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Elizabeth Tasker
Astrophysicist. Writer. Cat lover. Tea drinker. (Order may inaccurately reflect applitude)
Astrophysicist. Writer. Cat lover. Tea drinker. (Order may inaccurately reflect applitude)
Elizabeth's posts

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I was on the Astrophiz podcast this week, talking about the Trappist-1 Earth-sized planets, the problems with talking about habitability and JAXA's MMX mission to the moons of Mars. Thanks +brendan o'brien!

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Shouting about 'Earth 2.0' really undermines the true excitement of the TRAPPIST-1 system -- and that is, the planets may have a completely different formation history to our own.

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Seven (TRAPPIST-1 planets) for the Dwarf-lords in halls of stone

“Hey Elizabeth, do you know what the NASA press conference is about tomorrow?”

I hadn’t a clue. Having stepped off a plane from Japan the night before, I was twirling around in a swivel chair in one of the student offices at McMaster University while I tried to bully my brain into action. Until that moment, I wasn’t aware NASA had announced a press conference.

The NASA site did not reveal much. Tomorrow’s event was to “present new findings on planets that orbit stars other than our sun.” It was exoplanet news, but the lack of details left us speculating.

“It’s an atmosphere detection for Proxima Centauri-b!”

“Can’t be. The planet doesn’t transit.”

This fact made our nearest exoplanet something of a disappointment. Proxima Centauri-b had been found by detecting the slight wobble in the position of the star due to the planet’s gravity. However, without an orbit that took the planet between star and Earth, there was no opportunity to examine starlight passing through the planet’s atmospheric gases. Such a technique is known as ’atmospheric spectroscopy’ and can uncover which molecules are in the air to reveal processes that must be occurring on the planet’s surface — the location relevant to habitability. The next generation of telescopes including NASA’s JWST and ESA’s Ariel are focussed on using this method to finally probe planet surface conditions. The uselessly orientated orbit of Proxima Centauri-b however, removes it from the target selection lists.

This took us back to the problem of what NASA were about to announce.

“It can’t just be another planet.”

“It could be a possible biosignature?”

“… do we have anything that could measure that yet?”

This was the crux of the mystery. It is amazing that in the scant 25 years since the first exoplanet discoveries, finding a new world beyond our solar system has become insufficient to warrant a press conference. We now know of nearly 3,500 exoplanets, roughly a third of which are less than twice the size of the Earth. The news had to be bigger than a simple additional statistic.

However, a discovery of alien life seemed to be too premature. It is true that the presence of biological organisms may be detected by their influence on a planet’s atmosphere. It is also true that the Hubble Space Telescope (HST) can do atmospheric spectroscopy, although not nearly at the resolution of the future instruments. As far as I am aware, HST has examined the atmosphere of three super Earth-sized planets and only seen features in 55 Cancri-e, which orbits so close to its star that a year is done in hours. So … a biosignature was not impossible. It just would have meant we had got very very very very very lucky.

Nobody’s that lucky. Especially not in 2017.

We were evidently not alone in our speculation, since the news was leaked later that day. Seven Earth-sized planets had been discovered orbiting the ultracool dwarf star, TRAPPIST-1. It was a miniature solar system and NASA were about to infuriate me by gabbling non-stop about the prospect of life.

Let’s make something clear:

Apart from roughly the same number of planets (by astronomer standards, 7 basically equals 8. Or 9.) the TRAPPIST-1 system is very unlike our own.

That is what makes it cool.

Also, the system takes its name from a Belgian beer.

Last year, three planets were discovered around TRAPPIST-1. The star was named for the telescope that was used in the discovery, the robotic Belgian 60cm ‘TRAnsiting Planets and Planetesimal Small Telescope’. It sounds like a perfectly reasonable acronym until you learn that Trappist is a Belgian brewing company. Astronomers have no shame. It’s all kinds of wonderful.

The news was that further inspection of the system had added another four planets. The fresh observations had used a number of telescopes around the world and finished with an intensive stint on NASA’s infrared Spitzer Space Telescope.

(Interesting fact: Launched in 2003, Spitzer was never designed to be able to see planets. Some swanky engineering tricks from the ground allowed a 1000 times improvement in measuring star brightness that led to the tiny dip from a transiting planet being detectable. Cool stars like TRAPPIST-1 are a 1000 times brighter in the infrared than at optical wavelengths, making Spitzer a kick-ass planet grabbing machine.)

What was still more exiting is that all the planets transit, leaving the door wide-open for some rocky planet atmosphere spectroscopy rock n’ roll.

Were alien climates ammonia cloudy with a chance of methane meatballs? The next five years might reveal the answer to that question.

The planets were all on short orbits, with years lasting between 1.5 and 13 days. This close packed system meant that neighbouring planets would appear larger than the Moon in the night sky. The in-your-face sibling-ness also allowed for the planet masses to be measured.

While transit observations normally yield only the planet radius, the gravitational tugs from planets in the same system can vary the time between successive transits. These ‘transit timing variations’ can be used to estimate the size of the tug, and thereby measure the mass of the planets. With the exception of the outermost planet —whose single transit measurement is only enough for a radius estimate— the TRAPPIST-1 planets got both radius and mass measurements.

And you know what that means.

(Density. It means density.)

In fact, the mass measurements were not particular accurate, leading to error bars as large as the measured value except in the case of planet TRAPPIST-1f. However, all measurements hinted at (and Ms Accurate TRAPPIST-1f agreed) that these planets were on the fluffy side.

With sizes less than the empirical threshold value 1.6 Earth radii, the planets were unlikely to be Neptune-like gas worlds. But their low density suggests they do have a much higher fraction of volatiles than the Earth. They could even be downright watery.

This possibility is backed up in a less obvious way by the planet orbits. The inner six worlds are in resonance, meaning that the ratio between their orbital times can be expressed as two small integer numbers. So while the innermost world orbits the star 8 times, the outer planets orbit 5, 2 and 2 times.

Well… almost. And since we declared above that 7 basically equalled 8 or 9, I’d say we were good.

Strings of planets in resonance are completely unsurprising and utterly predictable.

… so long as you formed somewhere entirely different.

Resonant orbits between neighbouring planets occur when young planets migrate through the planet-forming gas disc. This gas migration can occur once the growing planet reaches the size of Mars and its gravity begins to pull on the surrounding gas, which pulls back. The net force usually sees the planet move towards the star. If multiple planets take this site-seeing tour of their system, their mutual gravity will pull on one another. These tugs only balance out when the orbital times form integer ratios, producing a resonance. The predicted result is a series of planets in resonant orbits close to the star — exactly what is seen in the TRAPPIST-1 system.

If the planets formed in cold outer reaches far from the star, then a substantial part of their mass would be in ice. As the planets moved towards the (ultracool but still a nuclear furnace and way hotter than Colin Firth in Pride and Prejudice) star, the ice would melt into water or vapour. This would explain the low densities compared to the Earth’s predominantly silicate composition.

Three of the TRAPPIST-1 planets stopped their mooch inwards within the star’s temperate zone (or ‘habitable zone’ if you must). This is the region around a star where an exact Earth clone could support liquid water on the surface.

Once more for the cheap seats at the back?


If you’re not an exact Earth clone, then the temperate zone guarantees as much as one of Nigel Farage’s Brexit bus adverts.

So how Earth-like are these temperate zone wannabes? On the plus side, they likely have plenty of water. On the down side, it’s quite likely too much.

While the majority of the Earth’s surface is covered with oceans, water makes up less than 0.1% of our planet’s mass. If we had formed further out where water freezes into ices (i.e. past the ‘ice line’), then that fraction could be nearer 50%. This would create huge oceans as the planet warmed, enveloping all land under a sea a bajillion fathoms deep (exact measurement. Prove me wrong.)

The bottom of such a monstrous ocean would be so high pressure than a thick layer of ice would separate the water from the rocky core. This would scupper the carbon-silicate cycle, preventing the quantity of carbon dioxide in the air responding like a thermostat to global changes in climate. This would mean anything other than the absolute perfect amount of stellar heat would render the planet uninhabitable. The temperate zone would shrink to a thin slice and any slight ellipticity in the planet orbit, or variation in the star’s heat, would fry or freeze everything in site.

It ain’t impossible for life, but it ain’t promising. It also ain’t Earth.

Even if the oceans were shallow enough to avoid this, the icy composition of the planet might burp out a crazy atmosphere. Our atmosphere was outgassed in volcanic eruptions during the Earth’s early years. But if the planet was made not of silicates, but of comet-like ices, then the gasses emerging from the volcanos would likely be mainly ammonia or methane. Not yummy. Also strong greenhouse gases, so could end up roasting planets within the temperate zone.

Since we’ve no analogue of such planets in our own solar system, it’s hard to speculate on their surface conditions. Could such a rocky ice mix produce a magnetic field? The icy Jovian moon, Ganymede, has a weak field, so it could be possible. If it is not, then any atmosphere might be stripped by the star’s stellar wind.

The fact we’ve not the foggiest idea of what these worlds would really be like is why they’re so exciting. Here we have 7 prime candidates for atmospheric studies and we’re hoping to see not the same thing as beneath our feet, but something entirely new. This would tell us about how planets form (really migration? really ices?) to how a completely non-terrestrial geology behaves. It’s going to be so much more awesome Ewoks.

So are we going to give these planets better names than TRAPPIST-1b, c, d, e, f and h? Speaking at the NASA press conference, lead author Michaël Gillon admitted,

“We have plenty of possibilities that are all related to Belgian beers, but we don’t think that they’ll become official!”


In better news, NASA has designed a new travel poster to mark the occasion. And there’s a google doodle. Yay.

#exoplanets #trappist-1 #it'satrap

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Media, get your shit together and read

My weekend slid downhill when I began an article that started:

"The discovery of alien life could be a step closer after scientists found a newly discovered planet is ‘likely' to harbour life forms."

My friends, that be pretty big talk for a planet for which we only know the minimum mass.

The planet in question is Proxima-b, whose discovery around our closest star was announced in August. You may remember its name from my previous editions of OMG-PLANET-NEWS-GET-UR-SHIT-TOGETHER:

This article (in the UK newspaper, the Independent) covered recent research published in the scientific journal, MNRAS. Unfortunately, it represents the work more poorly than the Hollywood adaption of your favourite novel. One you really, really liked.

Now, I too find reading research papers a drag: they’re dry, gloss over the exciting scrumptious bits in favour of a parameter space study and sometimes the graphs aren’t even in colour. But this particular paper was less than five pages. FIVE. And that includes the plots. And those five pages do not discuss the chances of Proxima-b being inhabited by anything.

Let’s take a look at “In innards of Proxima-b: how the movie differs from the book".

The research paper asks a simple question: If we assume Proxima-b is a rocky planet, what might it be like?

Did you notice that summary started with an “if”?

Because Proxima-b has not been observed passing in front of its star, we don’t know the planet’s radius. Instead, astronomers have measured the slight wobble in the star’s position due to the tug from the planet’s gravity. This tells us how much the planet is pulling the star towards the Earth and that gives us a handle on its mass. However, since we don’t know the angle of the planet’s orbit, we can’t tell whether the planet is pulling the star directly towards the Earth, or if only part of its tug is in our direction. The upshot is we know only the lowest possible mass for the planet, with its true value being potentially much higher.

Proxima-b’s minimum mass is ~1.3 Earth masses; a value that suggests (but still doesn’t guarantee) a rocky surface. The maximum value would make the planet a gas giant such as Neptune.

For this paper, the researchers are only interested in the outcomes for a rocky composition. This leads them to consider only the follow situation:

(1) The MINIMUM MASS of the planet is the TRUE mass. Since every measurement has errors, they actually consider the planet has a mass between 1.1 Earth masses - 1.46 Earth masses.

(2) The planet has a thin Earth-like atmosphere, not a thick envelope like Neptune.

(3) The rock composition is similar to that found in the solar system, with the planet having an iron core, silicate mantle and ice or water top layer.

There is no observational evidence at all for any of these points. A familiarly solid base for the planet is assumed, and then the research asks what permutations are possible. To say this suggests Proxima-b is like Earth is akin to filling a pen with red ink and then claiming this proves all pens write in red. It’s nonsensical and it’s not the point of the paper.

The paper considers three possible masses for Proxima-b, within the error bars that surround the minimum possible value: (1) 1.1 Earth mass planet, (2) 1.27 Earth mass planet and (3) 1.46 Earth mass planet. The authors then tweak the relative amounts of core, mantle and water to see what worlds result.

To put limits on the possibilities, the research assumes a mix of silicate, iron and water typical of planets, asteroids and comets found in the solar system. Planet models that have more water than most solar system objects, or huge cores are dismissed as implausible.

The authors placed down these boundaries as the solar system is only place where we have data on what ranges are reasonable. However, Proxima-b’s star is not like our sun. Instead, it’s a dim red dwarf with a different mix of elements. It could therefore be that the rocks available to build planets have a very different blend than those around our own sun. Such differences can lead to drastic changes in planet conditions, such as producing carbon worlds with diamond mantles and seas of tar.

However —again— we have to work with the data we have. Which is very little for the Proxima system.

The result of the paper is not a single favoured model, but a range of possibilities for a rocky Proxima-b. A Proxima-b with a 1.1 Earth mass but radii between 1.2 - 1.3 Earth size could contain 60 - 70% water, compared to our own Earth’s minute 0.05%. On the other hand, an Earth-sized planet of that mass could contain no water but a fat iron core. The total composition range (for conditions 1 - 3 above) is a planet made from 65% iron / 35% rocky silicates (matching a radius of 0.94 x Earth) to a 50% silicate / 50% water world (radius 1.4 x Earth), with 200 km deep liquid ocean.

While the inspiration of the paper was Proxima-b, there’s nothing really particular about this calculation that applies only to this planet. The results are true for any world around 1.3 Earth masses.

Should the radius of Proxima-b ever be measured, these models could help narrow down possible planet conditions or even rule out the planet being rocky at all. However, it’s worth noting that even for an exact radius and mass, different combinations of water, silicate and iron are still possible. At present, there is no way of selecting a more probably model amongst any of the options.

So do these possibilities say anything about habitability? Not a jot.

Should water be present, life would get a helpful medium for some biochemistry action. But this is only one of many many (many many) factors. The changing iron core size is liable to affect the magnetic field; a likely essential component of any planet orbiting a red dwarf. These dim stars may sound benign, but they are prone to violent outbursts of energy that could strip a planet’s atmosphere without the protection from some heavy duty magnetics. A thick mantle will have a baring on plate tectonics and is liable to determine the gases in the atmosphere. The deep water world may also have a thick layer of ice that cuts off the silicon surface from the ocean, preventing a carbon-silicate cycle of elements that helps control planet temperature on Earth.

We cannot be anymore quantitive about these properties, since we don’t know the surface conditions on any exoplanets. The next generation of instruments are just beginning to be able to sniff the atmospheres of these new worlds. This may provide us with the first clue of what the surfaces could be like.

This paper was a neat modelling experiment that drives home how varied a planet could be, even with a huge number of assumptions. So how did the news article get the message quite so wrong?

My guess is that the writer did not read the journal paper at all (despite quoting it as the source) but took the information from the very beginning of the press release by CNRS: the ‘Centre National de la Recherché Scientifique’ in France, and home institute of the research authors.

The press release overall isn’t bad, but the opening paragraphs are misleadingly phrased and contain the statement; “[Proxima-b] is likely to harbour liquid water at its surface and therefore to harbour life forms."

No dude, that just ain’t true. Water is a possibility for the planet’s composition, but the research doesn’t promise that any is there.

Taking this as the full research, the news article then quotes the lead author seemingly collaborating this statement. While it’s hard to know without hearing the interview verbatim, I suspect this is an example of poor editing. The author apparently told the newspaper:

"Among the thousands of exoplanets we have already discovered, Proxima-b is one of the best candidates to sustain life."

With only the minimum mass measured, there’s no reason Proxima-b is more likely to harbour life than many other exoplanet discoveries. However, it is true the proximity of the planet makes it an excellent candidate for more detailed observations.

"It is in the habitable zone of its star, [and] even if it is really close to the star the fact that Proxima Centauri is a red dwarf allows the planet to have a lower temperature and maybe liquid water."

Note, the author said “maybe” here. Like the Earth, it is unlikely that Proxima-b formed with liquid water: its location close to the star would have been too warm for ice to be incorporated into its body. Instead, the planet would need to form further out and move inwards, or receive a delivery of icy meteorites from further from the star. Both are possible; neither are certain.

"The fact there could still be life on the planet today, not only during its formation, is huge."

I guess this is true, but its unsubstantiated based on the research paper, so I don’t understand the motivation behind the comment. I’m inclined to blame selective editing once again.

"The interesting thing about Proxima-b is it is the closest exoplanet to Earth. It is really exciting to have the possibility that there is life just at the gates of our solar system."

Yes. This is the main reason Proxima-b is exciting. We don’t yet know if the planet is rocky. We certainly don’t know if its surface conditions are similar to Earth. But while the world is too far to visit with current technology, its relative proximity gives the next generation of telescopes the best chance at finding out more.

Research paper: Brugger, Mousis, Deleuil, Lunine, 2016

#Proximab #Exoplanets #journaljuice


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I'm on the Astrophiz podcast talking about my career and the planet discovery around Proxima Centauri!

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Yes, we've discovered a planet around our nearest star… but let's not lose our shit.

The third brightest star in the night sky is Alpha Centauri. It is our closest stellar neighbour, the fictional birth system of the Transformers, Small Furry Creatures and Pan Galactic Gargle Blasters… and now our closest exoplanet.

Alpha Centauri is actually a triple star system. It consists of a binary pair, Alpha Centauri A and B, and a more distant dwarf star called Proxima Centauri. It is around this dim third wheel that a planet has been detected.

The discovery has exploded my social media feeds. The new world has a minimum mass 30% larger than the Earth and receives a comparable amount of light and heat. Anyone familiar with exoplanet news knows this is sufficient to start packing a suitcase and buying shares in the inevitable Really-Star-Bucks coffee franchise.

Anyone familiar with my feed knows I am about to ice bucket challenge this baby.

To be fair, this is exciting. It's really exciting. In fact, I'm so excited I've ditched my morning chore of cleaning my newly empty apartment to sit on a cardboard box and write this post [+].

Let's start by disembowelling the description of those planet properties. Proxima Centauri b has been found by the Radial Velocity Technique: this is the slight wobble in the star's motion due to the pull from the planet's gravity. The bigger the wobble, the stronger the gravity of the planet and thus, the more massive the planet. All true, but it runs us into our first caveat:

CAVEAT #1: We only know the minimum mass

We can only measure the star's motion directly towards the Earth. This means we only see part of the planet's effect on the star.

This is like trying to judge how far a hot air balloon has moved by looking at its shadow. You'll probably underestimate the distance travelled, because the balloon has moved upwards (causing no change in its shadow position) as well as horizontally. You might think the balloon didn't need much gas to move such a short distance, but actually it needed a lot of fuel to climb vertically. Likewise, the Proxima Centauri's planet might be much more massive but most of its force is pulling the star 'upwards' compared to our line of sight.

How close our measurement of 1.3 Earth masses is to Proxima Centauri b's true mass depends on the orientation of its orbit around the star. If we're looking at the orbit exactly edge on, then 1.3 Earth masses is the true value. If it's nearer to face-on, then the mass could be 70 times the mass of the Earth; the regime of the gas giants. If we assume the orientation is completely random, then the planet is most likely to be about 2.6 Earth masses.

So… what does this mean?

To gain even a rough inkling about what Proxima Centauri b is really like, we need its density. A high density would indicate a world with a solid surface, while a low density would suggest a Neptune-like gas giant. For density, we need size.

For all those headlines out there that have been proclaiming "Earth-sized planet discovered!" — be ashamed. We don't know jack about Proxima Centauri b's dimensions. This means the planet could be a rocky super Earth, or a gaseous Neptune.

However… if we were to take a guess… a rocky planet is likely. There is empirical evidence that planets smaller than about 1.5 x Earth size are more typically rocky than gassy. This boundary corresponds to a planet mass of roughly 4.5 Earth masses, assuming an Earth-like silicate rock composition. This is bigger than the most probable mass for Proxima Centauri b. So let's be optimistic and say we have a planet with a solid surface, but remember this is an educated guess based on only one measurement.

Let's move on to talk about the light and heat issue. Proxima Centauri b is much much closer to its star than the Earth is to the sun. In fact, it orbits at just 5% of our distance. That's way nearer than Mercury, which sits at 40% of the Earth-sun distance. A year on Proxima Centauri b is over in just 11.2 days. However, Proxima Centauri is a weakling among stars. It's a red dwarf with just over 10% of our sun's mass. It therefore only delivers 2/3rds of the radiation to Proxima Centauri b that we get on Earth.

This means that if you were to coat Proxima Centauri b in Earth's atmosphere, the surface temperature would be chilly… but it could support liquid water. That is, Proxima Centauri b is squarely in the 'Habitable Zone'.

CAVEAT #2: The Habitable Zone does not say ANYTHING about habitability.

I hate the term 'Habitable Zone' because any rational individual would believe it marks a location suitable for life. You know, with the term HABITABLE being right there in the name.

It doesn't.

All the 'Habitable Zone' means is that a planet with an Earth-like atmosphere and surface pressure could host liquid water. Do we have ANY INDICATION WHATSOEVER THAT PROXIMA CENTAURI b HAS AN EARTH-LIKE ATMOSPHERE?

One guess. Two choices. And the answer isn't 'yes'.

The Habitable Zone tells us nothing at all about the planet, only about its location. If Jupiter sat at the Earth's position, it would be in the Habitable Zone, but certainly not any more habitable. The Habitable Zone is still interesting, since it can be used as part of a selection tool for follow-up studies: with a zoo of over 3,000 known planets, we need to pick out the best candidates for further observations. But this doesn't mean we're selecting Earths.

The Habitable Zone also doesn't tell us that much about the star. Which brings us to the third caveat:

CAVEAT #3: Red dwarfs have behavioural problems.

Red dwarfs make up for their poxy size by spewing strands of stellar material called 'flares'. The sun has flares too, but Proxima Centauri has way bigger ones and the planet is much much closer. The net result is an X-ray bath at Proxima Centauri b that is 400 times that on Earth. This value is the present-day one: the early years of the star would have been far more dangerous. Such radiation levels (past or present) could strip the atmosphere, evaporate any water and nuke all life on the planet.

… or it could not. The Earth is protected from the sun by its magnetic field. If Proxima Centauri b has a molten iron core and some plate tectonic action, then it may have wrapped itself in a magnetic safety vest. Do we know? Not a clue — even if the planet is rocky, its ingredient mix might be entirely different to Earth. Even if it's got the same rocky recipe as Earth, there may still be no magnetic field: Venus is an incredibly close match to us in size and mass, but has next to zilch in the magnetics department.

The flaring action of the star leads to another issue...

CAVEAT #4: The planet may not exist.

Flares, star spots and general star action can produce wobbles as the star rotates that can look an awful lot like a planet. Planets are so tiny compared to stars that it is terribly terribly easy to mistake their faint whisper among stellar groans and creaks. The more rambunctious the star, the harder the detection.

The published signal for Proxima Centauri b looks reasonable, but the star's activity has led to scepticism. Independent observations are needed before we can be certain the planet is definitely there. If it does turn out to be a false positive, it will be in good company: in 2012, a Earth mass planet was announced around Alpha Centauri B, but later retracted when a fresh analysis of the data caused the signal to vanish.

If the planet does exist, its close proximity to the star may lead to another problem: it might be tidally locked. Like the moon and the Earth, one side of Proxima Centauri b may permanently face the star, while the other side is a land of perpetual night. Whether this creates a split world of deathly roast and deathly cold depends again on the planet's atmosphere. If the air can circle around and redistribute the star's heat, surface conditions might be liveable. Alternatively, it might be the worst cooked Christmas turkey ever.

There is also the problem we've no real idea what it takes to be 'habitable'. With only the Earth as a reference point for hosting life, it's impossible to tell which conditions are the most key. For example, does the planet need to be in a system of worlds to have water delivered to its surface? Is having a moon important for heating? What happens if the planet's orbit is not circular, but a bent ellipse? (And if anyone has seen the ESI number discussed, just bleach your brain:

It's also worth remembering:

CAVEAT #5: We've seen similar planets.

Proxima Centauri b is not the nearest exoplanet to Earth in mass, nor is it the first found in the Habitable Zone. However...

CAVEAT #6: Yeah, OK, this is still big news...

Proxima Centauri b it is the nearest exoplanet that could exist [++] and that is the reason its discovery is incredibly exciting. The Kepler Space Telescope has given us fantastic statistics about the numeracy of planets and the architecture of these alien systems, but just a single radius measurement for the planet itself. To understand more about planet formation and the development of life, we desperately need details on these individual worlds. In particular, we need a rocky planet close enough to examine its atmosphere and begin to probe surface conditions. That candidate is very likely to be Proxima Centauri b.

Proxima Centauri b will never be "Earth-like", since its star is definitely not "sun-like". However, red dwarfs are the most common stars in our galactic neighbourhood and the planets around them some of the easiest to find. The science community has raged about whether such stars are the best targets (easy to find planets) or the worst (warm planets dangerously close to the star) to explore the prospect of habitability. Future observations of Proxima Centauri b will hopefully pour facts into a debate that has been speculation and models.

So what is next?

Absolutely ideally, we'd spot the planet crossing the star's surface. This is the 'Transit Technique' for planet detection: the planet blocks out a small amount of the star's light as it passes between the star and Earth on its orbit. The amount of light obscured and its duration gives a handle on the planet's diameter and confirms the orientation of its orbit. With that, we'd have an average density AND the possibility of glimpsing the contents of the planet's atmosphere as the starlight gleams around its edges.

Unfortunately, the probability of Proxima Centauri b actually transiting the star are low. Astronomers have been hopefully gazing at our nearest stars for so long, that we should have spotted the little blighter if it were acting as a periodic dimmer switch. This means that the planet either does not transit as viewed from Earth, or the transit is undetectable due to the frequent massive flares from the star.

However, another exciting prospect is direct imaging. Planets (fortunately) don't have the roaring inferno of stars, but they do emit some heat. If this can be detected, we'd actually be able to see the planet. Direct imaging is still in its infancy and normally only spots Jupiter-sized worlds far from the star. But the proximity of Proxima Centauri means we might just be able to catch a glimpse of it with our best telescopes now… or very soon with instruments such as the James Webb Space Telescope (Hubble's successor) and the ground-based European Extremely Large Telescope in the pipeline. Seeing the planet directly would also allow us to check out its atmosphere and potential surface environment. In my (OK, potentially slightly biased) view, this makes Proxima Centauri b THE most exciting target for these telescopes.

If Proxima Centauri b is so very close, could we visit?

CAVEAT #7: The closest possible exoplanet is still damn far.

Proxima Centauri is 4.24 light years away from Earth. The furthest humans have ever travelled is a loop around the moon: a teeny tiny 00000004 light years away. Voyager 1 —our furthest and currently fastest travelling space craft— would still take about 75,000 years to reach this system (and it's not pointed in the right direction).

That said… one of the craziest idea in the world recently got funding. Project 'Starshot', financed by Russian billionaire, Yuri Milner, is planning to develop a method to send a tiny probe to Alpha Centauri in 20 years. To describe this as a "long shot" is a joke on several levels. However, if it were to be workable, there is now the greatest of great destinations.

[+] I'm moving. Not to Proxima Centauri.
[++] Still orbiting a star

Image credit: ESO (artist's impression)

Nature paper (behind firewall):

Good reads on Proxima Centauri b by planetary scientists:

Jonti Horner and Tanya Hill @ The Conversation:

Sean Raymond @ Nautilus:

#Proximab #Exoplanets #journaljuice


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In the third act of "Swan Lake" the black swan performed 33 fouettés; bobbing and turning on one leg. Is this enchantment or physics?

(OK, it's physics, but that doesn't make it less cool)

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Anyone also on the wrong side of the globe (or hindered by cloud cover) and missed the Mercury transit? #NASA's got you covered with better telescopes. 

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How two planets can act as a weighing scale.

Planet mass is most commonly measured by the Radial Velocity technique. Despite the usual picture of a planet orbiting a stationary star, in truth, both objects orbit a common centre of mass. For a star and planet, that central point is very close to the star's own centre, since the star is so much more massive than the planet. The planet therefore sweeps a wide circle, while the star makes a small jiggle. Nevertheless, the size of that jiggle can give away the planet's mass. Bigger jiggle = bigger planet.

However, for Earth-sized planets, that jiggle may be undetectable. The mass of the planet is therefore unknown, unless it has a planet sibling. If two planets orbit a star and they both transit (that is, cross between the star's surface and the Earth to produce a small dip in the star light), then the mass can be measured using 'Transit Timing Variations' or TTV.

TTV works because the planets pull on one another, changing the time needed to circle the star. Measure the variations in the time taken for the planet to reappear and you can find the planet's mass.

Movie credit: NASA Ames / Kepler Mission

#exoplanets #planets 
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