study of polarized light (polarimetry). Using FORS2, the team observing the planet as it passed in front of its star (aka. made a transit), which revealed valuable spectra from its atmosphere.
After carefully analyzing the light that passed through its hazy clouds, the team was surprised to find trace amounts of titanium oxide (as well as sodium and water). As Elyar Sedaghati, who spent 2 years as a student with the ESO to work on this project, said of the discovery in an ES press release:
“Detecting such molecules is, however, no simple feat. Not only do we need data of exceptional quality, but we also need to perform a sophisticated analysis. We used an algorithm that explores many millions of spectra spanning a wide range of chemical compositions, temperatures, and cloud or haze properties in order to draw our conclusions.”
Titanium oxide is a very rare compound which is known to exist in the atmospheres of cool stars. In small quantities, it acts as a heat absorber, and is therefore likely to be partly responsible for WASP-19b experiencing such high temperatures. In large enough quantities, it can prevent heat from entering or escaping an atmosphere, causing what is known as thermal inversion.
This is a phenomena where temperatures are higher in the upper atmosphere and lower further down. On Earth, ozone plays a similar role, causing an inversion of temperatures in the stratosphere. But on gas giants, this is the opposite of what usually happens. Whereas Jupiter, Saturn, Uranus and Neptune experience colder temperatures in their upper atmospheres, temperatures are much hotter closer to the core due to increases in pressure.
The team believes that the presence of this compound could have a substantial effect on the atmosphere’s temperature, structure and circulation. What’s more, the fact that the team was able to detect this compound (a first for exoplanet researchers) is an indication of how exoplanet studies are achieving new levels of detail. All of this is likely to have a profound impact on future studies of exoplanet atmospheres.
The study would also have not been possible were it not for the FORS2 instrument, which was added to the VLT array in recent years. As Henri Boffin, the instrument scientist who led the refurbishment project, commented:
“This important discovery is the outcome of a refurbishment of the FORS2 instrument that was done exactly for this purpose. Since then, FORS2 has become the best instrument to perform this kind of study from the ground.”
Looking ahead, it is clear that the detection of metal oxides and other similar substances in exoplanet atmospheres will also allow for the creation of better atmospheric models. With these in hand, astronomers will be able to conduct far more detailed and accurate studies on exoplanet atmospheres, which will allow them to gauge with greater certainty whether or not any of them are habitable.
So while this latest planet has no chance of supporting life – you’d have better luck finding ice cubes in the Gobi desert! – its discovery could help point the way towards habitable exoplanets in the future. On step closer to finding a world that could support life, or possibly that elusive Earth 2.0!
In the past few decades, the search for extra-solar planets has turned up a wealth of discoveries. Between the many direct and indirect methods used by exoplanet-hunters, thousands of gas giants, rocky planets and other bodies have been found orbiting distant stars. Aside from learning more about the Universe we inhabit, one of the main driving forces behind these efforts has been the desire to find evidence of Extra-Terrestrial Intelligence (ETI).
This method consists of astronomers observing stars for periodic dips in brightness, which are attributed to planets passing (i.e. transiting) between them and the observer. For the sake of their study, Wells and his colleagues reversed the concept in order to determine if Earth would be visible to any species conducting observations from vantage points beyond our Solar System.
To answer this question, the team looked for parts of the sky from which one planet would be visible crossing the face of the Sun – aka. “transit zones”. Interestingly enough, they determined that the terrestrial planets that are closer to the Sun (Mercury, Venus, Earth and Mars) would easier to detect than the gas and ice giants – i.e. Jupiter, Saturn, Uranus and Neptune.
While considerably larger, the gas/ice giants would be more difficult to detect using the transit method because of their long-period orbits. From Jupiter to Neptune, these planets take about 12 to 165 years to complete a single orbit! But more important than that is the fact that they orbit the Sun at much greater distances than the terrestrial planets. As Robert Wells indicated in a Royal Astronomical Society press statement:
”Larger planets would naturally block out more light as they pass in front of their star. However the more important factor is actually how close the planet is to its parent star – since the terrestrial planets are much closer to the Sun than the gas giants, they’ll be more likely to be seen in transit.”
Ultimately, what the team found was that at most, three planets could be observed from anywhere outside of the Solar System, and that not all combinations of these three planets was possible. For the most part, an observer would see only planet making a transit, and it would most likely be a rocky one. As Katja Poppenhaeger, a lecturer at the School of Mathematics and Physics at Queen’s University Belfast and a co-author of the study, explained:
“We estimate that a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet. The probability of detecting at least two planets would be about ten times lower, and to detect three would be a further ten times smaller than this.”
What’s more, the team identified sixty-eight worlds where observers would be able to see one or more of the Solar planets making transits in front of the Sun. Nine of these planets are ideally situated to observe transits of the Earth, though none of them have been deemed to be habitable. These planets include HATS-11 b, 1RXS 1609 b, LKCA 15 b, WASP-68 b, WD 1145+017 b, and four planets in the WASP-47 system (b, c, d, e).
On top of that, they estimated (based on statistical analysis) that there could be as many as ten undiscovered and potentially habitable worlds in our galaxy which would be favorably located to detect Earth using our current level of technology. This last part is encouraging since, to date, not a single potentially habitable planet has been discovered where Earth could be seen making transits in front of the Sun.
The team also indicated that further discoveries made by the Kepler and K2 missions will reveal additional exoplanets that have “a favorable geometric perspective to allow transit detections in the Solar System”. In the future, Wells and his team plan to study these transit zones to search for exoplanets, which will hopefully reveal some that could also be habitable.
One of the defining characteristics in the Search for Extra-Terrestrial Intelligence (SETI) has been the act of guessing about what we don’t know based on what we do. In this respect, scientists are forced to consider what extra-terrestrial civilizations would be capable of based on what humans are currently capable of. This is similar to how our search for potentially habitable planets is limited since we know of only one where life exists (i.e. Earth).
While it might seem a bit anthropocentric, it’s actually in keeping with our current frame of reference. Assuming that intelligent species could be looking at Earth using the same methods we do is like looking for planets that orbit within their star’s habitable zones, have atmospheres and liquid water on the surfaces.
In other words, it’s the “low-hanging fruit” approach. But thanks to ongoing studies and new discoveries, our reach is slowly extending further!
Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.
The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.
The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.
For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.
Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.
Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.
Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.
“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”
From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.
In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.
For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.
In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.
In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.
In February of 2017, a team of European astronomers announced the discovery of a seven-planet system orbiting the nearby star TRAPPIST-1. Aside from the fact that all seven planets were rocky, there was the added bonus of three of them orbiting within TRAPPIST-1’s habitable zone. As such, multiple studies have been conducted that have sought to determine whether or not any planets in the system could be habitable.
When it comes to habitability studies, one of the key factors to consider is the age of the star system. Basically, young stars have a tendency to flare up and release harmful bursts of radiation while planets that orbit older stars have been subject to radiation for longer periods of time. Thanks to a new study by a pair of astronomers, it is now known that the TRAPPIST-1 system is twice as old as the Solar System.
The study, which will be published in The Astrophysical Journal under the title “On The Age Of The TRAPPIST-1 System“, was led by Adam Burgasser, an astronomer at the University of California San Diego (UCSD). He was joined by Eric Mamajek, the deputy program scientist for NASA’s Exoplanet Exploration Program (EEP) at the Jet Propulsion Laboratory.
Together, they consulted data on TRAPPIST-1s kinematics (i.e. the speed at which it orbits the center of the galaxy), its age, magnetic activity, density, absorption lines, surface gravity, metallicity, and the rate at which it experiences stellar flares. From all this, they determined that TRAPPIST-1 is quite old, somewhere between 5.4 and 9.8 billion years of age. This is up to twice as old as our own Solar System, which formed some 4.5 billion years ago.
These results contradict previously-held estimates, which were that the TRAPPIST-1 system was about 500 millions yeas old. This was based on the fact that it would have taken this long for a low-mass star like TRAPPIST-1 (which has roughly 8% the mass of our Sun) to contract to its minimum size. But with an upper age limit that is just under 10 billion years, this star system could be almost as old as the Universe itself!
“Our results really help constrain the evolution of the TRAPPIST-1 system, because the system has to have persisted for billions of years. This means the planets had to evolve together, otherwise the system would have fallen apart long ago.”
The implications of this could be very significant as far as habitability studies are concerned. For one, older stars experience less in the way of flareups compared to younger ones. From their study, Burgasser and Mamajek confirmed that TRAPPIST-1 is relatively quiet compared to other ultra-cool dwarf stars. However, since the planets around TRAPPIST-1 orbit so close to their star, they have been exposed to billions of years of radiation at this point.
As such, it is possible that most of the planets which orbit TRAPPIST-1 – expect for the outermost two, g and h – would probably have had their atmospheres stripped away – similar to what happened to Mars billions of years ago when it lost its protective magnetic field. This is certainly consistent with many recent studies, which concluded that TRAPPIST-1’s solar activity would not be conducive to life on any of its planets.
Whereas some of these studies addressed TRAPPIST-1s level of stellar flare, others examined the role magnetic fields would play. In the end, they concluded that TRAPPIST-1 was too variable, and that its own magnetic field would likely be connected to the fields of its planets, allowing particles from the star to flow directly onto the planets atmospheres (thus allowing them to be more easily stripped away).
However, the results were not entirely bad news. Since the TRAPPIST-1 planets have estimated densities that are lower than that of Earth, it is possible that they have large amounts of volatile elements (i.e. water, carbon dioxide, ammonia, methane, etc). These could have led to the formation of thick atmospheres that protected the surfaces from a lot of harmful radiation and redistributed heat across the tidally-locked planets.
Then again, a thick atmosphere could also have an effect akin to Venus, creating a runaway greenhouse effect that would have resulted in incredibly thick atmospheres and extremely hot surfaces. Under the circumstances, then, any life that emerged on these planets would have had to be extremely hardy in order to survive for billions of years.
Another positive thing to consider is TRAPPIST-1’s constant brightness and temperature, which are also typical of M-class (red dwarf) stars. Stars like our Sun have an estimated lifespan of 10 billion years (which it is almost halfway through) and grow steadily brighter and hotter with time. Red dwarfs, on the other hand, are believed to exist for as much as 10 trillion years – far longer than the Universe has existed – and do not change much in intensity.
Given the amount of time it took for complex life to have emerged on Earth (over 4.5 billion years), this longevity and consistency could make red dwarf star systems the best long-term bet for habitability. Such was the conclusion of one recent study, which was conducted by Prof. Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA). And as Mamajek explained:
“Stars much more massive than the Sun consume their fuel quickly, brightening over millions of years and exploding as supernovae. But TRAPPIST-1 is like a slow-burning candle that will shine for about 900 times longer than the current age of the universe.”
NASA has also expressed excitement over these findings. “These new results provide useful context for future observations of the TRAPPIST-1 planets, which could give us great insight into how planetary atmospheres form and evolve, and persist or not,” said Tiffany Kataria, an exoplanet scientist at JPL. At the moment, habitability studies of TRAPPIST-1 and other nearby star systems are confined to indirect methods.
However, in the near future, next-generation missions like the James Webb Space Telescope are expected to reveal additional information – such as whether or not these planets have atmospheres and what their compositions are. Future observations with the Hubble Space Telescope and the Spitzer Space Telescope are also expected to improve our understanding of these planets and possible conditions on their surface.