Large Reservoir of Liquid Water Found Deep Below the Surface of Mars

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Artist impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed on a radar cross section of the southern polar layered deposits. The leftmost white line is the radar echo from the Martian surface, while the light blue spots are highlighted radar echoes along the bottom of the ice.  Those highlighted areas measure very high reflectivity, interpreted as being caused by the presence of water. (ESA, INAF. Graphic rendering by Davide Coero Borga )

Far beneath the frigid surface of the South Pole of Mars is probably the last place where you might expect the first large body of Martian liquid water would be found.  It’s -170 F on the surface, there are no known geothermal sources that could warm the subterranean ice to make a meltwater lake, and the liquid water is calculated to be more than a mile below the surface.

Yet signs of that liquid water are what a team of Italian scientists detected — a finding that they say strongly suggests that there are other underground lakes and streams below the surface of Mars.  In a Science journal article released today, the scientists described the subterranean lake they found as being about 20 kilometers in diameter.

The detection adds significantly to the long-studied and long-debated question of how much surface water was once on Mars, a subject that has major implications for the question of whether life ever existed on the planet.

Finding the subterranean lake points to not only a wetter early Mars, said co-author Enrico Flamini of the Italian space agency, but also to a Mars that had a water cycle that collected and delivered the liquid water.  That would mean the presence of clouds, rain, evaporation, rivers, lakes and water to seep through surface cracks and pool underground.

Scientists have found many fossil waterways on Mars, minerals that can only be formed in the presence of water, and what might be the site of an ancient ocean.

But in terms of liquid water now on the planet, the record is thin.  Drops of water collected on the leg of NASA’s Phoenix Lander after it touched down in 2008, and what some have described as briny water appears to be flowing down some steep slopes in summertime.  Called recurrent slope lineae or RSLs, they appear at numerous locations when the temperatures rise and disappear when they drop.

This lake is different, however, and its detection is a major step forward in understanding the history of Mars.

Color photo mosaic of a portion of Planum Australe on Mars.  The subsurface reflective echo power is color coded and deep blue corresponds to the strongest reflections, which are interpreted as being caused by the presence of water. (USGS Astrogeology Science Center, Arizona State University, INAF)

The discovery was made analyzing echoes captured by the the radar instruments on the European Space Agency’s Mars Express, a satellite orbiting the planet since 2002.  The data for this discovery was collected from observation made between 2012 and 2015.

 

A schematic of how scientists used radar to find what they interpret to be liquid water beneath the surface of Mars. (ESA)

Antarctic researchers have long used radar on aircraft to search for lakes beneath the thick glaciers and ice layers,  and have found several hundred.  The largest is Lake Vostok, which is the sixth largest lake on Earth in terms of volume of water.  And it is two miles below the coldest spot on Earth.

So looking for a liquid lake below the southern pole of Mars wasn’t so peculiar after all.  In fact, lead author Roberto Orosei of the Institute of Radioastronomy of Bologna, Italy said that it was the ability to detect subsurface water beneath the ice of Antarctica and Greenland that helped inspire the team to look at Mars.

There are a number of ways to keep water liquid in the deep subsurface even when it is surrounded by ice.  As described by the Italian team and an accompanying Science Perspective article by Anja Diez of the Norwegian Polar Institute, the enormous pressure of the ice lowers the freezing point of water substantially.

Added to that pressure on Mars is the known presence of many salts, that the authors propose mix with the water to form a brine that lowers the freezing point further.

So the conditions are present for additional lakes and streams on Mars.  And according to Flamini, solar system exploration manager for the Italian space agency, the team is confident there are more and some of them larger than the one detected.  Finding them, however, is a difficult process and may be beyond the capabilities of the radar equipment now orbiting Mars.

 

Subsurface lakes and rivers in Antarctica. Now at least one similar lake has been found under the southern polar region of Mars. (NASA/JPL)

The view that subsurface water is present on Mars is hardly new.  Stephen Clifford, for many years a staff scientist at the Lunar and Planetary Institute, even wrote in 1987 that there could be liquid water at the base of the Martian poles due to the kind of high pressure environments he had studied in Greenland and Antarctica.

So you can imagine how gratifying it might be to learn, as he put it “of some evidence that shows that early theoretical work has some actual connection to reality.”

He considers the new findings to be “persuasive, but not definitive” — needing confirmation with other instruments.

Clifford’s wait has been long, indeed.  Many observations by teams using myriad instruments over the years did not produce the results of the Italian team.

Their discovery of liquid water is based on receiving particularly strong radar echoes from the base of the southern polar ice — echoes consistent with the higher radar reflectivity of water (as opposed to ice or rock.)

After analyzing the data in some novels ways and going through the many possible explanations other than the presence of a lake, Orosei said that none fit the results they had.  The explanation, then, was clear:  “We have to conclude there is liquid water on Mars.”

The depth of the lake — the distance from top to bottom — was impossible to measure, though the team concluded it was at least one meter and perhaps in the tens of meters.

Might the lake be a habitable?  Orosei said that because of the high salt levels “this is not a very pleasant environment for life.”

But who knows?  As he pointed out, Lake Vostok and other subglacial Antarctic lake, are known to be home to single-cell organisms that not only survive in their very salty world, but use the salt as part of their essential metabolism.

 

 

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Asteroid Remains Around Dead Stars Reveal the Likely Fate of Our Solar System

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Artist concept of an asteroid breaking up. (NASA/JPL-Caltech)

(This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS).  Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics.  Her book, “The Planet Factory,” came out last year.)

June 30th has been designated “Asteroid Day” to promote awareness of these small members of our solar system. But while asteroids are often discussed in the context of the risk they might pose to the Earth, their chewed up remains around other stars may also reveal the fate of our solar system.

It is 6.5 billion years into our future. The sun has fused hydrogen into a core of heavier helium. Compressed by its own gravity, the helium core releases heat and the sun begins to swell. It is the end of our star’s life, but what will happen to the solar system?

While very massive stars end their element-fusing days in a colossal explosion known as a supernovae, the majority of stars in our galaxy will take a less dramatic exit.

Our sun’s helium core will fuse to form carbon but there is not enough mass to achieve the crushing compression needed for the creation of heavier elements. Instead, the outer layers of the dying star will be blown away to leave a dense remnant with half the mass of our current sun, but squeezed down to the size of the Earth. This is a white dwarf; the most common of all stellar ends.

 

The life cycle of our sun

The white dwarf rapidly cools to become a dim twinkle in the sky. Within a few million years, our white dwarf will be less luminous that the sun today. Within 100 million years, it will be dimmer by a factor of 100. But examination of white dwarfs in our galaxy reveals this gentle dimming of the lights is not as peaceful as first appears.

The remnants of stars too light to fuse carbon, white dwarfs have atmospheres that should be thin shells of residue hydrogen and helium. Instead, observations have detected 20 different heavy elements in this envelope of gases that include rock-forming elements such as silicon and iron and volatiles such as carbon and nitrogen.

Infrared observations of over forty white dwarfs have additionally revealed compact dusty discs circling the dead stars. Sitting within the radius of a regular star, these could not have formed before the star shrank into a white dwarf. These must be the remains of what occurred as the star morphed from a regular fusion burner into a white dwarf.

This grizzly tale begins with the star’s expansion. Inflated by the heat from the helium core, our sun will increase to 230 times its current size. The outer layers will cool to emit a red hue that earns this bloated dying star the name “red giant”.

The outer layers of our red giant will sweep outwards and engulf Mercury and Venus, possibly stopping just short of the Earth’s position. But for any life remaining on our planet’s surface, the difference between envelopment and near-envelopment is rather moot.

The sun’s luminosity will peak at about 4000 times its current value, roasting Mars and triggering a whole new set of chemical reactions in Jupiter’s huge atmosphere. As the outer layers blow away and the red giant shrinks in mass, the surviving planets will drift outwards onto longer orbits, circling the white dwarf remnant at around twice their current distance from the sun.

The asteroids in our solar system discovered between 1980 – 2015. (Scott Manley)

But if the surviving planets are pushed outwards and the innermost worlds engulfed and vaporized, what is the origin of the compact disc and rocky pollutants? The answer, explains Dimitri Veras, explains Dimitri Veras, a planetary scientist at the University of Warwick in the UK, is asteroids.

Sitting between Mars and Jupiter, the asteroid belt is a band of rocky rubble left over from the planet formation process.

Occasionally, a kick from Jupiter’s gravity can send these space rocks skittering towards the Earth. These become known as “Near-Earth Objects” (NEOs) and are studied both for the potential threat to our planet should they collide, and also for their scientific value as time capsules from the earliest stages of planet formation.

At the moment, two missions are en-route to bring a sample from two different asteroids back to Earth. Japan’s Hayabusa2 mission has just arrived at asteroid Ryugu, returning stunning images of the asteroid to Earth. The NASA OSIRIS-REx mission is traveling to asteroid Bennu, and will arrive later this year.

 

Asteroid Ryugu images by the ONC-T camera onboard Hayabusa2 between June 18 – 20, 2018.  (JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)

 

But sitting further out than Mars, should not the majority of these small celestial bodies be unaffected by the sun’s demise? The problem turns out to be radiation.

Walk outside on a sunny afternoon and you are likely to notice that the ground beneath your feet is hottest at around 2pm in the afternoon, several hours after the sun has moved from directly overhead. This is because it takes time for the pavement to warm and re-emit the solar radiation as heat.

During that time, the Earth has rotated so that this heat radiation is released in a different direction to the absorbed radiation. Like catching a ball and throwing it away at an angle, this difference in direction gives the planet a small kick.

This kick is too small to make a difference to the Earth, but it can have a much more significant result on the evolution of an asteroid. The result is known as the YORP effect (standing for the Yarkovsky-O’Keefe-Radviesvki-Paddock effect, after the mouthful of researchers who developed the theory) and the related phenomenon named after the same first researcher, the Yarkovsky effect. Stemming from the push due to the uneven absorption and emission of radiation, the YORP effect causes a turning torque on asymmetric bodies while the Yarkovsky effect results in a push.

 

The Yarkovsky Effect describes how outgoing infrared radiation on an asteroid can speed up or slow down its motion, and in time change its orbit.  (A. Angelich, NRAO/AUI/NSF)

 

As radiation absorption and emission depends on the individual asteroid’s composition and topology, these forces are immensely hard to predict. This point was driven home in February 2013, when the world was primed for the close approach of asteroid Duende.

While everyone watched the sky in one direction, a second asteroid shot towards the Earth and exploded above Russia. This was the Chelyabinsk meteorite whose collisional path had not been anticipated. Studying the changes in an asteroid’s path due to radiation is therefore one of the primary goals of the OSIRIS-REx mission.

Given these challenges at the sun’s current level of radiation, it perhaps is not surprising that the red giant phase has more violent consequences.

Too small for gravity to pull them into a sphere, asteroids are typically lumpy rocks resembling potatoes or dumplings, like the rocky destination of Hayabusa2 and its predecessor which visited asteroid Itokawa. This asymmetry causes differences in the radiative force across the asteroid and creates a torque. This is the YORP effect and it spins the asteroid. As these small bodies typically have a weak tensile strength, the asteroid can self-destruct by spinning itself to pieces.

This effect is seen in our solar system as there is a sharp cut-off in the population for asteroids around 250m in size with rotation periods shorter than 2.33 hours.

As the radiation from our swollen red giant beats down on the asteroid belt, these space rocks will start to spin and fission. The pieces will form a disc of dust around the dying star as it becomes a white dwarf, slowly accreting onto the dead remnant to pollute its atmosphere .

So is this now the end of our tale? A white dwarf surrounded by the fissioned remains of the asteroid belt, orbited by our more distant planets on wide orbits? It could be, depending on the existence of Planet 9.

Proposed by Mike Brown and Konstantin Batygin at the California Institute of Technology, Planet 9 is a possible addition to our solar system that sits on a very distant orbit beyond Neptune. Its presence is suggested by the alignment of six small objects in the Kuiper belt, a second outer band of rocky rubble that includes the dwarf planet, Pluto.

How Planet 9 might have formed remains a subject of debate. A likely scenario is that the planet formed in the neighborhood of the gas giants, but was thrown outwards in a game of gravitational pinball during a chaotic period as our planet-forming disc was evaporating. If this is true, the planet may be able to enact a terrible revenge.

 

The six most distant objects in the solar system with orbits exclusively beyond Neptune (magenta) all line up in a single direction, indicating the presence of an outside force from an unseen Planet 9. (Caltech/R. Hurt; IPAC)

Running a set of 300 simulations, Veras discovered that the fate of Planet 9 will depend on the planet mass, the distance of its current orbit and how rapidly the sun loses its mass. In the most benign outcome, Planet 9 meets the same fate as the gas giants and drifts outwards onto an even longer orbit. However, there are two situations in which this expansion causes the orbit of Planet 9 to bend.

If a star loses mass gradually, then the orbiting planets will gently spiral outwards and keep their nearly circular paths. But if the stellar mass loss is more rapid, then very distant planets that are more loosely held by the star’s gravity may undergo a runaway expansion of their orbits. As the planet shoots away, its orbit can become bent into an ellipse.

Dimitri Veras is an astrophysicist who researches the contents of planetary systems, including our own, at the University of Warwick, United Kingdom.

Such distant worlds also risk becoming susceptible to the gravitational tug of the surrounding stars in the galaxy. Known as the “galactic tide”, this force is much too weak to affect the planets in their current positions. Yet if Planet 9 drifts too far outwards, then the tidal forces could become strong enough to bend the planet’s orbit.

On an elliptical path, Planet 9 could move from its distant location to swing into the neighborhood of the gas giants. If the planet is massive enough, this could result in either Uranus or Neptune being ejected from the solar system to become rogue worlds: a fitting, final revenge for Planet 9.

Veras’s calculations suggest the most risky discovery for internal harmony would be a Jupiter-sized Planet 9 on an orbit beyond 300 AU, or 300 times the current distance between the Earth and the sun. For comparison, Neptune sits at 30 AU and the dwarf planet Sedna is three times as far, at about 86 AU. Alternatively, a smaller super-Earth Planet 9 could pose a risk if it was further out than 3000 AU.

Observing the gory remains of this process in other star systems provides us with more than just an eerie snapshot of our future. The crushed up asteroids in the atmosphere of white dwarfs reveal the composition of that planetary system.

“There’s no other way of performing an exoplanet autopsy,” explains Veras.

The results can reveal whether the asteroids and planets that orbited the star have a similar composition to our own or something more exotic. So-called “carbon worlds” have been proposed to orbit stars more carbon-rich than our own, whose rocky base may contain graphite and diamond rather than silicates.

So far, the planet autopsy has shown Earth-like remnants, but this is one area in which we would love to see more dead remains.

 

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Know Thy Star, Know Thy Planet: How Gaia is Helping Nail Down Planet Sizes

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Gaia’s all-sky view of our Milky Way and neighboring galaxies. (ESA/Gaia/DPAC)

 

 

(This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS).  Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics.  Her book, “The Planet Factory,” came out last year.)

 

Last month, the European Space Agency’s Gaia mission released the most accurate catalogue to date of positions and motions for a staggering 1.3 billion stars.

Let’s do a few comparisons so we can be suitably amazed. The total number of stars you can see without a telescope is less than 10,000. This includes visible stars in both the northern and southern hemispheres, so looking up on a very dark night will allow you to count only about half this number.

The data just released from Gaia is accurate to 0.04 milli-arcseconds. This is a measurement of the angle on the sky, and corresponds to the width of a human hair at a distance of over 300 miles (500 km.) These results are from 22 months of observations and Gaia will ultimately whittle down the stellar positions to within 0.025 milli-arcseconds, the width of a human hair at nearly 680 miles (1000 km.)

OK, so we are now impressed. But why is knowing the precise location of stars exciting to planet hunters?

The reason is that when we claim to measure the radius or mass of a planet, we are almost always measuring the relative size compared to the star. This is true for all planets discovered via the radial velocity and transit techniques — the most common exoplanet detection methods that account for over 95% of planet discoveries.

It means that if we underestimate the star size, our true planet size may balloon from being a close match to the Earth to a giant the size of Jupiter. If this is true for many observed planets, then all our formation and evolution theories will be a mess.

The size of a star is estimated from its brightness. Brightness depends on distance, as a small, close star can appear as bright as a distant giant. Errors in the precise location of stars therefore make a big mess of exoplanet data.


An artist’s impression of the Gaia spacecraft — which is on a mission to chart a three-dimensional map of our Milky Way. In the process it will expand our understanding of the composition, formation and evolution of the galaxy. (ESA/D. Ducros)

This issue has been playing on the minds of exoplanet hunters.

In 2014, a journal paper authored by Fabienne Bastien from Vanderbilt University suggested that nearly half of the brightest stars observed by the Kepler Space Telescope are not regular stars like our sun, but actually are distant and much larger sub-giant stars. Such an error would mean planets around these stars are 20 – 30% larger than estimated, a particularly hard punch for the exoplanet community as planets around bright stars are prime targets for follow-up studies.

Previous improvements in the accuracy of the measured radii and other properties of stars have already proved their worth. In 2017, a journal paper led by Benjamin Fulton at the University of Hawaii revealed the presence of a gap in the distribution of sizes of super Earths orbiting close to their star. Planets 20% and 140% larger than the Earth appeared to be common, but there was a notable dearth of planets around twice the size of our own.

Super Earth planets with orbits of less than 100 days seem to come in two different sizes. (NASA/Ames/Caltech/University of Hawaii. (B.J.Fulton))

The most popular theory for this gap is that the peaks belong to planets with similar core sizes, but the planets with larger radii have deep atmospheres of hydrogen and helium. This would make the planets belonging to the smaller radii peak true rocky worlds, whereas the second peak would be mini Neptunes: the first evidence of a size distinction between these two regimes.

This split in the small planet population was spotted due to improved measurements of planet radii based on higher precision stellar observations made using the Keck Observatory. With a gap size of only half an Earth-radius, it had previously gone unnoticed due to the uncertainty in planet size measurements.

Both the concern of a significant error in planet sizes and the tantalizing glimpse at the insights that could be achieved with more accurate data is why Gaia is so exciting.

Launched on December 19, 2013, Gaia is a European Space Agency (ESA) space telescope for astrometry; the measurement of the position and motion of stars. The mission has the modest goal of creating a three-dimensional map of our galaxy to unprecedented precision.

Gaia measures the position of stars using a technique known as parallax, which involves looking at an object from different perspectives.

Parallax is easily demonstrated by holding up your finger and looking at it with one eye open and the other closed. Switch eyes, and you will see your finger moves in relation to the background. This movement is because you have viewed your finger from two different locations: the position of your left eye and that of your right.

Parallax is the apparent shift in the position of stars as the Earth orbits the sun. It can be used to determine distances between stars. (ESA/ATG medialab)

The degree of motion depends on the separation between your eyes and the distance to your finger: if you move your finger further from your eyes, its parallax motion will be less. By measuring the separation of your viewing locations and the amount of movement you see, the distance to an object can therefore be calculated.

Since stars are far more distant than a raised finger, we need widely separated viewing locations to detect the parallax. This can be done by observing the sky when the Earth is on opposite sides of its orbit. By measuring how far stars seem to move over a six month interval, we can calculate their distance and precisely estimate their size.

This measurement was first achieved by Friedrich Wilhelm Bessel in 1838, who calculated the distance to the star 61 Cygni. Bessel estimated the star was 10.3 light years from the Earth, just 10% lower than modern measurements which place the star at a distance of 11.4 light years.

However, measuring parallax from Earth can be challenging even with powerful telescopes. The first issue is that our atmosphere distorts light, making it difficult to measure tiny shifts in the position of more distant stars. The second problem is that the measured motion is always relative to other background stars. These more distant stars will also have a parallax motion, albeit smaller than stars closer to Earth.

As a result, the motion measured and hence the distance to a star, will depend on the parallax of the more distant stars in the same field of view. This background parallax varies over the sky, leaving no way on Earth of creating a consistent catalogue of stellar positions.

The Gaia spacecraft’s billion-pixel camera maps stars and other objects in the Milky Way. (C. Carreau/ESA)

These two conundrums are where Gaia has the advantage. Orbiting in space, Gaia simply avoids atmospheric distortion. The second issue of the background stars is tackled by a clever instrument design.

Gaia has two telescopes that point 106.4 degrees apart but project their images onto the same detector. This allows Gaia to see stars from different parts of the sky simultaneously. The telescopes slowly rotate so that each field of view is seen once by each telescope and overlaid with a field 106.4 degrees either clockwise or counter-clockwise to its position. The parallax motion of stars during Gaia’s orbit can therefore be compared both with stars in the same field of view, and with stars in two different directions.

Gaia repeats this across the sky, linking the fields of view together to globally compare stellar positions. This removes the problem of a parallax measurement depending on the motion of stars that just happen to be in the background.

The result is the relative position of all stars with respect to one another, but a reference point is needed to turn this into true distances. For this, Gaia compares the parallax motion to distant quasars.

Quasars are black holes that populate the center of galaxies and are surrounded by immensely luminous discs of gas. Being outside our Milky Way, the distance to quasars is so great that their parallax during the Earth’s orbit is negligibly small. Quasars are too rare to be within the field of view of most stars, but with stellar positions calibrated across the whole sky, Gaia can use any visible quasars to give the absolute distances to the stars.

What did these precisely measured stellar motions do to the properties of the orbiting planets? Did our small worlds vanish or the intriguing division in the sizes of super Earths disappear?

This was bravely investigated in a journal paper this month led by Travis Berger from the University of Hawaii. By matching the stars observed by Kepler to those in the Gaia catalogue, Berger confirmed that the majority of bright stars were indeed sun-like and not the suspected sub-giant population. However, the more precise stellar sizes were slightly larger on average, causing a small shift in the observed small planet radii towards bigger planets.

Planet radii derived from the new Gaia data and the Kepler (DR25) Stellar Properties Catalogue. Red points are confirmed planets while black points are planet candidates. Bottom panel shows the ratio between the two data sets. There is a small shift towards larger planets in the new Gaia data. (Figure 6 in Berger et al, 2018.)

The same result was found in a parallel study led by Fulton, who found a 0.4% increase in planet radii from Gaia compared with the (higher precision than Kepler, but less precision than Gaia) results using Keck.

The papers authored by Berger and Fulton investigated the split in super Earth sizes on short orbits, confirming that the two planet populations was still evident with the high precision Gaia data. Further exploration also revealed interesting new trends.

Fulton noticed that two peaks in the super Earth population appear at slightly larger radii for planets orbiting more massive stars. This is true irrespective of the level radiation the planets are receiving from the star, ruling out the possibility that more massive stars are simply better at evaporating away atmospheres on bigger planets. Instead, this trend implies that bigger stars build bigger planets.

Models proposed by Sheng Jin (Chinese Academy of Sciences) and Christoph Mordasini (the Max Planck Institute for Astronomy) in a paper last year proposed that the location of the split in the super Earth population could be linked to composition.

Planets made of lighter materials such as ices would need a larger size to retain their atmospheres, compared to planet cores of denser rock. If the planet size at the population split marks the transition from large rocky worlds without thick atmospheres to mini-Neptunes enveloped in gas, then it corresponds to the size needed to retain that gas.

Berger suggests that the gap between the planet populations seen in the new Gaia data is best explained by planets with an icy-rich composition. As these planets all have short orbits, this suggests these close-in worlds migrated inwards from a much colder region of the planetary system.

The high precision planet radii measurements from Gaia seem to leave our planet population intact, but suggest new trends worth exploring. This will be a great job for TESS, NASA’s recently launched planet hunter that is preparing to begin its first science run this summer. Gaia’s astrometry catalogue of stars will be ensuring we get the very best from this data.

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Exoplanet Fomalhaut b On the Move

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Enlarge and enjoy.  Fomalhaut b on its very long (1,700 year) and elliptica orbit, as seen here in five images taken by the Hubble Space Telescope over seven years.  The reference to “20 au” means that the bar shows a distance of 20 astronomical units, or 20 times the distance from the sun to the Earth. (Jason Wang/Paul Kalas; UC Berkeley)

Direct imaging of exoplanets remains in its infancy, but goodness what a treat it is already and what a promise of things to come.

Almost all of the 3,714 exoplanets confirmed so far were detected via the powerful but indirect transit and radial velocity methods — measures of slightly decreased light as a planet crosses in front of its star, or the measured wobble of a star caused by the gravitational pull of a planet.

But now 44 planets have also been detected by telescopes — in space and on the ground — looking directly at distant stars.  Using increasingly sophisticated coronagraphs to block out the blinding light of the stars, these tiny and often difficult-to-identify specks are nonetheless results that are precious to scientists and the public.

To me, they make exoplanet science accessible as perhaps nothing else so far.  Additionally, they strike me as moving — and I don’t mean in orbit.  Rather, as when you see your own insides via x-rays or MRIs, direct imaging of exoplanets provides a glimpse into the otherwise hidden realities of our world.

And in the years ahead – actually, most likely the decades ahead — this kind of direct imaging of our astronomical neighborhood will become increasingly powerful and common.

This is how the astronomers studying the Fomalhaut system describe what you are seeing:

“The Fomalhaut system harbors a large ring of rocky debris that is analogous to our Kuiper belt. Inside this ring, the planet Fomalhaut b is on a trajectory that will send it far beyond the ring in a highly elliptical orbit.

“The nature of the planet remains mysterious, with the leading theory being the planet is surrounded by its own ring or a sphere of dust.”

 

A simulation of one possible orbit for Fomalhaut b derived from the analysis of Hubble Space Telescope data between 2004 and 2012, presented in January 2013 by astronomers Paul Kalas and James Graham of Berkeley, Michael Fitzgerald of UCLA and Mark Clampin of NASA/Goddard. (Paul Kalas)

Fomalhaut b was first described in 2008 by Paul Kalas, James Graham and colleagues at the University of California, Berkeley.   If not the first object identified through direct imaging — a brown dwarf failed star preceded it, as well as other objects that remain planet candidates — Fomalhaut was among the very first.  The data came via the Advanced Camera for Surveys on the Hubble Space Telescope.

But Fomalhaut b is an unusual planet by any standard, and that resulted in a lot of early debate about whether it really was a planet.  Early efforts to confirm the presence of the planet failed, in part because the efforts were made in the infrared portion of the spectrum.

Instead, Fomalhaut b had been detected only in the optical portion of the spectrum, which is uncommon for a directly imaged planet. More specifically, it reflects bluish light, which again is unusual for a planet.  Some contended that the planet detection made by Hubble was actually a noise artifact.

A pretty serious debate ensued in 2011 but by 2013 the original Hubble data had been confirmed by two teams and its identity as a planet was broadly embraced, although the noise of the earlier debate to some extent remains.

As Kalas told me, this is probably because “no one likes to cover the end of a debate.”  Nonetheless, he said, it is over.

“Fomalhaut b at age 440 Myr (.44 billion years) is much older than the other directly imaged planets,” Kalas explained. “The younger the planet, the greater the infrared light it emits. Thus it is not particularly unusual that it is hard to image planets in the Fomalhaut system using infrared techniques.”

Kalas believes that a ring system around the planet could be reflecting the light.  Another possibility, he said, is that two dwarf planets collided and a compact dust cloud surrounding a dwarf planet is moving through the Fomalhaut system.

That scenario would be very difficult to test, he said, but the alternate possibility of a Saturnian exoplanet with a ring is something that the James Webb Space Telescope will be able to explore.

In any case, the issue of whether or not the possibly first directly-imaged planet is in fact a planet has been resolved for now.

When the International Astronomical Union held a global contest to name some of the better known exoplanets several years ago, one selected for naming was Fomalhaut b, which also now has the name “Dagon.”  The star Fomalhaut is the brightest in the constellation Pisces Australis — the Southern Fish — and Dagon was a fish god of the ancient Philistines.

 

This video of Beta Pictoris and its exoplanet was made using nine images taken with the Gemini Planet Imager over more than two years years.  The planet is expected to come our from behind its star later this year, and the GPI team hopes to capture that event. (Jason Wang; UC Berkeley, Gemini Planet Imager Exoplanet Survey)

While instruments on the W.M. Keck Observatory in Hawaii, the European Very Large Telescope in Chile and the Hubble Space Telescope have succeeded in directly imaging some planets, the attention has been most focused on the two relatively newcomers.  They are the Gemini Planet Imager (GPI), now on the Gemini South Telescope in Chile and funded largely by American organizations and universities, and the largely European Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument, also in Chile.

In real time, the two instruments correct for distorting atmospheric turbulences around Earth and also block the intense light of the host stars. Any residual incoming light is then scrutinized, and the brightest spots suggest a possible planet and can be photographed as such.

The ultimate goal is have similar instruments improved until they are powerful enough to read the atmospheres of the planets through spectroscopy, which has been done so far only to a limited extent.

Kalas, Graham and Jason Wang (a graduate student at Berkeley who put together the direct imaging movies ) are part of the GPI team, which since 2014 has been searching for Jupiter-sized and above planets orbiting some distance from their suns.  The group is a member of NASA’s NExSS initiative to encourage exoplanet scientists from many disciplines to work together.

While GPI has had successes detecting important exoplanets such as 51 Eridani b, it also studies already identified planets to increase understanding of their orbits and their characteristics.

The Gemini Planet Imager when it was being connected to the Gemini South Telescope in Chile. (Gemini Observatory)

GPI has been especially active in studying the planet Beta Pictoris b, a super Jupiter discovered using data collected by the European Southern Observatory Very Large Telescope.  While the data was first collected in 2003, it took five years to tease out the planet orbiting the young star and it took several more years to confirm the discovery and begin characterizing the planet.

GPI has followed Beta Pictoris b for several years now, compiling orbital and other data used for video above.

The planet is currently behind its sun and so cannot be observed.  But James Graham told me that the planet is expected to emerge late this year or early next year.  It remains unclear, Graham said, whether GPI will be able to capture that emergence because it will soon be moved from the Gemini telescope in Chile to the Gemini North Telescope on Hawaii.  But he certainly hopes that it will be allowed to operate until the planet reappears.

The planet 51 Eridani b was the first exoplanet discovered by the GPI and remains one of its most important.   The planet is a million times fainter than its parent star and shows the strongest methane signature ever detected on an alien planet, which should yield additional clues as to how the planet formed.

The four-year GPI campaign from Chile has not discovered as many Jupiter-and-greater sized planets as earlier expected.  Graham said that may well be because there are fewer of them than astronomers predicted, or it may be because direct imaging is difficult to do.

But Graham said the campaign is actually nowhere near over.  Much of the data collected since 2014 remains to be studied and teased apart, and other Jupiters and super Jupiters likely are hidden in the data.

Right now the exoplanet science community, and especially those active in direct imaging, are anxiously awaiting a decision by NASA, and then Congress, about the fate of the Wide Field Infrared Survey Telescope (WFIRST.)

Designed to be the first space telescope to carry a coronagraph and consequently a major step forward for direct imaging, it was scheduled to be NASA’s big new observatory of the 2020s.

But the Trump Administration cancelled the mission earlier this year, Congress then restored it but with the caveat that NASA had to provide a detailed plan for its science, its technology and its cost.  That plan remains an eagerly-awaited work in progress.

Meanwhile, here is another example of what direct imaging, with the help of soon-to-be Caltech postdoc Jason Wang, can provide.  The video of the HR 8799 system went viral when first made public in early last year.

 

The four planet system orbiting the planet HR 8977, first partially identified in 2008 by Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics and Bruce Macintosh of Stanford and others.   The video was created in 2017 after all four planets had been identified via direct imagine and their orbits had been followed for some years. (Jason Wang of UC Berkeley/Christian Marois of NRC Herzberg.)

The promise of direct imaging is enormous.  The collected photons can be used for spectroscopy that can potentially tell scientists about a planet’s radius, mass, age, effective temperature, clouds, molecular composition, rotation rate and atmospheric dynamics.

For a small, potentially habitable planet, direct imaging can measure surface temperate and pressure and determine whether it can support liquid water.  It can also potentially determine if the atmosphere is in the kind of disequilibrium regarding oxygen, ozone and perhaps methane that signal the presence of life.

But almost all this is in the future since none of the current instruments are powerful enough to collect that data.

In the meantime, researchers such as Berkeley graduate student Lea Hirsch, soon to be a Stanford postdoc,  are focused on using the strengths of the different detection methods to come up with constraints on exoplanetary characteristics (such as mass and radius) that one technique alone could not provide.

University of California at Berkeley astronomy grad student Lea Hirsch at Lick Observatory. She will be going soon to Stanford University for a postdoc with Gemini Planet Imager Principal Investigator Bruce Macintosh.

For instance, the transit technique works best for identifying planets close to their stars, direct imaging is the opposite and radial velocity is best that detecting large and relatively close-in planets.  Radial velocity gives a minimum (but not maximum) mass, while transits provide an exoplanet radius.

What Hirsch would like to do is determine constraints (limits) on the size of exoplanets using both radial velocity measurements and direct imaging.

As she explained, radial velocity will give that minimum mass, but nothing more in terms of size.  But in an indirect way for now, direct imaging can provide some maximum mass.

If, for instance, astronomers know through the radial velocity method that exoplanet X orbits a certain star and is twice the size of Jupiter, they can then look for it using direct imaging with confidence that something is there.  Let’s say the precision of the imaging is such that if a planet six times the size of Jupiter was present they would — over a period of time — detect it.

A detection would indeed be great and the planet’s mass (and more) would then be known.  But if no planet is detected — as often happens — then astronomers still collect important information.  They know that the planet they are looking for is less than six Jupiter masses.  Since the radial velocity method already determined it was at least larger than two Jupiters, scientists would then know that the planet has a mass of between two and six Jupiters.

“All the techniques in our toolkit {of exoplanet searching} have their strengths and weaknesses,” she said.  “But using those techniques together is part of our future because there’s a potential to know much more.”

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The Just-Approved European ARIEL Mission Will Be First Dedicated to Probing Exoplanet Atmospheres

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This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS).  Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics.  Her book, “The Planet Factory,” came out last year.

 

The Ariel space telescope will explore the atmospheres of exoplanets. (Artist impression, ESA)

The European Space Agency (ESA) has approved the ARIEL space mission—the world’s first dedicated exoplanet atmosphere sniffer— to fly in 2028.

ARIEL stands for the “Atmospheric Remote-sensing Infrared Exoplanet Large-Survey mission.” It is a space telescope that can detect which atoms and molecules are present in the atmosphere of an exoplanet.

The mission was selected as a medium class mission in the ESA Cosmic Vision program; the agency’s decadal plan for space missions that spans 2015 – 2025.

One of the central themes for Cosmic Vision is uncovering the conditions for planet formation and the origins of life. This has resulted in three dedicated exoplanet missions within the same decadal plan. ARIEL will join CHEOPS (in the small class mission category) and PLATO (another medium class mission) in studying worlds beyond our own sun.

Yet ARIEL is a different type of telescope from the other exoplanet-focused missions. To understand why, we need to examine what properties we can observe of these distance exo-worlds.

Exoplanet missions can be broadly divided into two types. The first type are the exoplanet hunter missions that search the skies for new worlds.

These are spacecraft and instruments such as the NASA Kepler Space Telescope. Since it launched in 2009, Kepler has been an incredibly prolific planet hunter. The telescope has found thousands of planets, modeled their orbits and told us about the distribution of their sizes.

From Kepler, we have learnt that planet formation is common, that it can occur around stars far different from our own sun, and that these worlds can have a vast range of sizes and myriad of orbits quite unlike our own Solar System.

 

Current and future (or proposed) space missions with capacities to identify and characterize exoplanets. (NASA,ESA: T. Wynne/JPL, composited by Barbara Aulicino)

 

However, the information Kepler is able to provide about individual planets is very limited. The telescope monitors stars for the tiny drop in light as the planet crosses (or “transits”) the star’s surface. From this, astronomers can measure the radius of the planet and its orbital period but nothing about the planet’s surface conditions.

The result is a little like knowing the number of students and distribution of grades in a particular school, but having no idea if the student who sits in the third row actually likes math.

The second medium-class mission in the ESA Cosmic Vision program, PLATO, is also a planet hunter. Like Kepler, PLATO will search stars for the periodic light dip that indicates the presence of a planet.

However, the telescope will explore a much larger region of the sky than Kepler, with an emphasis on detecting rocky planets on Earth-like orbits that receive a similar amount of radiation as our own planet (the so-called habitable zone).

While PLATO will not be able to tell if these planets are actually Earth-like (and not just Earth-sized on similar orbits), it will tell us a lot more about the statistics of solar systems like our own. I’m also going to throw CHEOPS into this category at well.

Technically, CHEOPS is not a planet hunter as the telescope will search for the transit light dip of stars already known to host planets. These worlds have been detected by the wobble of the star due to the presence of the orbiting planet, a method known as the radial velocity or Doppler wobble technique.

This method provides the minimum mass of a planet, but the true mass depends on the angle of the planet’s orbit. If a transit could be detected, then both the radius and the orbit inclination would be known, providing a density for these worlds. Bulk density is a very useful measurement for differentiating between rocky super Earths and gaseous mini Neptunes, providing a huge boost information for planet formation theories.

However, CHEOPS still will not know anything about the composition or surface conditions of these planets.

 

NASA’s Transiting Exoplanet Survey Satellite (TESS) is scheduled to launch next months. (Artist impression, NASA’s Goddard Space Flight Center)

By virtue of its imminent launch in the next month, the NASA TESS mission is one of the most exciting new entries in the planet hunter class. TESS will sweep over the whole sky, a huge coverage that means the telescope must focus on planets orbiting bright stars at a swift click of about 10 days per orbit.

In a two-year survey of the solar neighborhood, TESS will monitor more than 200,000 stars for planetary transits. This first-ever space-borne all-sky transit survey will identify planets ranging from Earth-sized to gas giants, around a wide range of stellar types and orbital distances.

If we compare the planet hunter missions to our school analogy, Kepler studied the grades of one school, TESS is checking a single class in multiple schools and PLATO is aiming for a full educational census. CHEOPS is adding in extra scores to greatly extend the useful statistics.

One of the key goals of TESS is to identify good targets for atmospheric follow-up missions —  a topic that brings us finally back to ARIEL.

 

Transmission spectroscopy: Molecules in an exoplanet’s atmosphere absorb different wavelengths of light, causing the atmosphere to go from transparent (left) to opaque (right). The observed planet radius therefore depends on the wavelength being observed. ARIEL can use this to determine atmospheric composition.

 

In contrast to Kepler, PLATO and TESS, ARIEL is not trying to find new planets.Instead, the telescope belongs to the second type of mission which probes conditions on the planet itself.

ARIEL will look at starlight that is passing through the atmosphere of known transiting planets. Molecules in the atmosphere absorb different wavelengths of light, turning the gases surrounding the planet opaque at these wavelengths. This produces a change in the planet’s radius as its atmosphere switches from transparent to opaque.

By measuring this apparent size change for different wavelengths, ARIEL can decipher what gases must be in the planet’s atmosphere.

This technique is known as transmission spectroscopy, where the term spectroscopy  refers to studying light split into its constituent wavelengths or spectrum. These molecules are the products of the planet’s composition, chemistry and —in the case of rocky planets— geological and potentially biological processes. This makes transmission spectroscopy a direct measure of what is going on within the planet.

 

Eclipse spectrometry: As the planet is eclipsed by the star, a secondary dip in luminosity is observed. This corresponds to the planet’s own radiated and reflected light and can also reveal atmospheric details.

 

In addition to exploring the light dip during the planet’s transit, ARIEL will also examine the tiny difference in radiation as the planet ducks behind the star.

We normally consider the planet as a dark object obscuring the star’s light during the transiting part of its orbit. However, the planet also emits radiation both due to its ownheat and reflected starlight. Just before the planet is eclipsed by the star, the observed luminosity peaks due to the combination of the star and fully illuminated planet.

When the planet moves behind the star and disappears from view, there is a sudden drop corresponding to the loss of the planet’s radiation. This fluctuation is more tiny than when the planet transits across the star, but its detection measures the planet’s own radiation.

The magnitude of this dip also depends on wavelength, as the structure and composition of the planet’s atmosphere will determine what wavelengths of radiation are being reflected and emitted.

Looking at the spectrum of this emitted (rather than transmitted) light gives ARIEL a second handle on planet conditions. The planet’s disappearance behind the star is known as the secondary or planetary eclipse giving this technique the name eclipse spectroscopy.

As well as studying the components of light as the planet and star eclipse one another, ARIEL will also monitor the luminosity of the planet during its whole orbit. This allows the telescope to see changes in radiation as different parts of the planet come into view, corresponding to a longitudinal map of planet temperature. Such changes are known as phase variations, referring to the star illuminating the planet at different angles just like phases of the moon.

An example of what can be drawn from this is found in the intriguing case of a planet known as 55 Cancri e. 55 Cancri e is a planet roughly twice the size of the Earth on an orbit so short that the scorching hot world circles its star in just 17 hours. This close proximity to the star makes 55 Cancri e tidally locked; like the moon orbiting the Earth, one side of 55 Cancri e always faces the star, giving a hemisphere of perpetual day and one of never ending night.

 

Super-Earth 55 Cancri e orbits in front of its parent star in this artist’s illustration. (ESA/Hubble, M. Kornmesser)

 

It would be logical to assume that the hottest spot on 55 Cancri e would be the center of the day side, known as the sub-stellar point. This would be visible just before the planet ducked behind the star.

However, a 2016 paper in the journal Nature, led by Brice-Olivier Demory from the University of Cambridge used the NASA Spitzer Space Telescope to map the phase variations of 55 Cancri e. To their surprise, the hot spot was shifted by 41 degrees east of the expected location. This suggested either 55 Cancri e had an atmosphere capable of redistributing the star’s heat or perhaps the baking surface was molten rock that pushed the hottest material along a river of lava.

The result from 55 Cancri e shows that ARIEL will not be the first attempt to study exoplanet atmospheres. However, it will be the first mission entirely dedicated to this task. This comes with serious advantages.

The first forages into exoplanet atmospheric data came from the Hubble and Spitzer Space Telescopes and ground-based observatories. These have been enticing but sparse, with data from only a handful of planets smaller than Neptune.

The upcoming (but, alas, just delayed until 2020) NASA James Webb Space Telescope (JWST) and European Extremely Large Telescope (E-ELT) will also be powerful observatories for exploring exoplanet atmospheres, but these are general purpose facilities whose high demand will give time for just a few tens of possible planetary targets.

To truly explore the composition of exoplanets, we need an order of magnitude more examples of atmospheric data. Without this, we risk building our models of planet formation on a handful of observed compositions that may not be typical, or only occur in particular situations. ARIEL aims to observe around 1000 exoplanet atmospheres, probing the gases enveloping planets from Jupiter to super-Earth in size.

“With the current data sets, we have unsettled questions including why some planets have flatter spectrum (less evidence of light being absorbed by different molecules) and others with distinct absorption features, and the diversity of atmospheric inventories and profiles is not well understood,” explains Yuka Fujii, Associate Professor at the Earth Life Science Institute (ELSI) at the Tokyo Institute of Technology. “The large number of planets Ariel will give us more clues.”

As a mission with a dedicated task, ARIEL is also fine tuned for the job.

The telescope can simultaneously examine a broad range of wavelengths covering all the major expected atmospheric gases such as water, carbon dioxide, methane, ammonia and hydrogen sulfide and cyanide through to the metallic compounds.

Simultaneous measurements are particularly exciting, as stars are rambunctious plasma balls and stellar activity in-between observations may interfere with being able to directly compare different wavelength observations.

While the atmosphere is potentially a good place to hunt out biosignatures, directly spotting biological action is not a target for ARIEL. The planets ARIEL will be studying are hot worlds which orbit close enough to their star to have equilibrium temperatures (the temperature at the top of the atmosphere) above 660°F (350°C).

 

ARIEL will focus on hot planets that orbit close to their star. (Artist impression, ESA/ATG medialab.)

 

The advantage of the high temperature is that the atmosphere should reflect the composition of the planet, whereas a cooler world might have many of its molecules in solid form or in condensed clouds, hidden from ARIEL.

Speaking to the journal Nature, the principal investor for the ARIEL mission, Giovanna Tinetti said that “ARIEL can really give us a full picture of what exoplanets are made of, how they form and how they evolve.”

Probing the main composition for the planet makes the data from ARIEL invaluable to understanding how and where planets formed. This last point is particularly intriguing, since results from our planet hunting telescopes strongly suggest planets do not stay where they are born.

The first discoveries of Jupiter-sized worlds on orbits much shorter than Mercury was highly surprising, since there should not be enough dusty building material so close to the star to create large planets. This advanced the idea of planetary migration, where planets form on one orbit and then move to another.

What is not clear is where these planets formed and when they began their migration. Unpicking the planet composition will help pin down their trajectories, since different elements condense into planet-building material at different distances from the star. A planet rich in water vapor, for example, likely formed far from the star where it was cold enough for ice to freeze.

Mapping the evolution of planetary systems has relevance for more temperate worlds, as the composition of planets on Earth-like orbits will dictate whether they can support processes we use for life, such as a carbon-silicate cycle and magnetic field. As the first dedicated atmospheric explorer, ARIEL will also act as a pathfinder for future missions that may one day be able to sniff the gases around a habitable world.

Whether your interests lie in composition, planet evolution or habitability, ARIEL will be the one to watch.

A wonderful overview of the different exoplanet missions, including ARIEL, Plato and TESS, can be found on David Kipping’s “Cool Worlds” YouTube series.

 

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