NASA’s Planet-Hunter TESS Has Just Been Launched to Check Out the Near Exoplanet Neighborhood


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 TESS exoplanet hunter telescope launched today on a SpaceX Falcon 9 rocket at the Cape Canaveral Air Force Station in Cape Canaveral, Fla. The space telescope will survey almost the entire sky, staring at the brightest and closest stars in an effort to find any planets that might be orbiting them. (AP Photo/John Raoux)

On January 5, 2010, NASA issued  landmark press release : the Kepler Space Telescope had discovered its first five new extra-solar planets.

The previous twenty years had seen the discovery of just over 400 planets beyond the solar system. The majority of these new worlds were Jupiter-mass gas giants, many bunched up against their star on orbits far shorter than that of Mercury. We had learnt that our planetary system was not alone in the Galaxy, but small rocky worlds on temperate orbits might still have been rare.

Based on just six weeks of data, these first discoveries from Kepler were also hot Jupiters; the easiest planets to find due to their large size and swiftly repeating signature as they zipped around the star. But expectations were high that this would be just the beginning.

“We expected Jupiter-size planets in short orbits to be the first planets Kepler could detect,” said Jon Morse, director of the Astrophysics Division at NASA Headquarters at the time the discovery was announced. “It’s only a matter of time before more Kepler observations lead to smaller planets with longer period orbits, coming closer and closer to the discovery of the first Earth analog.”

Morse’s prediction was to prove absolutely right. Now at the end of its life, the Kepler Space Telescope has found 2,343 confirmed planets, 30 of which are smaller than twice the size of the Earth and in the so-called “Habitable Zone”, meaning they receive similar levels of insolation –the amount of solar radiation reaching a given area–to our own planet.

Yet, the question remains: were any of these indeed Earth analogs?

In just a few decades, thanks to Kepler, the Hubble Space Telescope and scores of astronomers at ground-based observatories, we have gone from suspecting the presence of exoplanets to knowing there are more exoplanets than stars in our galaxy. (NASA/Ames Research Station; Jessie Dotson and Wendy Stenzel)

It was a question that Kepler was not equipped to answer. Kepler identifies the presence of a planet by looking for the periodic dip in starlight as a planet passes across the star’s surface. This “transit technique” reveals the planet’s radius and its distance from the star, which provides an estimate of the insolation level — the amount of solar radiation reaching a given area — but nothing about the planet surface conditions.

To distinguish between surfaces like those of Earth or Venus, a new generation of space telescopes is required.

These are the tasks before NASA’s long-awaited flagship James Webb Space Telescope (JWST) and  WFIRST  (if ultimately funded,)  Europe’s ARIEL mission and potentially what would be the 2030s flagship space telescope LUVOIR, if it is selected by NASA over three competitors. These telescopes will be able to probe exoplanet atmospheres and will have the capacity to measure the faint reflected light of the planets to study, via spectroscopy, their composition, geology and possibly biology.

But there is one big problem. While Kepler has found thousands of exoplanets, very few are suitable targets for these studies.

At the time of Kepler’s launch, we had no idea whether planet formation was common or anything about the distribution of planet sizes. Kepler therefore performed a planet census. By staring continuously at a small patch of the sky, Kepler waited out the time needed to see planets whose orbits took days, months and then years to complete.

From this, we discovered that planet formation takes place around the majority of stars, small planets are common and planets frequently get shoveled inwards onto short orbits close to the star. The cost of focusing on a small patch of sky is that many of the planets Kepler discovered were very distant. This is like staring into a forest; if you try to count 100 trees by looking in just one direction, many will be deep in the wood and far away from you.

Looping animated gif of the unique orbit TESS will fly. At 13.7 days, it is exactly half of the moon’s orbit, which lets the moon stabilize it. During the part of the orbit marked with blue, TESS will observe the sky, collecting science data. During the orange part, when TESS is closest to Earth, it will transmit that data to the ground. (NASA’s Goddard Space Flight Center)”

These distant planets are great for number counting, but they are too far away for their atmosphere or reflected light to be detected. In such cases, even enticing properties such as an orbit within the habitable zone have little meaning as follow-up studies that could probe signs of life are not possible.

Yet the census result that short-period planets were common allows for an entirely new type of mission. A survey to focus only on the bright, close stars whose planets would be near enough to detect their atmospheres with instruments such as the JWST. Prior to Kepler, we did not know such a telescope would find any planets. Now, we can be certain.

And that is why TESS was launched on Wednesday.

Standing for the Transiting Exoplanet Survey Satellite, TESS is a NASA mission to look for planets around bright stars less than 300 light years from Earth. All told, TESS will look at 200,000 stars spread over 85% of the sky in two years. For comparison, the field of view for Kepler had a sky coverage of just 0.25% and looked as deep as 3,000 light years into space.

Such a wide sweep means TESS cannot spend long staring at any one position. TESS will observe most of the sky for about 27 days, which is ample for detecting planets on ten day orbits, the most common orbital period found by Kepler. Over the ecliptic pole (90 degrees from the Sun’s position), TESS will observe somewhere between 27 and 351 days.  This region is where the JWST will be able to study planets throughout the year.

Image showing the planned viewing regions for the Transiting Exoplanet Survey Satellite mission. (Roland Vanderspek, Massachusetts Institute of Technology)

Bright and close by red dwarf stars, and the planets around them, are a prime target for TESS.  These stars are smaller and cooler than our sun, which makes it easier to spot the subtle dip in brightness from smaller planets. The cooler temperatures also mean that planets can orbit much closer to the star without roasting. A ten day orbit is still unlikely to be within the habitable zone, but orbits lasting between 20 – 40 days (which TESS will spot near the ecliptic poles) may receive similar insolation levels to the Earth.

A recent paper submitted to the Astrophysical Journal by Sarah Ballard, an exoplanet astronomer at MIT, estimated that TESS may find as many as 1000 planets orbiting red dwarfs and around 15 of these may be less than twice the size of the Earth and orbit within the habitable zone; ideal candidates for a JWST observation.

Previous predictions for TESS suggested the telescope will find a total (all orbits around all stars) of 500 planets less than twice the size of the Earth and 20,000 exoplanets over the first two years. Ballard’s new numbers for planets around red dwarfs are 1.5 times higher than previous predictions, so these totals look likely to be lower limits.

While future atmospheric studies with JWST are exciting, these observations will still be very challenging. Time on this multi-purpose telescope will also be limited and we have to wait until 2020 for the launch. However, the bright stars targeted by TESS are also perfect for a second type of planet hunting method: the radial velocity technique.

This second-most prolific planet-hunting technique looks for the slight shift in the wavelength of the light as the star wobbles due to the gravitational pull of the planet. As the star moves away from Earth, the light waves stretch and redden. The light shifts towards blue as the star wobbles back our way. The result is a measurement of the planet’s minimum mass. The true mass can be found if the inclination of the orbit is known, which can be measured if the planet is also seen to transit.

With both a transit measurement from TESS and a radial velocity measurement from another ground-based instrument such as HARPS, on Europe’s La Silla Telescope in Chile, the average density of the planet can be calculated.

The transit technique identifies planets by the tiny drop in starlight measured as a planet passes in front of the star.


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.

The planet density can reveal whether a world is gaseous or rocky or heavy in volatiles such as water. This is a particularly interesting question for the “super Earths” that are one of the most common class of planet found by Kepler, but for which we have no solar system analog. While an average density can only be a crude estimate of the planet interior, it can potentially be measured for a large number of the planets found by TESS and is an extremely useful guide for narrowing down planet formation theories.

But before TESS can find these planets, it first has to get into a rather unusual orbit. From launch on the SpaceX Falcon 9, TESS will boost its orbit using solid rocket motors (ignitable cylinders of solid propellent) until it is able to get a kick from the Moon’s gravity. The need for the lunar push was why the launch window for TESS was a very brief 30 seconds.

After the lunar shove, TESS will enter a highly elliptical orbit around the Earth, circling our planet every 13.7 days. This means TESS will orbit the Earth twice in the time it takes the Moon to orbit once: a situation known as a 2:1 resonance.

Planets that orbit in very close packed systems are often seen to be in similar resonant orbits. For examples, the TRAPPIST-1  worlds are in resonance and within our own solar system, the Jovian moons of Io, Europa and Ganymede orbit Jupiter in a 4:2:1 resonance.

This common occurrence is because resonant orbits are very stable, due to the pull from the gravity of the neighboring planets or moons exactly cancelling out. It is exactly for this reason that such an orbit has been chosen for TESS. With the gravitational tugs from the Moon cancelling out over an orbit, TESS’s path around the Earth will remain stable for decades. This potentially allows the mission to continue far beyond its designated two year lifespan.

TESS will take about 60 days to reach its final orbit and power-on, initialize and test its instruments. Science operations are expected to begin properly 68 days after launch. The first full data release from TESS is planned for next January, but with science operations starting in the summer we may hear the first results from TESS in the second half of this year.

Unlike with Kepler, this will be the data that will let us get to know our neighborhood.

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

Diamonds and Science: The Deep Earth, Deep Time, and Extraterrestrial Crystal Rain

Deep Earth diamond with garnet inside.  These inclusions, which occur during the diamond formation process, provide not only a way to date the diamonds, but also a window into conditions in deep Earth when they wee formed.  (M. Gress, VU Amsterdam)

We all know that cut diamonds sparkle and shine, one of the great aesthetic creations from nature.

Less well known is that diamonds and the bits of minerals, gases and water encased in them offer a unique opportunity to probe the deepest regions of our planet.

Thought to be some of the oldest available materials found on Earth — some dated at up to 3.5 billion years old — they crystallize at great depth and under great pressure.

But from the point of view of those who study them, it’s the inclusions that loom large because allow them to know the age and depth of the diamond’s formation. And some think they can ultimately provide important clues to major scientific questions about the origin of water on Earth and even the origin of life.

The strange and remarkable subterranean world where the diamonds are formed has, of course, never been visited, but has been intensively studied using a variety of indirect measurements.  And this field has in recent weeks gotten some important discoveries based on those diamond inclusions.

First is the identification by Fabrizio Nestola of the Department of Geosciences at the University of Padua and colleagues of a mineral that has been theorized to be the fourth most  common on Earth, yet had never been found in nature or successfully synthesized in a laboratory.  As reported in the journal Nature, the mineral is a variant of calcium silicate (CaSiO3), created at a high pressure that gives it a uniquely deep-earth crystal structure called “perovskite,” which is the name of a mineral, too.

Mineral science does not allow a specimen to be named until it has actually been found in name, and now this very common form of mineral finally will get a name. But more important, it moves forward our understanding of what happens far below the Earth’s surface.



Where diamonds are formed and found on Earth. The super-deep are produced very far into the mantle and are pushed up by volcanoes and convection  The lithospheric diamonds are from the rigid upper mantle and crust and the alluvial diamonds are those which came to the surface and then were transported elsewhere by natural forces. (Fabrio Nestola, Joseph R Smyth)


The additional discovery was of a tiny bit of water ice known as ICE-VII inside several other deep diamonds.  While samples of H2O ice have been identified in diamonds before, none were ICE-VII which is formed only under tremendous deep-Earth pressure.

In addition to being a first, the ICE-VII discovery adds to the growing confidence of scientists that much H2O remains deep underground, with some inferring as much deep subsurface water as found in the surface oceans.  That paper was authored by University of Nevada, Las Vegas geoscientist Oliver Tschauner and colleagues, and appeared in the journal Science.

Diamonds are a solid form of carbon with a distinctive cubic crystal structure.  They are generally formed at depths of 100 to 150 miles in the Earth’s mantle, although a few have come from as deep as 500 to 600 miles down.  (And some come from space, as described in this article below and in a just published Nature Communications article about diamonds in the Almahata Sitta meteorite that crashed in Sudan in 2006.)

Those super-deep Earth diamonds form in a cauldron up to 1,000 degrees F and at 240,000 times the atmospheric pressure at sea level.  They are made from carbon-containing fluids that dissolve minerals and replace them with what over time become diamonds.

Much more recently (tens to hundreds of million years ago), the would have been pushed to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites (blue-tinged in color and coarse grained) and lamproites (rich in potassium and also from deep in the mantle.)

The mantle – which makes up more than 80 percent of the Earth’s volume – is made of silicate minerals containing iron, aluminum, and calcium among others.  Blue diamonds are that color because of the presence of the trace mineral boron in the mantle.

And now we can add water the list as well.


Professor of Mineralogy Fabrizio Nestola presented on his recent work in advances in X-ray crystallography on diamonds and their inclusions in a talk title “Diamond, A Journey to the Center of the Earth.” One of his collaborators on the recent high-pressure calcium silicate paper is mantle geochemist Graham Pearson of the University of Alberta, where Nestola was recently a visiting professor. (

Nestola, who has been conducting his deep-Earth studies with a major grant from the European Union, is eager to take his already substantial work much further.

First he is looking for answers to the basic question of the origin of water on Earth (from incoming asteroids and comets or an integral component at formation) and ultimately to the origin of life.  Diamonds, he says, offer a pathway to study both subjects.

For water, his goal is to find a range of diamond-encircled samples that can be measured for their deuterium to hydrogen ratio — a key diagnostic to determining where in the solar system an object and its H2O originated,  Deuterium, or heavy hydrogen, is an isotope of hydrogen with an extra neutron.

An example of a super-deep diamond from the Cullinan Mine, where scientists recently discovered a diamond that provides first evidence in nature of Earth’s fourth most abundant mineral–calcium silicate perovskite. (Petra Diamonds)

As the number of diamond samples with evidence of water grows, Nestola says it will be possible to determine how the D/H ratio changes over time and as a result gain a better understanding of where the Earth’s water came from.

When it comes to clues regarding the origin of life, Nestola will be looking for carbon isotopes in the diamonds.

“Actually, it cannot be excluded that carbon from a primordial organic matter can even travel to the lower mantle,” he told me. “The oldest diamonds were dated 3.5-3.6 billion years, so it would be fantastic to detect a carbon isotope signature of surface carbon in a 3.5 billion years diamond.  This could provide very strong input for the origin of life.”

Regarding the high-pressure form of calcium silicate that he and his colleagues recently identified, Nestola said that many scientists have tried to reproduce it in their labs but have found there’s no way to keep the mineral stable at surface pressures.  So the discovery had to be made from inside the nearly impermeable container of a diamond.

The diamond that contained the common yet never before found mineral was just 0.031 millimeters across, is also a super-rare specimen.

Adding to the interest in this discovery is that other trace minerals and elements found in the inclusion strongly suggest that the material was once on the Earth’s crust.  The logic is that it would have been subducted as a function of plate tectonics billions of years ago, then encased in a forming diamond deep in the mantle, and ultimately sent back up near the surface again.

Most diamonds are born much closer to Earth’s surface, between 93 and 124 miles deep. But this particular diamond would have formed at a depth of around 500 miles, the researchers said.

“The diamond keeps the mineral at the pressure where it was formed, and so it tells us a lot about the ancient deep-Earth environment,” Nestola said.  “This is how we’ll learn about deep Earth and ancient Earth.  And we hope about those central origin questions too.”


A South African diamond crystal on kimberlite, an igneous rock formed deep in the mantle and famous for the frequency with which it contains diamonds. (Shutterstock)

For his ICE-VII study, Tschauner used diamonds found in China, the Republic of South Africa, and Botswana that had been pushed up from inside Earth.  He believes the range of locations strongly suggests that the presence of the ICE-VII is a global phenomenon.

Scientists theorize the diamonds used in the study were born in the mantle under temperatures reaching more than 1,000-degrees Fahrenheit.

“One essential question that we are working on is how much water is actually stored in the mantle.  Is it oceans, or just a bit?” Tschauner said. “This work shows there can be free excess fluids in the mantle, which is important.”

The mantle is a vast reservoir of mostly solid and very hot rock under immense pressure beneath the crust. It has an upper layer, a transition zone, and a lower layer. The upper layer has a little bit of water, but scientist estimate 10 times more water may be in the transition zone, where the enormous pressure is changing crystal structures and minerals seem to be more soluble. Minerals in the lower layer don’t seem to hold water as well.

There’s already evidence of water in the mantle in different forms, such as water that has been broken up and incorporated into other minerals. But these diamonds contain water frozen into a special kind of ice crystal. There are lots of different ways water can crystallize into ice, but ice-VII is formed under higher pressures.

While the diamond was forming, it must have encapsulated some liquid water from around the transition zone. The high temperatures prevented this water from crystalizing under the high pressures. As geologic activity moved the diamonds to the surface, they maintained the high pressures in their rigid crystal structures—but the temperature dropped. This would have caused the water to freeze into ice-VII.

The discovery of Ice-VII in the diamonds is the first known natural occurrence of the aqueous fluid from the deep mantle. Ice-VII had been found as a solid in prior lab testing of materials under intense pressure. As described before,  it begins as a liquid in the mantle.

“These discoveries are important in understanding that water-rich regions in the Earth’s interior can play a role in the global water budget and the movement of heat-generating radioactive elements,” Tschauner said.

This discovery can help scientists create new, more accurate models of what’s going on inside the Earth, specifically how and where heat is generated under the Earth’s crust.

 “It’s another piece of the puzzle in understanding how our planet works,” Tschauner said.

A polished and enlarged section of the Esquel pallasite meteoritemeteorite that delivered tiny nano-diamonds to Earth. This is a common occurrence, as there is believed to be substantial amounts of high-pressure carbon in the galaxies, and thus some diamonds. (Trustees of the NHM, London)

The diamonds studied by researchers such as Nestola and Tschauner not the sort that would ever go to the market.  “They are very bad diamonds, bad for jewelers,” Nestola said, “but precious for geologists.”

Diamonds are by no means exclusive to Earth, and are becoming a significant area of study for planetary exoplanet scientists, too.

Not only are they contained in minute form in meteorites, but atmospheric data for the gas giant planets indicates that carbon is abundant in its famous hard crystal form elsewhere in the solar system and no doubt beyond.

Lightning storms turn methane into sooty carbon which, as it falls, hardens under great pressure into graphite and then diamond.

These diamond “hail stones” eventually melt into a liquid sea in the planets’ hot cores, researchers told a an American Astronomical Society conference in 2013.

The biggest diamonds would likely be about a centimeter in diameter – “big enough to put on a ring, although of course they would be uncut,” says Dr Kevin Baines, of the University of Wisconsin-Madison and NASA’s Jet Propulsion Laboratory.

“The bottom line is that 1,000 tons of diamonds a year are being created on Saturn. People ask me – how can you really tell? Because there’s no way you can go and observe it.

“It all boils down to the chemistry. And we think we’re pretty certain.”

These potential raining diamonds, and all sorts of other extraterrestrial diamonds including possible diamond worlds, doubtless have their own scientifically compelling and important stories to tell.








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

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



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.


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

A Reprieve for Space Science?

View of WFIRST focusing on supernova SN1995E in NGC 2441. The high-priority but embattled space telescope would, if congressional support continues, add greatly to knowledge about dark energy and dark matter, supernovae, and exoplanets.  (NASA)

A quick update on a recent column about whether our “golden age” of space science and discovery was in peril because of cost overruns and Trump administration budget priorities that emphasized human space travel over science.

The 2018 omnibus spending bill that was passed Wednesday night by the House of Representatives and Thursday night by the Senate represents a major push back against the administration’s earlier NASA budget proposals.  Not only would the agency receive $1.6 billion more funding than proposed by the administration, but numerous projects that had been specifically eliminated in that proposal are back among the living.

They include four Earth science satellites, a lander to accompany the Europa Clipper mission to that potentially habitable moon and, perhaps most important, the Wide Field Infrared Survey Telescope (WFIRST) space telescope.

Funding for that mission, which was the top priority of the space science community and the National Academy of Sciences for the 2020s, was eliminated in the proposed 2019 Trump budget, but WFIRST received $150 million in the just-passed omnibus bill.

A report accompanying the omnibus bill is silent about the proposed cancellation and instructs NASA to provide to Congress in 60 days a cost estimate for the full life cycle of the mission, including any additions that might be needed.  So there appears to be a strong congressional desire to see WFIRST launch and operate.

Still hanging fire is the fate of the James Webb Space Telescope, which has fallen behind schedule again and is in danger of crossing the $8 billion cap put into place by Congress in 2011.  NASA officials said this week that they will soon announce their determination about whether a breach of the program’s cost cap will occur as a result of further delays.

NASA has a fleet of 18 Earth science missions in space, supported by aircraft, ships and ground observations. Together they have revolutionized understanding of the planet’s atmosphere, the oceans, the climate and weather. The Obama administration emphasized Earth studies, but the Trump administration has sought to eliminate future Earth missions. This visualization shows the NASA fleet in 2017, from low Earth orbit all the way out to the DSCOVR satellite taking in the million-mile view. (Goddard Space Flight Center/Matthew R. Radclif)

Four of the five Earth science programs the administration sought to cancel are specifically named for funding in the omnibus bill — the Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission, the CLARREO Pathfinder and Orbiting Carbon Observatory 3 instruments and the Earth observation instruments on the Deep Space Climate Observatory spacecraft. A fifth program was already cancelled by NASA earlier this year for technical reasons.

In all, the Science Mission Directorate would receive $6,221 million, an increase of $456  million.  Language in the bill explicitly “reiterates the importance of the decadal survey process and rejects the cancellation of scientific priorities.”

While all this is promising and hopeful, it may well be a short-term reprieve — as reported in that earlier column.

A two-year budget deal reached earlier this year raised spending caps substantially for both defense and non-defense programs, freeing up additional funding that may or may not be available in future years. The 2019 budget needs to be passed in six months, and funds could easily be stripped out then or in subsequent years.

But most important, the administration’s plans to focus on sending astronauts to the moon and establish a colony there could and almost certainly would, in time, eat up large portions of the space science budget.

Under the omnibus bill, NASA would receive $4.79 billion for space exploration efforts, up $466 million over 2017 funding levels.  This includes $2.15 million for the heavy-lift Space Launch System and $1.35 for the Orion space capsule.

The bill also provides $350 million to build a second mobile launch platform at the Kennedy Space Center. NASA considered, but did not request, funding in its 2019 proposal for a second platform.  If built, it could substantially shorten the gap between the first and second launches of SLS by eliminating the delays that would inevitably come at the launch site as it is modified to handle subsequent larger rockets.

Illustration of the Space Launch System as it will appear on the launch pad. In development for almost decade, it is now scheduled for a maiden launch in 2019. (NASA)

In some of its funding, the omnibus bill seems almost too good to be true.

The planetary science program, for instance, received $300 million more than last year.  The $2.2 billion total includes $595 million for work on the Europa Clipper mission and for a follow-on lander — a scientifically exciting aspect of the Europa program, but one that had earlier been cancelled.

The bill also keeps earlier plans to use the SLS to launch Europa Clipper by 2022 and the lander by 2024. An SLS launch would halve the number of years it would take to get the spacecraft to Europa, a moon of Jupiter.

But NASA’s assessment of the SLS program make it highly unlikely that the rockets will be ready for those launches, and there are competing plans to use the second SLS launch to send humans into orbit.

As a kind of added treat, the omnibus bill also provides $23 million for a proposed helicopter NASA has under consideration for the the Mars 2020 rover mission.

The Trump administration has shown great interest in manned missions and little interest in space science and especially Earth science.

Clearly, many members of Congress have very different views, informed no doubt by a highly mobilized space science community.  And for now, at least, they appear to have carried the day.


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

Space Science In Peril


NASA’s decades-long success at enabling ground-breaking discoveries about our planet, our solar system, our galaxy, our origins and the billions of other planets out there is one of the crown jewels of our nation’s collective inventiveness and will, and surely of our global soft power.

Others have of course made major contributions as well.  But from the Viking Mars landings of the 1970s on to the grand space observatories Hubble and Spitzer and Chandra, to the planetary explorations such as Cassini (Saturn), Galileo and Juno (Jupiter), New Horizons (Pluto and beyond) and Curiosity (Mars), to the pioneering exoplanet census of Kepler, the myriad spacecraft enhancing our understanding of our own planet and the sun, and the pipeline confidently filled with of missions to come, NASA has been the consistent and essential world leader.

What we know of our world writ large has just exploded in these decades, and we’re far richer for it.

But of late, the future of these efforts to ever expand our knowledge of the logic and make-up of our universe has become worryingly unclear.

First there are the recently revealed new problems with the James Webb Space Telescope, initially scheduled to launch years ago and now reportedly unlikely to meet its launch date next year.  It is also over budget again and under serious threat.

This news came as Congress wrestled with the White House decision to scuttle the WFIRST dark energy, planet and star formation, and exoplanet mission, planned as NASA’s major flagship mission of the 2020s.

And perhaps most worrisome, NASA now wants to fold its Space Technology Mission Directorate into the Human Exploration and Operations Directorate, surely to support the administration’s goal of setting up a human colony on the moon.

This is an Apollo-sized, many-year and very costly effort that would have to take funds away from potential space science missions unless the NASA budget was growing substantially. But the proposed 2019 NASA budget would cap spending for the next four years.

Might our Golden Era of space discovery be winding down?

An illustration of the James Webb Space Telescope after deploying in space.  The pioneering technology of the JWST is both its great promise and recurring pitfall. (NASA)

First the JWST situation.  The telescope, far more powerful and complex than anything sent into space, is expected to open up new understandings about the origins of the universe, xxx, and exoplants.

But late last month, the General Accounting Office released a report that said:

“The James Webb Space Telescope, the planned successor to the Hubble Telescope, is one of NASA’s most complex and expensive projects.

“NASA recently announced that JWST’s launch would be delayed several months, from October 2018 to no later than June 2019, because components of the telescope are taking longer to integrate than planned.

“Based on the amount of work NASA has to complete before JWST is ready to launch, we found that it’s likely the launch date will be delayed again. If that happens, the project will be at risk of exceeding the $8 billion cost cap set by Congress.”

That cost cap was put in place in 2011, after a House subcommittee voted to end the project entirely because of overruns.  The full Congress then agreed to continue funding but only to the $8 billion mark.

Will Congress agree to more money if the agency needs more time to complete launch preparations?  Or will the money have to come out of the existing NASA budget?  It seems highly unlikely that the project will be halted but all the overruns and delays — often based on the difficulties associated with new technologies — cast a pall of sorts over plans for big space science projects in the decades ahead.

The long-term ramifications of the JWST delays and overruns could be substantial.  The space community began pushing in the 1970s for the launching of a new grand space observatory every decade, and the science and public engagement results have been tremendous.  The process of selecting a grand observatory mission for the 2030s is underway now, with teams of scientists and engineers feverishly gathering ideas, data, technology know-how and cost predictions for four contenders.

Two focus on astrophysics and questions about the make-up and origins of the universe and two on exoplanets and the effort to determine if some might have the conditions that could support life and, perhaps, might actually do so.  Those two are the Habitable-Exoplanet Imaging Mission (HabEx), and Large Ultraviolet-Optical-Infrared Surveyor (LUVOIR).

Both are likely to be quite costly, and LUVOIR in particular.  But unlike HabEx, LUVOIR would have the power and kinds of instruments needed to determine not only if life might be possible on an exoplanet, but potentially if that life is present.  It would be a Hubble on steroids — a dream observatory that would have the ability to transform (or greatly deepen) space science.

If it is restored to the NASA budget, WFIRST would survey distant galaxies looking for the effects of dark matter, that mysterious stuff that can’t be seen or touched but outnumbers normal matter by roughly 5 to 1. The telescope would study Type Ia supernovas to track dark energy, that strange repulsive force that is causing the universe to expand faster and faster. The observatory could  use its instruments to explore the planets around other stars and to better understand how stars and planets are formed . (NASA)

But the enormous promise of a LUVOIR or HabEx helps explain some of the scientific dismay about the administration’s decision to cancel NASA’s  “flagship” observatory of the 2020s, the Wide Field Infrared Survey Telescope (WFIRST.)

Selected in 2010 by the space science community and later the National Academy of Sciences as the priority mission of the 2020s,  WFIRST would focus on the nature of dark matter, the expansion of the universe, and would push forward some exoplanet observing as well.

So cancelling of the mission — if Congress now allows that to happen — would not only eliminate an important observatory that would keep NASA in the forefront of space astrophysics, but would also send a message that even being selected as the top priority space mission for the decade does not provide ironclad protection.

At space subcommittee hearing of the House Science Committee with NASA Acting Administrator Robert Lightfoot, Rep. Ami Bera (D. Calif.) voiced that concern earlier this month.

“The decadal survey has served us well, and not looking at this scientific-based prioritization and moving away from that can certainty set a dangerous precedent,” Bera warned.

James Irwin on the moon during the Apollo 15 mission of the summer of 1971.  While Apollo was an enormous success, it took up large percentages of the NASA budget between 1964 and 1972.  The peak was 1967, when it accounted for 70 percent of the NASA budget.  In all, the program cost the 2016 equivalent of $107 billion.  (NASA)

The elephant in the room in this discussion is easy to identify — the administration’s well-publicized desire to set up an on-going human colony of Americans on the moon, or at least to get American astronauts back on the lunar surface during the 2020s.  The stated goals are exploration, commercial and international joint ventures and geopolitics, with seldom a mention of science.

The proposed 2019 budget does not set aside a great deal of money for the moon project, but it does do something that worries many former NASA leaders and NASA followers — the funding for space technology and innovation ($1 billion) will now be housed within the human exploration directorate, as “Exploration Research and Technology.”

The stated logic is that technological advancement should be directed toward human space exploration.

“The FY 2019 budget is restructured to align with the Administration’s new space exploration policy by consolidating and refocusing existing NASA technology development activities on space exploration,” the budget document reads.

This will inevitably take some funds away from technology projects that could be useful across NASA’s directorates, but more important sets the stage for a ramp up in funding for moon missions in the years ahead.  And since the proposed 2019 budget would cap NASA funds for the next four years, other NASA programs would have to suffer — most notably Earth sciences and other science exploration unrelated to the moon.

Seldom discussed by those excited by the prospect of continuing the legacy of the Apollo program and having Americans return to the moon is that Apollo was extraordinarily expensive and required great national sacrifice.

During the 1960s the NASA budget (which was directed in large part into the Mercury and Apollo manned missions) took up as much as four percent of the federal budget (the equivalent of $40 billion today.)  For six years it took up three percent or more of the budget.  The NASA budget is now at its lowest point since 1959 as a percentage of the federal budget — less than one-half of one percent of the budget —  and provides less than $20 billion and has for decades.

It seems pretty clear that ambitious humans-on-the-moon project would mean fewer Cassinis, fewer Hubbles, fewer Keplers.

Another sign of the lowering profile of NASA science is the proposal in the 2019 budget to launch the other NASA flagship science mission of the 2020s, the flyby of Jupiter’s moon Europa, on a commercial heavy-lift rocket rather than NASA’s Space Launch System.  The SLS was sold to Congress as the vehicle that could send spacecraft speedily to outer planets, but now both production delays and a desire to quickly get astronauts into space on the SLS has made that far less likely and some years further out, if it happens at all.

Heavy lift rockets other than SLS—including SpaceX’s Falcon Heavy and the Delta IV from United Launch Alliance —lack the power to blast the Europa Clipper directly from Earth to Jupiter. A conventional rocket would rely on three gravity assists from Earth and one from Venus, increasing the transit time from about 3 years to at least 6 years.

The search for life, or habitable conditions, beyond Earth in the 2020s will continue on Mars and is scheduled to expand to Jupiter’s moon Europa.  The moon orbits Jupiter every 3.5 days and that proximity, coupled with the fact that Europa has a slightly elliptical rather than circular orbit, creates the tidal “flexing” and resulting heating that can keep water liquid beneath its surface of ice. The Europa Clipper mission was set by Congress to launch in 2022, but that date looks near impossible.  A plan to have an accompanying lander was sidelined because of cost. (NASA)

Missions happen when they are a priority, and clearly now not just a scientific priority.  Nothing is settled, but the warning signs are there that the moon program will force space science down the priority list unless NASA suddenly gets a lot more money.

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