We’re used to thinking about small asteroid-like bodies emitting gaseous tails: They’re called comets. But could there be such things as cometary planets? Two studies regarding the escaping atmospheres of hot exoplanets have been published this month in the journal Science (1,2). The first is a transiting warm Neptune-mass exoplanet located 20 times nearer its host star than the Earth is to the Sun. The tail of helium being blasted away from this planet by radiation from its orange dwarf host star extends some five planetary radii out. The planet, known as HAT-P-11b, is blown up like a helium balloon, according to the researchers who have been studying it (3).
However, HAT-P-11 is not a young star still blasting away at the primordial atmosphere of a new Neptune-sized world, as you might expect. Instead, HAT-P-11 is 6.5 billion years old; almost 2 billion years old than our own Sun. So, why is it still managing to have such a devastating effect upon the Neptune-sized exoplanet in its midst? Common sense would dictate that you can’t have such an effect going on for 6.5 billion years, as the planet would have been eradicated long ago. Loosely bound helium held in this gaseous ‘envelope’ would surely leak out into space in considerable quantities over time? Like with comets repeatedly transiting around their stars at perihelion, you would think that at some point the volatile gases would all get blown away. Perhaps HAT-P-11b was once a much greater hot Jupiter world which has shrunk to Neptune proportions over time. Or, perhaps this is a case of inwards migration of this world from further out in the star system.
A similar tail of atmospheric helium is being blasted back from another giant exoplanet, this time known as WASP-69b (5). This world is about a quarter of the mass of Jupiter, making it a sub-‘hot Jupiter’ object. The discovery of this helium tail was also made using the Carmenes instrument, installed on the 3.5-meter telescope of the Calar Alto Observatory in Spain. In this case, the host star, is 2 billion years old and just a little smaller than our own Sun (6), making it another orange dwarf. So, again, this effect isn’t due to the young nature of the system – there is a sustained effect taking place over a long period of time if the system has been static for that entire time. How long can such helium leakage be sustained from this ‘evaporating exoplanet’? Previous discoveries of such leaking exoplanets have included a ‘hot Neptune’ planet whizzing around a ~9 billion year old red dwarf, Gliese 436 (7). Read More…
The shard-like asteroid from deep space which shot through the solar system last years, known as ‘Oumuamua, set many an astronomer’s heart racing. The peculiar body was determined to be the first confirmed interstellar asteroid to have been observed (1). It’s possible, though, that other comets which pursue so-called hyperbolic orbits (moving fast enough to escape the solar system) also have an interstellar origin, rather than having originated from the Oort Cloud. A team of Spanish astrophysicists, who have more than a passing interest in the topic of Planet X, have performed powerful computer simulations to build up a picture of the trajectories and spatial origins of various hyperbolic comets (2). The objects they chose to consider have inbound velocities greater than 1km/s
Following adjustment for the Sun’s own movement through space towards the Solar Apex, interstellar visitors would likely have a more or less random distribution to their radiants (the position in the sky from which they came, rather like meteor showers striking the Earth’s atmosphere). The Spanish team carried out statistical analysis on the emerging sky maps of these radiants, and looked for patterns or clusters of these origin points. Statistically significant patterns did indeed emerge from the data. A particularly large source was located in the zodiacal constellation Gemini. Such a clustering might indicate a number of possibilities, which the astrophysicists explore in their paper.
One possibility is a close flyby of a star in the past which could have disrupted the outer edges of the distant Oort Cloud, sending comets in-bound towards the Sun. Looking at the tracking of candidate flybys in the (by Cosmic standards) relatively recent past, Carlos de la Fuente Marcos, Raul de la Fuente Marcos & S. J. Aarseth argue that there is a possible correlation between this cluster of hyperbolic orbit radiants in Gemini, and a close flyby of a neighbouring binary red dwarf system known as Scholz’s star some 70,000 years ago (2). At a current distance of about 20 light years, Scholz’s star may be a close neighbour to the Sun relatively speaking, but even so it took a while for it to be discovered. This was probably because of a combination of factors: Its proximity to the Galactic plane, its relative dimness, and its slow relative movement across the sky (3). Its distance was less than a light year 70,000 years ago, and its rapid movement away from us in the intervening time helps to explain why it was difficult to detect as a neighbouring binary star: Its retreating motion is mostly along our line of sight, making it difficult to differentiate from background stars. Read More…
Brown dwarfs are notoriously hard to find. It’s not so bad when they are first born: They come into the Universe with a blast, shedding light and heat in an infantile display of vigour. But within just a few million years, they have burned their available nuclear fuels, and settle down to consume their leaner elemental pickings. Their visible light dims considerably with time to perhaps just a magenta shimmer. But they still produce heat, and the older they get, the more likely that a direct detection of a brown dwarf will have to be in the infra-red spectrum.
This doesn’t make them much easier to detect, though, because to catch these faint heat signatures in the night sky, you first need to have a cold night sky. A very cold night sky. Worse, water vapour in the atmosphere absorbs infra-red light along multiple stretches of the spectrum. The warmth and humidity of the Earth’s atmosphere heavily obscures infra-red searches, even in frigid climates, and so astronomers wishing to search in the infra-red either have to build IR telescopes atop desert mountains (like in Chile’s Atacama desert), or else resort to the use of space-based platforms. The downside of the latter is that the telescopes tend to lose liquid helium supplies rather quickly, shortening their lifespan considerably compared to space-based optical telescopes.
The first major sky search using a space telescope was IRAS, back in the 1980s. Then came Spitzer at the turn of the century, followed by Herschel, and then WISE about five years ago. Some infra-red telescopes conduct broad searches across the sky for heat traces, others zoom in on candidate objects for closer inspection. Each telescope exceeds the last in performance, sometimes by orders of magnitude, which means that faint objects that might have been missed by early searches stand more of a chance of being picked up in the newer searches.
The next big thing in infra-red astronomy is the James Webb Space Telescope (JSWT), due for launch in Spring 2019. The JSWT should provide the kind of observational power provided by the Hubble Space telescope – but this time in infra-red. The reason why astronomers want to view the universe in detail using infra-red wavelengths is that very distant objects are red-shifted to such a degree that their light tends to be found in the infra-red spectrum, generally outside Hubble’s operational parameters (1). Essentially, the JWST will be able to see deeper into space (and, therefore, look for objects sending their light to us from further back in time when the first stars and galaxies emerged). Read More…
Jupiter, the solar system’s largest planet, is turning out to be as majestic as its ancient name implies. High definition images taken of its poles, transmitted back to Earth by the space probe Juno, show a vibrant, churning cloudscape which appear to have been artistically generated in oils (1). The gnarly appearance of the storms and tempests which are woven into this mind-blowingly immense vista seem peaceful enough from space, but the ferocity of their winds can only be imagined. Although the colours have been enhanced to a certain extent artificially (2), Juno’s imaging equipment has captured the incredibly beautiful blue colours of the polar zones and the immense set of storms swirling within.
(Image credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt/Seán Doran)
These dramatic regions contrast strongly with the generally dull series of beige bands wrapping around the more familiar equatorial region (although Juno has also allowed us to better appreciate the intricate patterns of these banded zones, too) . These blues are more reminiscent of the ice giants Neptune and Uranus, and perhaps even of Earth – although the constituent gases of the atmospheres of these worlds can differ significantly from Jupiter’s. which give the clouds. The different colours and properties of Jupiter’s clouds can be attributed to their constituent gases – mostly hydrogen and helium, but also water, ammonia, methane and sulphur.
It seems to me that the solar system is starting to come to life – not in the way of biological life, although that may yet come to be, but instead in terms of our appreciation of its rich complexity and visuality. The Pioneer and Voyager space probes provided what were incredible images of the outer solar system planets back in their day. But limitations in the image-capturing technology also created a sense in those images of dull uniformity.
In the decades before the space-probe images had been sent back from the outer solar system, sci-fi writers, film-makers and scientists had created an amazing array of ideas about what these worlds might be like. This potential had become ingrained within the public collective consciousness, and to some extent helped drive NASA’s ambitious space programme forward. This was enhanced by a sense of mystery – and a hope of alien life. However, the images returning to our television screens in the latter part of the 20th century clearly did not do these worlds justice. So, although obtaining the planetary images were astonishing achievements in themselves, the disappointing lack of features within them dashed many hopes, and provided the public with a new view of the outer solar system. Like lifeless Mars and overheated Venus, the outer solar system consisted of a rather mundane set of giant planets distinctly lacking in the vibrant complexity of our own Earth. Read More…
For some time, astrophysicists have argued over how many Dark Stars there might be in the galaxy, with varying opinions. (Note that astronomers use several different names for these objects: sub-brown dwarfs, Y Dwarfs, ‘planemos’). In this short article, I argue that new evidence presented about the stellar populations of open star clusters point towards there being more Dark Stars than stars in our galaxy.
When I use the term ‘Dark Star’ in my book (1) and internet articles, I’m generally referring to gas giant planets/ultra-cool dwarf stars which are several times more massive than Jupiter, up to perhaps ~13 times as massive (at this point, the gas giant begins to burn deuterium and is reclassified as a brown dwarf). Most examples of these objects (perhaps more than a few million years old) are essentially dark. By contrast, very young examples light up more brightly, because they still retain some heat from their formation. It’s a curious quirk of nature that these sub-brown dwarfs are actually smaller in size than Jupiter, despite being heavier. Because these objects are so small, and so dim, they are extraordinarily difficult to observe. Some have been found, but they are usually either extremely young (and therefore still burning brightly), or are exoplanets discovered orbiting parent stars (and so detectable through gravitational ‘wobble’ effects, or other means of finding massive exoplanets).
It has been my contention for some time that the populations of these objects are significantly underestimated. It is recognised generally that these ultra-cool dwarf stars may be free-floating objects in inter-stellar space, often as a result of having been ejected from young star systems as the fledgling planets in those systems jostle for position. Opinions about their numbers vary greatly among astrophysicists. There may be twice as many of these objects as stars, according to studies involving gravitational microlensing surveys of the galactic bulge (2). Other studies conflict with this conclusion, arguing that there may be as few as 1 object of 5-15 MJup size per 20-50 stars in a cluster (3). This discrepancy is important because the difference is perhaps as high as two orders of magnitude, and this ultimately affects our understanding of how many free-floating Dark Stars we can expect to find out there.
Their mass, lying between that of Jupiter and the deuterium-burning limit at about 13 MJup (4) seems to single Dark Stars out as rather special objects:
“An abrupt change in the mass function at about a Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.” (2)
Therefore, if the number of free-floating sub-brown dwarfs (also sometimes known as “planemos”) is on the high end of expectation, then it means that there are also likely to be far more of these objects in wide, distant orbits around their parent stars. This, in turn, increases the likelihood of there being a similar Dark Star object (or more) in our own immediate solar neighbourhood. Read More…
NASA made a big announcement this week about new exoplanets found orbiting the dwarf star TRAPPIST-1 some 39 light years away. I’ve discussed this particular dwarf star system before (1), as it was already known to have three terrestrial planets in attendance orbiting very close to this cool, fairly dim star (2,3). The dwarf star is approximately one tenth the size of the Sun, and it’s mass places it on the border between a brown dwarf and a red dwarf star. Unusually for a star this small, TRAPPIST-1 has a high metallicity, which actually exceeds that of the Sun (4).
Now, an international team of astronomers, using the Belgian TRAPPIST telescope in Chile and the Spitzer infra-red space telescope, have released details about a further four terrestrial planets in this mini-star system, three of which (e, f and g) are located within it’s habitable zone, where temperatures favour the presence of liquid water (5):
“Researchers led by Michaël Gillon, of the University of Liège in Belgium, have been studying the infrared light emitted by this miniature star and have detected drops in luminosity characteristic of transits, i.e. the passage of astronomical bodies moving across its face. As early as 2015, the first three planets (dubbed b, c and d) had been identified. Tracking the system using TRAPPIST and the space telescope Spitzer, the team was then able to identify four others planets (e, f, g and h) in 2016. Based on the frequency of these transits and the degree of reduction in luminosity of the star, they have demonstrated that these seven planets are all comparable in size to Earth (to within 15%), and orbit very close to their star.” (6)
I’ve spent many years extolling the virtues of life on a cold brown dwarf moon. Similar to the Galilean moons of Jupiter, a moon orbiting a sub-brown dwarf would be warmed internally by the tidal forces generated by its proximity to such a powerful gravitational force. Additionally, the sub-brown dwarf itself might provide some local heating, or at least an abundance of charged-particle strewn local magnetic fields to energise the sub-stellar environment. So, a habitable environment on a moon seems a likely scenario. If a cold, dark sub-brown dwarf were to be found orbiting the Sun at a great distance, then it neatly provides the grounding for extraterrestrial life on our doorstep (1).
This seems to me to be the simplest scenario for life in a sub-brown dwarf system. There are complexities – tidally-locked moons (2), lack of light, and so on. But the basics are there.
Another exotic possibility is that the sub-brown dwarf itself might harbour life. The complex cloud systems in these failed stars can contain layers which are at room temperature, and abundant in water and other chemical goodies which could form the building blocks of life. A team of astronomers from Edinburgh University have been considering this very point, wondering whether very simple life might be able to get going in the clouds of a cold brown dwarf (3). This life might arise in two ways – either somehow evolving from scratch in the cloud environment, or originally being seeded into it by an impacting asteroid or comet. Either way, conditions for life might be good, except for the lack of a solid surface to dwell on:
“Floating out by themselves in the Milky Way galaxy are perhaps a billion cold brown dwarfs, objects many times as massive as Jupiter but not big enough to ignite as a star. According to a new study, layers of their upper atmospheres sit at temperatures and pressures resembling those on Earth, and could host microbes that surf on thermal updrafts...Observations of cold brown dwarf atmospheres reveal most of the ingredients Earth life depends on: carbon, hydrogen, nitrogen, and oxygen, though perhaps not phosphorous.“ (4)
These ideas build upon work done by the late, great Carl Sagan (with his Cornell colleague E. E. Salpeter) on the potential for life in the clouds of the gas giant Jupiter, first considered back in the 1970s (5). They envisioned giant ‘floaters’ filled with hydrogen bobbing through the Jovian atmosphere, tiny ‘sinkers’ and self-propelled ‘hunters’ which had evolved from the lazy floaters (6). All very speculative, but presented in Dr Sagan’s inimitably compelling fashion. Read More…
Not so long ago, brown dwarfs (failed stars caught in an awkward in-betweener zone between stars and planets) were hypothetical bodies. Their low stellar masses allow for only a very short period of light-emission in their early years, after which they cool and darken considerably.
“[A] brown dwarf has too little mass to ignite the thermonuclear reactions by which ordinary stars shine. However, it emits heat released by its slow gravitational contraction and shines with a reddish colour, albeit much less brightly than a star.” (1)
It was recognised early on that if they existed at all, they would be very difficult to spot – and so it proved. In recent years, the ability to detect these objects has improved considerably, including more effective infra-red sky surveys. As they have become more common, the frontier of sub-stellar bodies has dropped in mass into the ultra-cool stellar bodies known as sub-brown dwarfs – many of which would equally properly be designated as rogue gas giant planets. These objects tend to have masses below 13 times that of Jupiter (13Mj) (2). These objects have always interested me greatly, and very early on in my own research efforts I was advocating the potential importance of sub-brown dwarfs in the hunt for additional planets orbiting our own Sun at great distances (3). I used the term ‘Dark Star’ to describe these ultra-cool objects; a term suggested by a friend of mine. Some can be found orbiting stars (usually beyond 50AU) while others are free-floating entities in their own right.
A couple of brown dwarfs have been discovered in a close binary system some 240 light years away, whose two stars circle each other at a distance of about 19AU, similar to that of Uranus around the Sun. The two new exoplanets orbit close to the primary Sun-like star HD 87646 (1). These two sub-stellar companions are HD 87646b, which is a minimum 12MJupiter sub-brown dwarf (a ‘hot Jupiter’-type exoplanet) orbiting every 13 days just 0.117AU from the star (2); and HD 87646c, which is a 57MJupiter brown dwarf circling the star every 673 days (1). The orbital eccentricity of the brown dwarf is greater than that of the inner sub-brown dwarf, which is in keeping with other observations of brown dwarfs orbiting stars.
Image Credit: Janella Williams, Penn State University
The international team that discovered this remarkable system is perplexed as to how it might have come about:
“Given the fact that HD 87646 is the first known system to have two massive substellar objects orbiting a star in a close binary and the masses of the two objects are close to the minimum masses for burning deuterium and hydrogen, these peculiarities raise questions about the system’s formation and evolution.
“”The large masses of these two substellar objects suggest that they could be formed as stars with their binary hosts: a large molecular cloud collapsed and fragmented into four pieces; the larger two successfully became stars and formed the HD 87646 binary, and the other smaller ones failed to form stars and became the substellar objects in this system. This scenario might be relevant for the binary stars but seems problematic for the two substellar objects on orbits within one AU because it is unclear whether fragmentation on such a small scale can occur,” the paper reads (1)
“Other hypothesis offered by the scientists is that the two newly discovered giant objects were formed like giant planet in a protoplanetary disk around HD 87646A. However, they added that such massive disks are rare in close binaries, and further investigation is needed to confirm this explanation.” (3)