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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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

Probing The Insides of Mars to Learn How Rocky Planets Are Formed

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An artist illustration of the InSight lander on Mars. InSight, short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, is designed to look for tectonic activity and meteorite impacts, study how much heat is still flowing through the planet, and track Mars’ wobble as it orbits the sun. While InSight is a Mars mission, it will help answer key questions about the formation of the other rocky planets of the solar system and exoplanets beyond. (NASA/JPL-Caltech)

In the known history of our 4.5-billion-year-old solar system,  the insides of but one planet have been explored and studied.  While there’s a lot left to know about the crust, the mantle and the core of the Earth, there is a large and vibrant field dedicated to that learning.

Sometime next month, an extensive survey of the insides of a second solar system planet will begin.  That planet is Mars and, assuming safe arrival, the work will start after the InSight lander touches down on November 26.

This is not a mission that will produce dazzling images and headlines about the search for life on Mars.  But in terms of the hard science it is designed to perform, InSight has the potential to tell us an enormous amount about the makeup of Mars, how it formed, and possibly why is it but one-third the size of its terrestrial cousins, Earth and Venus.

“We know a lot about the surface of Mars, we know a lot about its atmosphere and even about its ionosphere,” says Bruce Banerdt, the mission’s principal investigator, in a NASA video. “But we don’t know very much about what goes on a mile below the surface, much less 2,000 miles below the surface.”

The goal of InSight is to fill that knowledge gap, helping NASA map out the deep structure of Mars.  And along the way, learn about the inferred formation and interiors of exoplanets, too.

Equitorial Mars and the InSight landing site, with noting of other sites. (NASA)

The lander will touch down at Elysium Planitia, a flat expanse due north of the Curiosity landing site.  The destination was selected because it is about as safe as a Mars landing site could be, and InSight did not need to be a more complex site with a compelling surface to explore.

“While I’m looking forward to those first images from the surface, I am even more eager to see the first data sets revealing what is happening deep below our landing pads.” Barerdt said. “The beauty of this mission is happening below the surface. Elysium Planitia is perfect.”

By studying the size, thickness, density and overall structure of the Martian core, mantle and crust, as well as the rate at which heat escapes from the planet’s interior, the InSight mission will provide glimpses into the evolutionary processes of all of the rocky planets in the inner solar system.

That’s because in terms of fundamental processes that shape planetary formation, Mars is an ideal subject.

It is big enough to have undergone the earliest internal heating and differentiation (separation of the crust, mantle and core) processes that shaped the terrestrial planets (Earth, Venus, Mercury, our moon), but small enough to have retained the signature of those processes over the next four billion years.

So Mars may contain the most in-depth and accurate record in the solar system of these processes. And because Mars has been less geologically active than the Earth — it does not have plate tectonics, for example —  it has retains a more complete evolutionary record in its own basic planetary building blocks.  In terms of deep planet geophysics,  it is often described as something of a fossil.

 

An artist rendering of the insides of rocky body like Mars.  The manner in which the different layers form and differentiate is seen as a central factor in whether the planet can become habitable.  (NASA)

 

By using geophysical instruments like those used on Earth, InSight will measure the fingerprints of the processes of terrestrial planet formation, as well as measuring the planet’s “vital signs.” They include the  “pulse” (seismology), “temperature” (heat flow probe), and “reflexes” (precision tracking).

One promising way InSight will peer into the Martian interior is by studying motion underground — what we know as marsquakes.

NASA has not attempted to do this kind of science since the Viking mission. Both Viking landers had their seismometers on top of the spacecraft, where they produced noisy data. InSight’s seismometer will be placed directly on the Martian surface, which will provide much cleaner data.

As described by the agency, “NASA have seen a lot of evidence suggesting Mars has quakes. But unlike quakes on Earth, which are mostly caused by tectonic plates moving around, marsquakes would be caused by other types of tectonic activity, such as volcanism and cracks forming in the planet’s crust.

“In addition, meteor impacts can create seismic waves, which InSight will try to detect.

“Each marsquake would be like a flashbulb that illuminates the structure of the planet’s interior. By studying how seismic waves pass through the different layers of the planet (the crust, mantle and core), scientists can deduce the depths of these layers and what they’re made of. In this way, seismology is like taking an X-ray of the interior of Mars.”

 

The InSight seismometer, developed by European partners and JPL, consists of a total of six seismic sensors that record the vibrations of the Martian soil in three directions in space and at two different frequency ranges. ges allows them to be mathematically combined into a single extremely broadband seismometer.  In order to protect the seismometer against wind and strong temperature fluctuations, a protective dome (Wind and Thermal Shield, WTS) will be placed over it. (German Aerospace Center)

 

Scientists think it’s likely they’ll see between a dozen and a hundred marsquakes over the course of two Earth years. The quakes are likely to be no bigger than a 6.0 on the Richter scale, which would be plenty of energy for revealing secrets about the planet’s interior.

Another area of scientific interest involves whether or not the core of Mars is liquid. InSight’s Rotation and Interior Structure Experiment, RISE, will help answer that question by tracking the location of the lander to determine just how much Mars’ North Pole wobbles as it orbits the sun.

These observations will provide information on the size of Mars’ iron-rich core and will help determine whether the core is liquid.  It will also help determine which other elements, besides iron, may be present.

The InSight science effort includes a self-hammering heat probe that will burrow down to 16 feet into the Martian soil and will for the first time measure the heat flow from the planet’s interior. Combining the rate of heat flow with other InSight data will reveal how energy within the planet drives changes on the surface.

This is especially important in trying to understand the presence and size of some of the solar system’s largest shield volcanoes in the solar system, a region known as Tharsis Mons.  Heat escaping from deep within the planet drives the formation of these types of features, as well as many others on rocky planets.

 

The Tharsis region of Mars has some of the largest volcanoes in the solar system. They include Olympus Mons, which is 375 miles in diameter and as much as 16 miles high. (U.S. Geological Survey)

InSight is not an astrobiology mission — no searching for life beyond Earth.

But it definitely is part of the process by which scientists will learn what planet formation and the dynamics of their interiors says about whether a planet can be home to life.

 

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

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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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

What Would Happen If Mars And Venus Swapped Places?

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Venus, Earth and Mars (ESA).

 

What would happen if you switched the orbits of Mars and Venus? Would our solar system have more habitable worlds?

It was a question raised at the “Comparative Climatology of Terrestrial Planets III”; a meeting held in Houston at the end of August. It brought together scientists from disciplines that included astronomers, climate science, geophysics and biology to build a picture of what affects the environment on rocky worlds in our solar system and far beyond.

The question regarding Venus and Mars was proposed as a gedankenexperiment or “thought experiment”; a favorite of Albert Einstein to conceptually understand a topic. Dropping such a problem before the interdisciplinary group in Houston was meat before lions: the elements of this question were about to be ripped apart.

The Earth’s orbit is sandwiched between that of Venus and Mars, with Venus orbiting closer to the sun and Mars orbiting further out. While both our neighbors are rocky worlds, neither are top picks for holiday destinations.

Mars has a mass of just one-tenth that of Earth, with a thin atmosphere that is being stripped by the solar wind; a stream of high energy particles that flows from the sun. Without a significant blanket of gases to trap heat, temperatures on the Martian surface average at -80°F (-60°C). Notably, Mars orbits within the boundaries of the classical habitable zone (where an Earth-like planet could maintain surface water)  but the tiny planet is not able to regulate its temperature as well as the Earth might in the same location.

 

The classical habitable zone around our sun marks where an Earth-like planet could support liquid water on the surface (Cornell University).

 

Unlike Mars, Venus has nearly the same mass as the Earth. However, the planet is suffocated by a thick atmosphere consisting principally of carbon dioxide. The heat-trapping abilities of these gases soar surface temperatures to above a lead-melting 860°F (460°C).

But what if we could switch the orbits of these planets to put Mars on a warmer path and Venus on a cooler one? Would we find that we were no longer the only habitable world in the solar system?

“Modern Mars at Venus’s orbit would be fairly toasty by Earth standards,” suggests Chris Colose, a climate scientist based at the NASA Goddard Institute for Space Studies and who proposed the topic for discussion.

Dragging the current Mars into Venus’s orbit would increase the amount of sunlight hitting the red planet. As the thin atmosphere does little to affect the surface temperature, average conditions should rise to about 90°F (32°C), similar to the Earth’s tropics. However, Mars’s thin atmosphere continues to present a problem.

Colose noted that without a thicker atmosphere or ocean, heat would not be transported efficiently around Mars. This would lead to extreme seasons and temperature gradients between the day and night. Mars’s thin atmosphere produces a surface pressure of just 6 millibars, compared to 1 bar on Earth. At such low pressures, the boiling point of water plummets to leave all pure surface water frozen or vaporized.

Mars does have have ice caps consisting of frozen carbon dioxide, with more of the greenhouse gas sunk into the soils. A brief glimmer of hope for the small world arose in the discussion with the suggestion these would be released at the higher temperatures in Venus’s orbit, providing Mars with a thicker atmosphere.

 

The surface of Mars captured by a selfie taken by the Curiosity rover at a site named Mojave. (NASA/JPL-Caltech/MSSS.)

 

However, recent research suggests there is not enough trapped carbon dioxide to provide a substantial atmosphere on Mars. In an article published in Nature Astronomy, Bruce Jakosky from the University of Colorado and Christopher Edwards at Northern Arizona University estimate that melting the ice caps would offer a maximum of a 15 millibars atmosphere.

The carbon dioxide trapped in the Martian rocks would require temperatures exceeding 300°C to be liberated, a value too high for Mars even at Venus’s orbit. 15 millibars doubles the pressure of the current atmosphere on Mars and surpasses the so-called “triple point” of water that should permit liquid water to exist. However, Jakosky and Edwards note that evaporation would be rapid in the dry martian air. Then we hit another problem: Mars is not good at holding onto atmosphere.

Orbiting Mars is NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN). Data from MAVEN has revealed that Mars’s atmosphere has been stripped away by the solar wind. It is a problem that would be exacerbated at Venus’s orbit.

“Atmospheric loss would be faster at Venus’s current position as the solar wind dynamic pressure would increase,” said Chuanfei Dong from Princeton University, who had modeled atmospheric loss on Mars and extrasolar planets.

Artist’s rendering of a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere (credit: NASA/GSFC).

This “dynamic pressure” is the combination of the density of particles from the solar wind and their velocity. The velocity does not change greatly between Mars and Venus —explained Dong— but Venus’s closer proximity to the sun boosts the density by almost a factor of 4.5. This would mean that atmosphere on Mars would be lost even more rapidly than at its current position.

“I suspect it would just be a warmer rock,” Colose concluded.

While Mars seems to fare no better at Venus’s location, what if Venus were to be towed outwards to Mars’s current orbit? Situated in the habitable zone, would this Earth-sized planet cool-off to become a second habitable world?

Surprisingly, cooling Venus might not be as simple as reducing the sunlight. Venus has a very high albedo, meaning that the planet reflects roughly 75% of the radiation it receives. The stifling temperatures at the planet surface are due not to a high level of sunlight but to the thickness of the atmosphere. Conditions on the planet may therefore not be immediately affected if Venus orbited in Mars’s cooler location.

“Venus’s atmosphere is in equilibrium,” pointed out Kevin McGouldrick from the University of Colorado and contributing scientist to Japan’s Akatsuki mission to explore Venus’s atmosphere. “Meaning that its current structure does depend on the radiation from the sun. If you change that radiation then the atmosphere will eventually adjust but it’s not likely to be quick.”

 

The surface of Venus captured from the former Soviet Union’s Venera 13 spacecraft, which touched down in March 1982. (NASA)

 

Exactly what would happen to Venus’s 90 bar atmosphere in the long term is not obvious. It may be that the planet would slowly cool to more temperate conditions. Alternatively, the planet’s shiny albedo may decrease as the upper atmosphere cools. This would allow Venus to absorb a larger fraction of the radiation that reached its new orbit and help maintain the stifling surface conditions. To really cool the planet down, Venus may have to be dragged out beyond the habitable zone.

“Past about 1.3 au, carbon dioxide will begin to condense into clouds and also onto the surface as ice,” said Ramses Ramirez from the Earth-Life Sciences Institute (ELSI) in Tokyo, who specializes in modelling the edges of the habitable zone. (An “au” is an astronomical unit, which is the distance from our sun to Earth.)

Once carbon dioxide condenses, it can no longer act as a greenhouse gas and trap heat. Instead, the ice and clouds typically reflect heat away from the surface. This defines the outer edge of the classical habitable zone when the carbon dioxide should have mainly condensed out of the atmosphere at about 1.7 au. The result should be a rapid cooling for Venus. However, this outer limit for the habitable zone was calculated for an Earth-like atmosphere.

The thick atmosphere of Venus captured by the Akatsuki orbiter. (JAXA)

“Venus has other things going on in its atmosphere compared to Earth, such as sulphuric acid clouds,” noted Ramirez. “and it is much drier, so this point (where carbon dioxide condenses) may be different for Venus.”

If Venus was continually dragged outwards, even the planet’s considerable heat supply would become exhausted.

“If you flung Venus out of the solar system as a rogue planet, it would eventually cool-off!” pointed out Max Parks, a research assistant at NASA Goddard.

It seems that simply switching the orbits of the current Venus and Mars would not produce a second habitable world. But what if the two planets formed in opposite locations? Mars is unlikely to have fared any better, but would Venus have avoided forming its lead-melting atmosphere and become a second Earth?

At first glance, this seems very probable. If the Earth was pushed inwards to Venus’s orbit, then water would start to rapidly evaporate. Like carbon dioxide, water vapour is a greenhouse gas and helps trap heat. The planet’s temperature would therefore keep increasing in a runaway cycle until all water had evaporated. This “runaway greenhouse effect” is a possible history for Venus, explaining its horrifying surface conditions. If the planet had instead formed within the habitable zone, this runaway process should be avoided as it had been for the Earth.

“When I suggested this topic, I wondered whether two inhabited planets would exist (the Earth and Venus) if Mars and Venus formed in opposite locations,” Colose said. “Being at Mars’s orbit would avoid the runaway greenhouse and a Venus-sized planet wouldn’t have its atmosphere stripped as easily as Mars.”

 

Artist impression of a terraformed Mars. (NASA GSFC)

 

But discussion within the group revealed that it is very hard to offer any guarantees that a planet will end up habitable. One example of the resultant roulette game is the planet crust. The crust of Venus is a continuous lid and not series of fragmented plates as on Earth. Our plates allow a process known as plate tectonics, whereby nutrients are cycled through the Earth’s surface and mantle to help support life. Yet, it is not clear why the Earth formed this way but Venus did not.

One theory is that the warmer Venusian crust healed breaks rapidly, preventing the formation of separate plates. However, research done by Matt Weller at the University of Texas suggests that the formation of plate tectonics might be predominantly down to luck. Small, random fluctuations might send two otherwise identical planets down different evolutionary paths, with one developing plate tectonics and the other a stagnant lid. If true, even forming the Earth in exactly the same position could result in a tectonic-less planet.

A rotating globe with tectonic plate boundaries indicated as cyan lines (credit: NASA/Goddard Space Flight Center Scientific Visualization Studio).

Venus’s warmer orbit may have shortened the time period in which plate tectonics could develop, but moving the planet to Mars’s orbit offers no guarantees of a nutrient-moving crust.

Yet whether plate tectonics is definitely needed for habitability is also not known. It was pointed out during the discussion that both Mars and Venus show signs of past volcanic activity, which might be enough action to produce a habitable surface under the right conditions.

Of course, moving a planet’s orbit is beyond our technological abilities. There are other techniques that could be tried, such as an idea by Jim Green, the NASA chief scientist and Dong involving artificially shielding Mars’s atmosphere from the solar wind.

“We reached the opposite conclusion to Bruce’s paper,” Dong noted cheerfully. “That is might be possible to use technology to give Mars an atmosphere. But it is fun to hear different voices and this is the reason why science is so interesting!”

 

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Elizabeth Tasker
Elizabeth Tasker is an astrophysicist and science communicator at the Japan Aerospace Exploration Agency (JAXA). Her research explores the formation of stars and planets, while her science articles have covered topics from Egyptian coffins to deep sea drilling (but mainly focus on exoplanets and space missions!). While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

Prepare For Lift-off! BepiColombo Launches For Mercury

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Artist illustration of the BepiColombo orbiters, MIO and Bepi, around Mercury (JAXA).

This Friday (October 19) at 10:45pm local time in French Guinea, a spacecraft is set to launch for Mercury. This is the BepiColombo mission which will begin its seven year journey to our solar system’s innermost planet. Surprisingly, the science goals for investigating this boiling hot world are intimately linked to habitability.

Mercury orbits the sun at an average distance of 35 million miles (57 million km); just 39% of the distance between the sun and the Earth. The planet therefore completes a year in just 88 Earth days.

The close proximity to the sun puts Mercury in a 3:2 tidal lock, meaning the planet rotates three times for every two orbits around the sun. (By contrast, our moon is in a 1:1 tidal lock and rotates once for every orbit around the Earth.) With only a tenuous atmosphere to redistribute heat, this orbit results in extreme temperatures between about -290°F and 800°F (-180°C to 427°C). The overall picture is one of the most inhospitable of worlds, so what do we hope to learn from this barren and baked land?

BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It consists of two orbiters, one built by each space agency. The mission is named after Giuseppe “Bepi” Colombo, an Italian mathematician who calculated the orbit of the first mission to Mercury —NASA’s Mariner 10— such that it could make repeated fly-bys of the planet.

When Mariner 10 reached Mercury in the mid-1970s, it made an astonishing discovery:  the planet had a weak magnetic field. The Earth also has a magnetic field that is driven by movement in its molten iron core.

However, with a mass of only 5.5% that of the Earth, the interior of Mercury was expected to have cooled sufficiently since its formation for the core to have solidified and jammed the breaks on magnetic field generation. This is thought to have happened to Mars, which is significantly larger than Mercury with a mass around 10% that of the Earth. So how does Mercury hold onto its field?

The discoveries only got stranger with the arrival of NASA’s MESSENGER mission in 2011. MESSENGER discovery that Mercury’s magnetic field was off-set, with the center shifted northwards by a distance equal to 20% of the planet’s radius.

The mysteries also do not end with Mercury’s wonky magnetic field. The planet’s density is very high, suggesting a much larger iron core relative to its volume compared to the Earth.

The thin atmosphere is mysteriously rich in sodium and there also appears to be more volatiles such as water ice than is expected for a planet that dances so close to the sun. All this points to a formation and evolution that we do not yet understand.

Artist impression of the JAXA orbiter, MIO, around Mercury (credit: JAXA).

The two BepiColombo orbiters will sweep around the planet to pick at these questions. The pair will get a global view of Mercury, in contrast to MESSENGER whose orbit did not allow a good view over the southern hemisphere.

“Getting data from the southern hemisphere to complement the details from MESSENGER is a logical next step to investigating the nature of Mercury’s magnetic field,” commented Masaki Fujimoto, Deputy Director General at JAXA’s Institute of Space and Astronautical Sciences (ISAS).

The European orbiter is the “Mercury Planetary Orbiter” (MPO), with “Bepi” as a nickname. Bepi will take a relatively close orbit around Mercury, with an altitude between 300 – 930 miles (480 – 1500 km). The main focus of the probe is the planet’s surface topology and composition, as well as a precise measurement of the gravitational field that reveals information about Mercury’s internal structure.

The Japanese orbiter is the “Mercury Magnetospheric Orbiter” (MMO) and was given the nickname “MIO” through a public contest held earlier this year and translates to “waterway” in Japanese.

Masaki Fujimoto, Deputy Director of ISAS, JAXA.

“Water related names received many votes,” explained Go Murakami, BepiColombo MIO project scientist. “Because in the Japanese language, Mercury is written ‘水星’ (suisei) meaning ‘water planet’.”

The focus for MIO is Mercury’s magnetic field and the interaction with the solar wind; a stream of high energy particles that comes from the sun. This requires exploration of the region around Mercury and MIO will take a correspondingly wider orbit than Bepi, with an altitude between 250 – 7500 miles (400 – 12,000km).

While Mercury itself is interesting, understanding the planet’s history has wide ranging implications for the search for habitable worlds around other stars.

The easiest exoplanets to spot are those on close orbits around dim red dwarf (also known as M-dwarf) stars. As they are far less luminous than our sun, even planets on close orbits around red dwarfs may receive a similar level of radiation to the Earth, placing them in the so-called “habitable zone.” An important example of this are the TRAPPIST-1 worlds, whose three habitable-zone planets have orbits lasting 6, 9 and 12 Earth days.

Go Murakami, BepiColombo MIO project scientist

However, the close proximity to the star comes with risks. Red dwarfs are particularly rambunctious, emitting flares that can strip the atmosphere of an orbiting planet. Mars is a classic example of this process.

Even orbiting a relatively quiet star at a distance further from the Earth, the thin atmosphere of Mars is being pulled away by the solar wind. Unless the TRAPPIST-1 worlds and those like them can protect their gases with a magnetic field, their surfaces may always be sterile.

While we know the Earth avoids this fate with its own magnetic field, it is not clear whether it would fare as well closer to the sun or with a weaker magnetic field. Mercury with its weak field and in the full blast of the solar wind offers an extreme comparison point.

A second insight Mercury could provide is that of the origin of rock. Planetary formation theories suggest there must have been mixing of dust grains in the planet-forming disc that circled the young sun. This would have shuffled up the elements that were condensing into solids at different temperatures within the disc. The exact nature and result of the shuffling remains a big question, yet it controls the composition of inner rocky planets that includes the Earth.

“The subject of planetary origins is very intriguing to me,” remarks Fujimoto. “JAXA’s asteroid sample return mission, Hayabusa2, is asking the question of where the water on Earth came from. BepiColombo will ask the complimentary question of how our planet’s rocky body was made.”

Together, the two orbiters cover a wide range of science of addressing these questions. They can also work as a pair by taking simultaneous measurements from different locations. This is particularly useful for analyzing time-varying events and also allows the planetary magnetic field to be separated out from the magnetic field carried by the solar wind.

The launch date for BepiColombo has been pushed back several times over the last few years. However, this has allowed for engineering improvements, and discoveries such as the TRAPPIST-1 planets have only added to the excitement of the mission.

“We are not unhappy about the launch delays,” said Fujimoto. “What has happened in planetary science during that period has made the expectation for BepiColombo even higher!”

The journey to the innermost planet is not a quick one. Due to arrive in 2025, the long duration is actually not due to distance but the need to brake. The pull from the sun’s gravity at such close proximity makes it hard for BepiColombo to slow sufficiently for the two probes to enter Mercury’s orbit.

The spacecraft therefore does nine planetary fly-bys; one by the Earth in April next year, then two for Venus and six for Mercury. The gravity of the planet can be used to slow down the spacecraft and allow Bepi and MIO to begin their main mission.

To my complete delight, ESA have started an animated series of shorts for the mission, similar to the cartoons for the Rosetta mission to comet 67P in 2014. These informative little videos depict the adventures of Bepi, MIO and the Mercury Transfer Module (MTM) that provides the propulsion to reach Mercury.

In addition to the videos, all three probes (and the mission itself) have twitter accounts @BepiColombo (main mission account), @esa_bepi (character account for Bepi which tweets in English), @jaxa_mmo (character account for MIO that tweets in English and Japanese) and @esa_mtm that tweets in… I’ll let you find that out!

The live launch feed from ESA is due to begin at 21:38 EDT on Friday, October 19. Good luck, BepiColombo!

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Elizabeth Tasker on Twitter
Elizabeth Tasker
Elizabeth Tasker is an astrophysicist and science communicator at the Japan Aerospace Exploration Agency (JAXA). Her research explores the formation of stars and planets, while her science articles have covered topics from Egyptian coffins to deep sea drilling (but mainly focus on exoplanets and space missions!). While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.