The Gale Winds of Venus Suggest How Locked Exoplanets Could Escape a Fate of Extreme Heat and Brutal Cold

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Two images of the nightside of Venus captured by the IR2 camera on the Akatsuki orbiter in September 2016 (JAXA).

 

More than two decades before the first exoplanet was discovered, an experiment was performed using a moving flame and liquid mercury that could hold the key to habitability on tidally locked worlds.

The paper was published in a 1969 edition of the international journal, Science, by researchers Schubert and Whitehead. The pair reported that when a Bunsen flame was rotated beneath a cylindrical container of mercury, the liquid began to flow around the container in the opposite direction at speeds up to four times greater than the rotation of the flame. The scientists speculated that such a phenomenon might explain the rapid winds on Venus.

On the Earth, the warm equator and cool poles set up a pressure difference that creates our global winds. These winds are deflected westward by the rotation of the planet (the so-called Coriolis force) promoting a zonal (east-west) air flow around the globe. But what would happen if our planet’s rotation slowed? Would our winds just cycle north and south between the equator and poles?

The Moon is tidally locked to the Earth, so only one hemisphere is visible from our planet (Smurrayinchester / wikipedia commons).

Such a slow-rotating scenario may be the lot of almost all rocky exoplanets discovered to date. Planets such as the TRAPPIST-1 system and Proxima Centauri-b all orbit much closer to their star than Mercury, making their faint presence easier to detect but likely resulting in tidal lock. Like the moon orbiting the Earth, planets in tidal lock have one side permanently facing the star, creating a day that is equal to the planet’s year.

The dim stars orbited by these planets can mean they receive a similar level of radiation as the Earth, placing them within the so-called “habitable zone.” However, tidal lock comes with the risk of horrific atmospheric collapse. On the planet side perpetually facing away from the star, temperatures can drop low enough to freeze an Earth-like atmosphere. The air from the dayside would then rush around the planet to fill the void, freezing in turn and causing the planet to lose its atmosphere even within the habitable zone.

The only way this could be prevented is if winds circulating around the planet could redistribute the heat sufficiently to prevent freeze-out. But without a strong Coriolis force from the planet’s rotation, can such winds exist?

A planet whose wind speeds exceed the rotation speed of the planet is said to have a “super-rotating” atmosphere. Global climate models of tidally locked planets have suggested that temperate conditions might be maintained by winds circulating between the night and day side in the same way as the Earth’s winds are generated between the equator and poles.

However, global climate models are extremely tricky, being computationally expensive and sensitive to a multitude of factors that are unmeasurable for exoplanets. As a result, it has not been possible to test if the climates produced by the computer could really exist, leaving the fate of tidally locked worlds uncertain.

 

Orbiting close to their star, the TRAPPIST-1 Earth-sized worlds are probably tidally locked, rotating just once per orbital period. This is an artist’s concept of the system, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018 (NASA/JPL-Caltech).

 

But there is a slowly rotating planet where one mechanism for super rotation can be explored. Venus is the only other Earth-sized planet we can reach by spacecraft and the planet has super rotating winds whose origin has been hotly debated for decades.

While Venus is not in tidal lock with the sun, its rotation is extremely slow. Our neighboring world takes 225 days to orbit the sun and rotates once every 243 Earth days, making the Venusian day (one rotation) longer than its year.

The planet’s thick carbon dioxide atmosphere provides Venus with the most powerful greenhouse effect in the solar system. This prevents nights on Venus from freezing and the planet maintains a fairly uniform surface temperature that is hot enough to melt lead. Yet despite the lack of a temperature gradient to drive winds, the upper atmosphere of Venus is a blistering gale. Winds whip around the planet with speeds that exceed 100 m/s (224 mph), traveling sixty times faster than the surface rotation. What could be driving this weather system?

 

A false color image of Venus with the IR2 camera on Akatsuki. 2.26 micron radiation (used in this work) is shown in red.

It is a question that returns us to the Bunsen flame experiment. Schubert and Whitehead speculated that the sun could replace the Bunsen flame in driving the Venusian winds rapidly around the planet. However, skeptics to this claim argued that the sun could only influence the cloud tops of Venus, whereas super-rotation had been observed to extend much deeper into the planet’s thick cloak of gases.

The alternative theory was that small contrasts in heat on the planet’s surface could set up circulations to drive the super-rotating winds. A challenge here was that such models appeared very sensitive to the exact starting conditions, suggesting that Venus’s super-rotating winds were a rare outcome for the planet. If true, the same mechanism was not likely to be acting on tidally locked worlds around other stars.

At the end of last year, a paper in the Astrophysical Journal Supplement Series was published that provided a heap of new data about Venus’s atmosphere. The research was led by Javier Peralta, a postdoctoral fellow at the Japan Aerospace Exploration Agency (JAXA). Using data from JAXA’s Venus orbiter, Akatsuki, Peralta had painstakingly tracked the Venusian winds.

The IR2 camera onboard Akatsuki captures images in the infrared at a wavelength of 2.26 microns. On the dayside of Venus, clouds in the upper atmosphere sitting at 60 – 70 km (37 – 44 miles) above the planet’s surface strongly reflect ultraviolet and infrared from the Sun. However, the nightside illumination comes from infrared heat emanating from Venus’s hot surface. This is partially blocked by clouds deeper in the atmosphere at altitudes between 48 – 60 km (30 – 37 miles). As the clouds have a varying transparency to this infrared glow, their shapes become visible when viewed through Akatsuki’s IR2 camera. It was these deeper, nightside clouds, that Peralta tracked.

Dr Javier Peralta, lead author of this work, is a postdoctoral researcher at JAXA.

By comparing results from 2,947 wind measurements, Peralta spotted a pattern. The winds acceleration was tied to the position of the sun, suggesting that the giant Bunsen-flame of our nearest star was indeed driving super-rotation deep in the Venusian atmosphere. It was a result that suggested this super-rotation mechanism could be common on many more planets, as it required only the heat from the star.

Yet, Peralta was quick to note that this was not a “case closed” for the Venusian winds. While they had not detected a north-south component to the winds that would have supported the alternative theory of a surface-driven origin for the super-rotating, it might have just been below the level they could detect.

“This was one reason why we made our wind measurements publicly available,” Peralta said. “Sharing measurements is critical nowadays since the new generation of computer models are able to incorporate this observational data to predict how an atmosphere will evolve.”

Peralta hopes that results from Venus can used with climate models for slow rotating worlds, helping scientists understand both our nearest neighbor and the conditions that might be present on tidally locked planets by providing comparative measurements for at least one type of super rotation generation.

 

Artist’s impression of JAXA’s Akatsuki Venus Climate Orbiter at Venus (JAXA / Akihiro Ikeshita)

As well as using results from Akatsuki, Peralta also looked at wind speed measurements from previous missions and ground observations of Venus. Comparing data since the late 1970s, his team noted a variation in the recorded wind speeds. While it is challenging to compare results from different instruments (which have different error estimates), this might suggest that the Venusian weather pattern has varied over time scales of decades.

Such variation could also support the sun being the main driver for the winds. Changes in the cloud’s reflectivity would alter how much solar radiation is absorbed, adjusting the efficiency of this driving force. If true, this could be used to calibrate models further for different reflectivity conditions.

Peralta’s results underline the importance of our solar system in understanding exoplanets. The comparison of the weird and wonderful climates among our neighboring worlds can help us explore the next Earth-sized discovery and these worlds are within the reach of our spaceships.

 

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Weird Planets

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Artist rendering of an “eyeball world,” where one side of a tidally locked planet is always hot on the sun-facing side and the back side is frozen cold.  Definitely a tough environment, but  might some of the the planets be habitable at the edges?  Or might winds carry sufficient heat from the front to the back?  (NASA/JPL-Caltech)

The very first planet detected outside our solar system powerfully made clear that our prior understanding of what planets and solar systems could be like was sorely mistaken.

51 Pegasi was a Jupiter-like massive gas planet, but it was burning hot rather than freezing cold because it orbited close to its host star — circling in 4.23 days.  Given the understandings of the time, its existence was essentially impossible. 

Yet there it was, introducing us to what would become a large and growing menagerie of weird planets.

Hot Jupiters, water worlds, Tatooine planets orbiting binary stars, diamond worlds (later downgraded to carbon worlds), seven-planet solar systems with planets that all orbit closer than Mercury orbits our sun.  And this is really only a brief peak at what’s out there — almost 4,000 exoplanets confirmed but billions upon billions more to find and hopefully characterize.

I thought it might be useful — and fun — to take a look at some of the unusual planets found to learn what they tell us about planet formation, solar systems and the cosmos.

 


Artist’s conception of a hot Jupiter, CoRoT-2a. The first planet discovered beyond our solar system was a hot Jupiter similar to this, and this surprised astronomers and led to the view that many hot Jupiters may exist. That hypothesis has been revised as the Kepler Space Telescope found very few distant hot Jupiters and now astronomers estimate that only about 1 percent of planets are hot Jupiters. (NASA/Ames/JPL-Caltech)

 

Let’s start with the seven Trappist-1 planets.  The first three were detected two decades ago, circling a”ultra-cool” red dwarf star a close-by 40 light years away.  Observations via the Hubble Space Telescope led astronomers conclude that two of the planets did not have hydrogen-helium envelopes around them, which means the probability increased that the planets are rocky (rather than gaseous) and could potentially hold water on their surfaces.

Then in 2016 a Belgian team, using  the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, found three more planets, and the solar system got named Trappist-1.  The detection of an additional outer planet was announced the next year, and in total three of the seven planets were deemed to be within the host star’s habitable zone — where liquid water could conceivably be present.

So, we have a most interesting 7-planet solar system quite close to us, and not surprisingly it has become the focus of much observation and analysis.

But consider this:  all seven of those planets orbits Trappist-1 at a distance much smaller than from our sun to the first planet, Mercury. The furthest out planets orbits the star in 19 days, while Mercury orbits in 88 days.

 

 

The Trappist-1 solar system, with the transit data used to detect the presence of seven planets, each one blocking the light curve at different locations. (NASA/JPL-Caltech)

 

Given this proximity, then, why are the Trappist-1 planets so interesting, especially in terms of habitability?  Because Trappist-1 puts out but .05 percent as much energy as our sun, and the furthest out planet (though very close to the star by the standards of our solar system) is nonetheless likely to be frozen.

So Trappist-1 a mini-system, with seven tidally-locked (never-rotating) planets that happen to orbit in resonance to each other.  Just because it is so different from our system doesn’t mean it isn’t fascinating, instructive, and even possibly the home of planets that could potentially support life.

And since red dwarf stars are the most common type of star in the Milky way (by lot), red dwarf solar system research is an especially hot field.

So there are mini planets and systems and massive planets in what used to be considered the impossibly wrong place.  And then there are planets with highly eccentric orbits — very different from the largely circular orbits of planets in our system.

The eccentricity of HD20782b superimposed onto our circular-orbiting inner solar system planets. (Stephen Kane)

The most extreme eccentric orbit found so far is HD 20782, measured at an eccentricity of .96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach. 

Many exoplanets have eccentricities far greater than what’s found in our solar system planets but nothing like this most unusual traveler, which has a path seemingly more like a comet than a planet.

Researchers have concluded that the eccentricity of a planet tends to relate to the number of planets in the system, with many-planeted systems having far more regularly orbiting planets.  (Ours and the Trappist-1 system are examples.)

Unusual planets come in many other categories, such as the chemical makeup of their atmospheres, surfaces and cores.  Most of the mass of stars, planets and living things consists of hydrogen and helium, with oxygen, carbon, iron and nitrogen trailing far behind.

Solid elements are exceptionally rare in the overall scheme of the solar system. Despite being predominant on Earth, they constitute less than 1 percent of the total elements in the solar system, primarily because the amount of gas in the sun and gas giants is so great.  What is generally considered the most important of these precious solid elements is iron, which is inferred to be in the core of almost all terrestrial planet.

The amount of iron or carbon or sulfur or magnesium on or around a planet generally depends on the amount of these “metals” present in the host star, and then in molecular protoplanetary disc remains of the star’s formation.  And this is where some of the outliers, the apparent oddities, come in.

A super-Earth, planet 55 Cancri e, was reported to be the first known planet to have huge layers of diamond, due in part to the high carbon-to-oxygen ratio of its host star. That conclusion has been disputed,  but the planet is nonetheless unusual.  Above is an artist’s concept of the diamond hypothesis. (Haven Giguere/Yale University)

The planet 55 Cancri e, for instance, was dubbed a “diamond planet” in 2012 because the amount of carbon relative to oxygen in the star appeared to be quite high.  Based on this measurement, a team hypothesized that the surface presence of abundant carbon likely created a graphite surface on the scalding super-Earth, with a layer of diamond beneath it created by the great pressures.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” lead researcher Nikku Madhusudhan of Yale University said in a statement at the time. “The surface of this planet is likely covered in graphite and diamond rather than water and granite.”

As tends to happen in this early phase of exoplanet characterization, subsequent measurements cast some doubt on the diamond hypothesis.  And in 2016, researchers came up with a different scenario — 55 Cancri e was likely covered in lava.  But because of heavy cloud and dust cover over the planet, a subsequent group raised doubts about the lava explanation. 

But despite all this back and forth, there is a growing consensus that 55 Cancri e has an atmosphere, which is pretty remarkable given its that its “cold” side has temperatures that average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere.

 

Could super-Earth HD 219134 b be a sapphire planet? (Thibaut Roger/University of Zurich)

And then there’s another super-earth, HD 219134, that late last year was described as a planet potentially featuring vast collections of different precious stones.

To back up for a second, researchers study the formation of planets using theoretical models and compare their results with data from observations. It is known that during their formation, stars such as the sun were surrounded by a disc of gas and dust in which planets were born. Rocky planets like the Earth were formed out of the solid bodies left over when the protoplanetary gas disc cooled and dispersed.

Unlike the Earth however, HD 219134 most likely does not have a massive core of iron — a conclusion flowing from measurements of its density.  Instead, through modeling of formation scenarios for a scalding super-Earth close to its host star, the researchers conclude the planet is likely to be rich in calcium and aluminum, along with magnesium and silicon.

This chemical composition would allow the existence of large quantities of aluminum oxides. On Earth, crystalline aluminum oxide forms the mineral corundum. If the aluminum oxide contains traces of iron, titanium, cobalt or chromium, it will form the noble varieties of corundum, gemstones like the blue sapphire and the red ruby.

“Perhaps it shimmers red to blue like rubies and sapphires, because these gemstones are aluminum oxides which are common on the exoplanet,” said Caroline Dorn, astrophysicist at the Institute for Computational Science of the University of Zurich.

 

 

A variation on the “eyeball planet” is a water world where the star-facing side is able to maintain a liquid-water ocean, while the rest of the surface is ice. (eburacum45/DeviantArt)

 

Super-Earths, which are defined as having a size between that of Earth and Neptune, are also inferred to be the most likely to be water worlds.

At a Goldschmidt Conference in Boston last year, a study was presented that suggests that some super-Earth exoplanets are likely extremely wet with water – much more so than Earth. Astronomers found more specifically that exoplanets which are between two and four times the size of Earth are likely to have water as a dominant component.  Most are thought to be rocky and to have atmospheres, and now it seems that many have ocean, as well.

The new findings are based on data from the Kepler Space Telescope and the Gaia mission, which show that many of the already known planets of this type (out of more than 4,000 exoplanets confirmed so far) could contain as much as 50 percent water. That upper limit is an enormous amount, compared to 0.02 percent of the water content of Earth.

This potentially wide distribution of water worlds is perhaps not so surprising given conditions in our solar system, where Earth is wet, Venus and Mars were once wet, Neptune and Uranus are ice giants and moons such as Europa and Enceladus as global oceans beneath their crusts of ice.

 

Might this be the strangest planet of all? (NASA)

 

As is apparent with the planetary types described so far, whether a planet is typical or atypical is very much up in the air.  What is atypical this year may be found to be common in the days ahead.

The Kepler mission concluded that small, terrestrial planets are likely more common than gas giants, but our technology doesn’t let us identify and characterize many of those smaller, Earth-sized planets.

Many of the planets discovered so far are quite close to their host stars and thus are scalding hot. Planets orbiting red dwarf stars are an exception, but if you’re looking for habitable planets — and many astronomers are — then red dwarf planets come with other problems in terms of habitability.  They are usually tidally locked and they start their days bathed in very high-energy radiation that could stertilize the surface for all time.

A prime goal of the Kepler mission had been to find a planet close enough in character to Earth to be considered a twin.  While they have some terrestrial candidates that could be habitable, no twin was found.  This may be a function of lacking the necessary technology, or it’s certainly possible (if unlikely) that no Earth twins are out there.  Or at least none with quite our collection of conditions favorable to habitability and life. 

With this in mind, my own current candidate for an especially unusual planet is, well, our own.   Planet-hunting over the past almost quarter-century leads to that conclusion — for now, at least.

And it may be that solar systems like ours are highly unusual, too.  Pretty surprising, given that not long ago it was considered the norm.

 

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Does Proxima Centauri Create an Environment Too Horrifying for Life?

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Artist’s impression of the exoplanet Proxima Centauri b. (ESO/M. Kornmesser)

 

In 2016, the La Silla Observatory in Chile spotted evidence of possibly the most eagerly anticipated exoplanet in the Galaxy. It was a world orbiting the nearest star to the sun, Proxima Centauri, making this our closest possible exoplanet neighbour. Moreover, the planet might even be rocky and temperate.

Proxima Centauri b had been discovered by discerning a periodic wobble in the motion of the star. This revealed a planet with a minimum mass 30% larger than the Earth and an orbital period of 11.2 days. Around our sun, this would be a baking hot world.

But Proxima Centauri is a dim red dwarf star and bathes its closely orbiting planet in a level of radiation similar to that received by the Earth. If the true mass of the planet was close to the measured minimum mass, this meant Proxima Centauri b would likely be a rocky world orbiting within the habitable zone.

 

Comparison of the orbit of Proxima Centauri  b with the same region of the solar system. Proxima Centauri is smaller and cooler than the sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone. (ESO/M. Kornmesser/G. Coleman.)

Sitting 4.2 light years from our sun, a journey to Proxima Centauri b is still prohibitively long.

But as our nearest neighbor, the exoplanet is a prime target for the upcoming generation of telescopes that will attempt to directly image small worlds. Its existence was also inspiration for privately funded projects to develop faster space travel for interstellar distances.

Yet observations taken around the same time as the La Silla Observatory discovery were painting a very different picture of Proxima Centauri. It was a star with issues.

This set of observations were taken with Evryscope; an array of small telescopes that was watching stars in the southern hemisphere. What Evryscope spotted was a flare from Proxima Centauri that was so bright that the dim red dwarf star became briefly visible to the naked eye.

Flares are the sudden brightening in the atmosphere of a star that release a strong burst of energy. They are often accompanied by a large expulsion of plasma from the star known as a “coronal mass ejection”. Flares from the sun are typically between 1027 – 1032 erg of energy, released in a few tens of minutes.

For comparison, a hydrogen bomb releases the equivalent of about 10 megatons of TNT or a mere 4 x 1023 erg. Hitting the Earth, energy from solar flares and coronal mass ejections can disrupt communication equipment and create a spectacular aurora.

A solar flare erupting from the right side of the sun. (NASA/SDO)

But the Proxima super-flare spotted by Evryscope was well beyond a regular stellar flare.

On March 18 in 2016, this tiny red dwarf emitted an energy belch of 1033.5 erg. The flare consisted of one major event and three weaker ones and lasted approximately one hour, during which time Proxima Centauri became 68 times brighter.

A sudden, colossal increase in the brightness of a star does not bode well for any closely orbiting planets.

However, such a major flare might well be rare. If the star was normally fairly quiet, perhaps a planet could recover from a single very disruptive flare in the same way the Earth has survived mass extinction events.

Led by graduate student Ward Howard at the University of North Carolina, Chapel Hill, the discovering team used Evryscope to monitor Proxima Centauri for flares for a total of 1344 hours between January 2016 and March 2018. What they found was a horrifying environment, as reported in The Astrophysical Journal Letters.

While an event on the scale of the Proxima super-flare was only seen once, 24 large eruptions were spotted from the red dwarf, with energies from 1030.5 to 1032.4 erg. Allowing for the fact the star had only been observed for a small part of the year, this pattern of energy outbursts meant that a massive super-flare (1033 erg) was likely to occur at least five times annually.

 

Artist’s impression of the surface of the planet Proxima Centauri b. But what would conditions be like so close to a flaring star? (ESO/M. Kornmesser)

 

But how important is this for the planet?

The Earth is protected from flares from our sun by our atmosphere. The ozone layer absorbs harmful ultraviolet radiation with wavelengths between about 2400 – 2800 Angstroms (10-10 m), preventing it reaching the surface. So what if Proxima Centauri b had a similar protective layer of gases as the Earth?

To answer this question, Howard and his team ran simulations of an Earth-like atmosphere on Proxima Centauri b.

As is the case for the sun, the team assumed that large flares would be frequently accompanied by a coronal mass ejection. Radiation and stellar material then flooded over an Earth-like Proxima Centauri b at the observed rate. And the atmosphere crumbled.

 

Ward Howard, astrophysicist at the University of North Carolina.

High energy particles in the coronal mass ejections split the nitrogen molecules (N2) in the atmosphere, which reacted with the ozone (O3) to form nitrogen oxide (NO2). After just 5 years, 90% of the ozone in the atmosphere was lost and the amount was still decreasing.

Without ozone, the surface of Proxima Centauri b would be stripped of its protection from UV radiation. During the Proxima super-flare, the radiation dose without the protective ozone would be 65 times larger than that needed to kill 90% of one of the most UV-resilient organisms on Earth.

“Life would have to undergo extreme adaptation to UV or exist underground or underwater,” Howard notes. “Only the most resistant organisms could survive on the surface in this environment.”

The simulation does assume that Proxima Centauri b does not have a magnetic field. Such a shield could channel the particles from the coronal mass ejection to the poles, forming the aurora as on Earth and reducing the damage to the atmosphere.

However, orbiting so close to the star, Proxima Centauri b is likely to be in tidal lock as the moon is to the Earth. This is expected to weaken the magnetic field, as the slower rotation makes it harder to create a magnetic dynamo within the planet.

So if the protective shields are lowered on Proxima Centauri b, is our nearest planet a world populated by highly resistant UV organisms? Or have we seen evidence that rather than warming the planet to allow life to exist, this star has snuffed it out?

 

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Barnard’s Star, The “Great White Whale” of Planet Hunting, Has Surrendered Its Secret

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Barnard’s Star is the closest single star to our sun, and the most fast moving. It has long been attractive to planet hunters because it is so close and so bright, especially in the infared section of the spectrum. But until now, the exoplanets of this “great white whale” have avoided detection.

 

Astronomers have found that Barnard’s star — a very close, fast-moving, and long studied red dwarf — has a super-Earth sized planet orbiting just beyond its habitable zone.

The discovery relied on data collected over many years using the tried-and-true radial velocity method, which searches for wobbles in the movement of the host star.

But this detection was something big for radial velocity astronomers because Barnard-b was among the smallest planet ever found using the technique, and it was the furthest out from its host star as well — orbiting its star every 233 days.

For more than a century, astronomers have studied Barnard’s star as the most likely place to find an extrasolar planet.

Ultimately, said Ignasi Rablis of Spain’s Institute of Space Studies of Catalonia, lead author of the paper in journal Nature, the discovery was the result of 771 observations, an extremely high number.

And now, he said, “after a very careful analysis, we are over 99 percent confident the planet is there.”

The planet is at least 3.2 times the size of Earth and orbits near the snowline of the system, where water cannot be expected to ever be liquid.  That means is it a frozen world (an estimated -150 degrees Celsius) and highly unlikely to support life.

But Rablis and others on the large team say it also an extremely good candidate for future direct imaging and next-generation observing.

 

An artist’s rendering of the Barnard’s star planet at sunset. (Martin Kornmesser/ESO)

 

Thousands of exoplanets have been identified by now, and hundreds using the radial velocity method.  But this one is different.

“Barnard’s star is the ‘great white whale’ of planet hunting,” said Paul Butler, senior scientist at the Carnegie Institution, a radial velocity pioneer, and one of the numerous authors of the paper.

Because the star is so close (but 6 light-years away) and as a result so tempting, it has been the subject of exoplanet searches for 100 years, Butler said.  But until the radial velocity breakthroughs of the mid 1990s, the techniques used could not find a planet.

Nonetheless, an early exoplanet hunter, the Dutch-American astronomer Peter van de Kamp of Swarthmore College, thought that he had indeed found two gas giant planets around Barnard’s star in the 1960s.  He used a different technique based on the movement of the host star, and the findings even made it into some textbooks.  But later the detection was found to be incorrect.

Even after the modern exoplanet era began Barnard’s star kept its planetary secret close.

As Butler explained it, the combination of the planet’s size and distance from the star ultimately pushed the technology (and astronomers) to the very limit — requiring a measurement of  1.2 meters per second of “wobble.”

In contrast, the first planets were found by radial velocity that would detect 70 meter per second of wobble caused by the gravitational pull of a planet, and 30 years ago the best instruments could detect only 300 meters per second.

 

The radial velocity technique identifies planets via the shift in the wavelength of the light of a star as it wobbles due to the presence of a planet.  When a celestial object moves away from us, the light we observe becomes slightly less energetic and redder.  The opposite — light becomes slightly more energetic and bluer — happens when the star moves toward us.

 

The detected planet (which remains a “candidate” until further confirmed) was ultimately found following concerted effort by a large team of astronomers around the world.  It was co-led and organized by Guillem Anglada-Escudé of the Queen Mary University of London.  The young astronomer had made a major splash in 2016 with the detection of a planet orbiting Proxima Centauri, the closest star to our own.

That discovery was part of the “Pale Red Dot” campaign, which had the goal of detecting rocky planets around red dwarf stars.  After the Proxima discovery Barnard’s star went to the top of Anglada-Escudé list with the renamed “Red Dots” collaboration — which is supported by the European Southern Observatory and universities in Chile, the United Kingdom, Spain and Germany.

The Red Dots campaign is a collaboration including the European Southern Observatory, Queen Mary University of London, and several European and South American institutions.

By 2015, there was already almost 18 years of modern data collected regarding a possible planet orbiting the star, and a faint but clearly present signal had been detected.  But more was needed to confidently report a discovery, and the Red Dots effort took up the challenge.

To see if the result could be confirmed, astronomers regularly monitored Barnard’s star with high precision spectrometers such as the CARMENES (Calar Alto Observatory in Spain), and also the HARPS  (High Accuracy Radial velocity Planet Searcher.)

Ultimately, the team used observations from seven different instruments taken over 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies.

“We all have worked very hard on this result,” said Anglada-Escudé. “This is the result of a large collaboration organized in the context of the Red Dots project, which is why it has contributions from teams all over the world including semi-professional astronomers.”

Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía and co-author of the paper, said of the significance of the finding grow over decades.

Guillem Anglada-Escudé was a leader of the Barnard’s star collaboration, as he was in the successful campaign to detect a planet orbiting of Proxima Centauri.

“This discovery means a boost to continue on searching for exoplanets around our closest stellar neighbors, in the hope that eventually we will come upon one that has the right conditions to host life,” she said.

The next pr0ject for the Red Dots campaign is to study the star Ross 154, at 9.69 light-years away another of the closest stars to us.

The dramatically increased (and increasing) precision in radial velocity measurements is expected to continue with the next generation of ground-based telescopes and spectrometers.

Butler, for instance,  said that Carnegie is in the process of upgrading its Planet Finding Spectrograph at the Las Campanas Observatory in Chile to reach a 0.5-meters-per-second measurement. Other groups including the European Southern Observatory and American teams based at Pennsylvania State and Yale Universities have similar efforts under way.

If they succeed, Butler said, it may well be possible to find potentially habitable planets around sun-like and other categories of stars using the radial velocity method.

 

 

Barnard’s star is the fourth closest to our sun, and the closest single star. It lies 6 light-years from us, as opposed to a little more than 4 light-years for the Alpha Centauri/Proxima Centauri threesome. (NASA Photojournal)

 

Barnard’s a very-low-mass red dwarf star in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Celestial hemisphere.

Despite its proximity, the star is too faint to be seen with the unaided eye, though it is quite visible with an amateur 8-inch telescope.  It is much brighter in the infrared than in visible light.  Although Barnard’s Star is an ancient star, it still experiences star flare events, one being observed in 1998.

The star is named after the American astronomer E. E. Barnard.  He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion –the apparent angular motion of a star across the sky with respect to more distant stars — as 10.3 arcseconds per year relative to the sun.

This is likely to be the fastest star in terms of proper motion, as its proximity to the sun, as well as its high velocity, make it unlikely any faster object will be discovered.

Barnard’s Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard’s Star has focused on measuring its stellar characteristics and its astrometry — which involves precise measurements of the positions and movements of stars and other celestial bodies on the plane of the sky.

When planet hunters use astrometry, they look for a minute but regular wobble in a star’s position as seen in images.  Van de Kamp, for instance, used astrometry to study Barnard’s star and (incorrectly) detected those two gas giants around it.

In contrast, radial (or Doppler) velocities look for the wobble of the star perpendicular to the plane sky, and astronomers have regularly, and now once again, made history with that method.

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The Kepler Space Telescope Mission Is Ending But Its Legacy Will Keep Growing.

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An illustration of the Kepler Space Telescope, which is on its very last legs.  As of October 2018, the planet-hunting spacecraft has been in space for nearly a decade. (NASA via AP)

 

The Kepler Space Telescope is dead.  Long live the Kepler.

NASA officials announced on Tuesday that the pioneering exoplanet survey telescope — which had led to the identification of almost 2,700 exoplanets — had finally reached its end, having essentially run out of fuel.  This is after nine years of observing, after a malfunctioning steering system required a complex fix and change of plants, and after the hydrazine fuel levels reached empty.

While the sheer number of exoplanets discovered is impressive the telescope did substantially more:  it proved once and for all that the galaxy is filled with planets orbiting distant stars.  Before Kepler this was speculated, but now it is firmly established thanks to the Kepler run.

It also provided data for thousands of papers exploring the logic and characteristics of exoplanets.  And that’s why the Kepler will indeed live long in the world of space science.

“As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.

“Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.”

 

 


The Kepler Space Telescope was focused on hunting for planets in this patch of the Milky Way. After two of its four spinning reaction wheels failed, it could no longer remain steady enough to stare that those distant stars but was reconfigured to look elsewhere and at a different angle for the K2 mission. (Carter Roberts/NASA)

 

Kepler was initially the unlikely brainchild of William Borucki, its founding principal investigator who is now retired from NASA’s Ames Research Center in California’s Silicon Valley.

When he began thinking of designing and proposing a space telescope that could potentially tell us how common distant exoplanets were — and especially smaller terrestrial exoplanets like Earth – the science of extra solar planets was at a very different stage.

William Borucki, originally the main champion for the Kepler idea and later the principal investigator of the mission. His work at NASA went back to the Apollo days. (NASA)

“When we started conceiving this mission 35 years ago we didn’t know of a single planet outside our solar system,” Borucki said.  “Now that we know planets are everywhere, Kepler has set us on a new course that’s full of promise for future generations to explore our galaxy.”

The space telescope was launched in 2009.  While Kepler did not find the first exoplanets — that required the work of astronomers using a different technique of observing based on the “wobble” of stars caused by orbiting planets — it did change the exoplanet paradigm substantially.

Not only did it prove that exoplanets are common, it found that planets outnumber stars in our galaxy (which has hundreds of billions of those stars.)

In addition it found that small, terrestrial-size planets are common as well, with some 20 to 50 percent of stars likely to have planets of that size and type.  And what menagerie of planets it found out there.

Astrophysicist Natalie Batalha was the Kepler project and mission scientist for a decade. She left NASA recently for the University of California at Santa Cruz “to carry on the Kepler legacy” by creating an interdisciplinary center for the study of planetary habitability.

Among the greatest surprises:  The Kepler mission provided data showing that the most common sized planets in the galaxy fall somewhere between Earth and Neptune, a type of planet that isn’t present in our solar system.

It found solar systems of all sizes as well, including some with many planets (as many as eight) orbiting close to their host star.

The discovery of these compact systems, generally orbiting a red dwarf star, raised questions about how solar systems form: Are these planets “born” close to their parent star, or do they form farther out and migrate in?

So far, more than 2,500 peer-reviewed papers have been published using Kepler data, with substantial amounts of that data still unmined.

Natalie Batalha was the project and mission scientist for Kepler for much of its run, and I asked her about its legacy.

“When I think of Kepler’s influence across all of astrophysics, I’m amazed at what such a simple experiment accomplished,” she wrote in an email. “You’d be hard-pressed to come up with a more boring mandate — to unblinkingly measure the brightnesses of the same stars for years on end. No beautiful images. No fancy spectra. No landscapes. Just dots in a scatter plot.

“And yet time-domain astronomy exploded. We’d never looked at the Universe quite this way before. We saw lava worlds and water worlds and disintegrating planets and heart-beat stars and supernova shock waves and the spinning cores of stars and planets the age of the galaxy itself… all from those dots.”

 

The Kepler-62 system is put one of many solar systems detected by the space telescope. The planets within the green discs are in the habitable zones of the stars — where water could be liquid at times. (NASA)

 

While Kepler provided remarkable answers to questions about the overall planetary makeup of our galaxy, it did not identify smaller planets that will be directly imaged, the evolving gold standard for characterizing exoplanets.  The 150,000 stars that the telescope was observing were very distant, in the range of a few hundred to a few thousand light-years away. One light year is about 6 trillion (6,000,000,000,000) miles.

Nonetheless, Kepler was able to detect  the presence of a handful of Earth-sized planets in the habitable zones of their stars.  The Kepler-62 system held one of them, and it is 1200 light-years away.  In contrast, the four Earth-sized planets in the habitable zone of the much-studied Trappist-1 system are 39 light-years away.

Kepler made its observations using the the transit technique, which looks for tiny dips in the amount of light coming from a star caused by the presence of a planet passing in front of the star.  While the inference that exoplanets are ubiquitous came from Kepler results, the telescope was actually observing but a small bit of the sky.  It has been estimated that it would require around 400 space telescopes like Kepler to cover the whole sky.

What’s more, only planets whose orbits are seen edge-on from Earth can be detected via the transit method, and that rules out a vast number of exoplanets.

The bulk of the stars that were selected for close Kepler observation were more or less sun-like, but a sampling of other stars occurred as well. One of the most important factors was brightness. Detecting minuscule changes in brightness caused by transiting planet is impossible if the star is too dim.

 

The artist’s concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. (NASA Ames/SETI Institute/JPL-Caltech)

 

Four years into the mission, after the primary mission objectives had been met, mechanical failures temporarily halted observations. The mission team was able to devise a fix, switching the spacecraft’s field of view roughly every three months. This enabled an extended mission for the spacecraft, dubbed K2, which lasted as long as the first mission and bumped Kepler’s count of surveyed stars up to more than 500,000.

But it was inevitable that the mission would come to an end sooner rather than later because of that dwindling fuel supply, needed to keep the telescope properly pointed.

Kepler cannot be refueled because NASA decided to place the telescope in an orbit around the sun that is well beyond the influence of the Earth and moon — to simplify operations and ensure an extremely quiet, stable environment for scientific observations.  So Kepler was beyond the reach of any refueling vessel.  The Kepler team compensated by flying considerably more fuel than was necessary to meet the mission objectives.

The video below explains what will happen to the Kepler capsule once it is decommissioned.  But a NASA release explains that the final commands “will be to turn off the spacecraft transmitters and disable the onboard fault protection that would turn them back on. While the spacecraft is a long way from Earth and requires enormous antennas to communicate with it, it is good practice to turn off transmitters when they are no longer being used, and not pollute the airwaves with potential interference.”

 

 

And so Kepler will actually continue orbiting for many decades, just as its legacy will continue long after operations cease.

Kepler’s follow-on exoplanet surveyor — the Transiting Exoplanet Survey Satellite or TESS — was launched this year and has begun sending back data.  Its primary mission objective is to survey the brightest stars near the Earth for transiting exoplanets. The TESS satellite uses an array of wide-field cameras to survey some 85% of the sky, and is planned to last for two years.

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