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

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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 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.

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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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Storming the One-Meter-Per-Second Barrier

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Kitt Peak National Observatory mountain top at Dusk looking north. Visible in the picture are the NOAO 4-meter Mayall, the Steward Observatory 90-inch, the University of Arizona Lunar and Planetary Laboratory Spacewatch Telescopes, LOTIS, 0.4-meter Visitor Center Telescope, Case Western Reserve University Observatory and the SARA Observatory. Credit: P. Marenfeld (NOAO/AURA/NSF)
The Kitt Peak National Observatory, on the Tohono O’odham reservation outside Tucson, will be home to a next-generation spectrometer and related system which will allow astronomers to detect much smaller exoplanets through the radial velocity method.  P. Marenfeld (NOAO/AURA/NSF)

When the first exoplanet was identified via the radial velocity method, the Swiss team was able to detect a wobble in the star 51 Pegasi at a rate of 50 meters per second.   The wobble is the star’s movement back and forth caused by the gravitational pull of the planet, and in that first case it was dramatic — the effects of a giant Jupiter-sized planet orbiting extremely close to the star.

Many of the early exoplanet discoveries were of similarly large planets close to their host stars, but it wasn’t because there are so many of them in the cosmos.  Rather, it was a function of the capabilities of the spectrographs and other instruments used to view the star.  They were pioneering breakthroughs, but they didn’t have the precision needed to measure wobbles other than the large, dramatic ones caused by a close-in, huge planet.

That was the mid 1990s, and radial velocity astronomers have worked tirelessly since to “beat down” that 50 meters per second number.  And twenty years later, RV astronomers using far more precise instruments and more refined techniques have succeeded substantially:  1 meter per second of wobble is now achieved for the quietest stars.  That has vastly improved their ability to find smaller exoplanets further from their stars and is a major achievement.  But it has nonetheless been a major frustration for astronomers because to detect terrestrial exoplanets in the Earth-sized range, they have to get much more precise  — in the range of tens of centimeters per second.

A number of efforts to build systems that can get that low are underway, most notably the ESPRESSO spectrograph scheduled to begin work on the High Accuracy Radial Vlocity Planet Searcher (HARPS) in Chile next year. Then earlier this month an ambitious NASA-National Science Foundation project was awarded to Penn State University to join the race.  The next-generation spectrograph is scheduled to be finished in 2019 and installed at the Kitt Peak National Observatory in Arizona, and its stated goal is to reach the 20 to 30 centimeters per second range.

Suvrath Mahadevan, an assistant professor at Penn State, is principal investigator for the project.  It is called NEID, which means ‘to see’ in the language of the Tohono O’odham, on whose land the Kitt Peak observatory is located.

“For many reasons, the (radial velocity) community has been desperate for an instrument that would allow for detections of smaller planets, and ones in habitable zones,” he said.  “We’re confident that the instrument we’re building will — in time — provide that capability.”

Las Cumbres Observatory Global Telescope Network.
A illustration of how the radial velocity method of planet hunting works.  The wobble of the stars is far away miniscule in galactic terms, making extreme precision essential in measuring the movement. (Las Cumbres Observatory Global Telescope Network)

Project scientist Jason Wright, associate professor of astronomy and astrophysics at Penn State, put it this way:  “NEID will be more stable than any existing spectrograph, allowing astronomers around the world to make the precise measurements of the motions of nearby, Sun-like stars.”  He said his Penn State team will use the instrument “to discover and measure the orbits of rocky planets at the right distances from their stars to host liquid water on their surfaces.”

NASA and the NSF wanted the new spectrograph built on an aggressive timetable to meet major coming opportunities and needs, Mahadevan said.

The speedy three-year finish date is a function of the role that radial velocity detection plays in exoplanet research.  While many planets have been, and will be, first detected through the technique, it is also essential in the confirming of candidate planets identified by NASA space telescopes such as Kepler, the soon-to-be launched TESS (the Transiting Exoplanet Survey Satellite) and others into the future.  There is a huge backlog of planets to be confirmed, and many more expected in the relatively near future.

What’s more, as Mahadevan explained, an instrument like NEID could significantly help NASA’s planning for a possible 2030s Flagship space telescope mission focused on exoplanets.  Two of the four NASA contenders under study are in that category — LUVOIR (Large Ultraviolet Visible Infrared) Surveyor and Hab-Ex — and their capabilities, technologies, timetables and cost are all now under consideration.

If NEID can identify some clearly Earth-sized planets in habitable zones, he said, then the planning for LUVOIR or Hab-Ex could be more focused (and the proposal potentially less costly.)  This is because the observatory could be designed to look at a limited number of exoplanets and their host stars, rather than scanning the skies for a clearly Earth-like planet.

“Right now we have no definite Earth-sized planets in a habitable zone, so a LUVOIR or Hab_ex design would have to include a blind search.  But if we know of maybe 15 planets we’re pretty sure are in their habitable zones, the targets get more limited and the project becomes a lot cheaper.”

Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)
Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)

These possibilities, however, are for the future.  Now, Mahadevan said, the Penn State team has to build a re-considered spectrograph, a significant advance on what has come before.  With its track record of approaching their work through interdisciplinary collaboration, the Penn State team will be joined by collaborators from NASA Goddard Space Flight Center, University of Colorado, National Institute of Standards and Technology, Macquarie University in Australia, Australian Astronomical Observatory, and Physical Research Laboratory in India.  Much of the work will be done over the next three years at Penn State, but some at the partner institutions as well.

Key to their assembly approach is that the instrument will be put together in vacuum-sealed environment and will have no vibrating or moving parts.  This design stability will prevent, or minimize, instrument-based misreadings of the very distant starlight being analyzed.

A major issue confronting radial velocity astronomers is that light from stars can fluctuate for many reasons other than a nearby planet — from sunspots, storms, and other magnetic phenomena.  The NEID instrument will try to minimize these stellar disruptors by providing the broadest wavelength coverage so far in an exoplanet spectrograph, Mahadevan said, collecting light from well into the blue range of the spectrum to almost the end of the red.

“We’re not really building a spectorograph but a radial velocity system, he said.  That includes upgrades to the telescope port, the data pipeline and more.

This is how Lori Allen, Associate Director for Kitt Peak, described that new “system”: “The extreme precision (of NEID) results from numerous design factors including the extreme stability of the spectrometer environment, image stabilization at the telescope, innovative fiber optic design, as well as state-of-the-art calibration and data reduction techniques”.

 

The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. Operated by National Optical Astronomy Observatory, the $10 million project is a collaboration of NASA and the National Science Foundation.
The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. The site is managed by the National Optical Astronomy Observatory, and $10 million spectrograph project is a collaboration of NASA and the National Science Foundation.

Sixteen teams ultimately competed to build the spectrograph, and the final two contenders were Penn State and MIT.  Mahadevan said that, in addition to its spectrograph design, he believed several factors helped the Penn State proposal prevail.

His team has worked for several years on another advanced spectrograph for the Hobby-Eberly Telescope in Texas, one that required complex vacuum-sealed and very cold temperature construction.  Although the challenges slowed the design, the team ultimately succeeded in demonstrating the environmental stability in the lab.  So Penn State had a track record. What’s more, the school and its Center for Exoplanets and Habitable Worlds have a history of working in an interdisciplinary manner, and have been part of several NASA Astrobiology Institute projects. (The instrument has a blog of its own: NEID.)

The Kitt Peak observatory, which saw first light in 1994, has been the sight of many discoveries, but in recent years has faced cutbacks in NSF funding.  There was some discussion of reducing its use, and the NASA-NSF decision t0 upgrade the spectrograph was in part an effort to make it highly relevant again.  And given the scientific need to confirm so many planets — a need that will grow substantially after TESS launches in 2017 or 2018 and begins sending back information on thousands of additional transiting exoplanets — enhancing the capabilities of the Kitt Peak 3.5 meter telescope made sense.

Kitt Peak is unusual in being open to all comers with a great proposal, whether they’re from the U.S. or abroad.  The Penn State team and partners will get a certain number of dedicated night to observe, but many others will be allocated through competitive reviews.  And so when NEID is completed, astronomers from around will have a shot at using this state-of-the-art planet finder.

 

 

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Hunting for Exoplanets Via TESS

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The TESS satellite, which will launch in 2017, will use four cameras to search for exoplanets around bright nearby stars. MIT
The TESS satellite, which will launch in 2017, will use four cameras to search for exoplanets around bright nearby stars. MIT initially proposed the mission, and it was approved in 2013.  (MIT)

Seven years ago this month the Kepler spacecraft launched into space – the first NASA mission dedicated to searching for planets around distant stars. The goal was to conduct a census of these exoplanets, to learn whether planets are common or rare. And in particular, to understand whether planets like Earth are common or rare.

With the discovery and confirmation of over 1,000 exoplanets (and thousands more exoplanet candidates that have not yet been confirmed), Kepler has taught us that planets are indeed common, and scientists have been able to make new inferences about how planetary systems form and evolve. But the planets found by Kepler are almost exclusively around distant, faint stars, and the observations needed to further study and characterize these planets are challenging. Enter TESS.

The Transiting Exoplanet Survey Satellite (TESS) is a NASA Explorer mission designed to search for new exoplanets around bright, nearby stars. The method that TESS will use is identical to that used by Kepler – it looks for planets that transit in front of their host star. Imagine that you’re looking at a star, and that star has planets around it.

If the orbit of the planet is aligned correctly, then once per “year” of the planet (i.e. once per orbit), the planet will pass in front of the star. As the planet moves in front of the star, it blocks a small fraction of the light, so the star appears to get slightly fainter. As the planet moves out of transit, the star returns to normal brightness. We can see an example of this in our own solar system on May 9, 2016, as Mercury passes in front of the Sun.

Tranit
A small dip in the amount of light emanating from a star tells astronomers that a planet may well be crossing in front of it.

We can learn a lot from observing the transits of a planet. First, we can learn the size of a planet – the bigger the planet, the more light it will block, and the larger the “dip” in the brightness of the host star. Second, we can learn how long the planet’s year is – since it only passes in front of the star once per orbit, the time between transits is the planet’s year.

The duration of the year, in combination with the properties of the host star, also allows us to determine if a planet might be habitable. With high precision measurements, we can also infer much more about the orbit of the planet (e.g., the eccentricity of the orbit). And, in fact, in some cases, we can look at small changes in the apparent year of the planet to discover additional planets in the system that do not transit (Transit Timing Variations).

To observe these transits, TESS will use four identical, extremely precise cameras mounted behind four identical 8-inch telescopes. Each one of these cameras will be sensitive to changes in the brightness of a star as small as about 40 parts per million, allowing TESS to detect planets even smaller than our planet.

Earth, transiting the sun, would produce a dip of about 100 parts per million. Each of the four cameras has a field-of-view of 24°×24°, and the fields of the four cameras are adjacent so that TESS will instantaneously observe a 24°×96° swath of the sky (referred to as an observation sector). Within this field, TESS will collect “postage stamp” images of about 8,000 stars every two minutes – the postage stamps are small sub-images, nominally about 10×10 pixels.

Over the course of two years, TESS will survey nearly the entire sky looking for transiting exoplanets. Each observing sector covers a patch of sky 24°×96° for 27 days; where sectors overlap, TESS will be able to observe planets for a long as nearly a year.
Over the course of two years, TESS will survey nearly the entire sky looking for transiting exoplanets. Each observing sector covers a patch of sky 24°×96° for 27 days; where sectors overlap, TESS will be able to observe planets for a long as nearly a year. (Ricker et al)

TESS will stare continuously at each of these observation sectors for 27 days before moving to the next sector; over the course of one year, this will give TESS coverage of almost one entire hemisphere, with postage stamp data on approximately 100,000 stars. In the second year of the TESS mission, 13 additional sectors will cover the other hemisphere of the sky, resulting in observations of about 200,000 stars.

The method used for these postage stamp-sized observations is very similar to that used for Kepler, but the survey itself is different. While TESS is conducting an all-sky survey (about 40,000 square degrees), Kepler looked at only a relatively small patch of the sky (115 square degrees). But with a telescope seven times larger than those on TESS, Kepler was able to look much further away – TESS surveys stars within only about 200 light years, compared to 3,000 light years for Kepler.

This underscores the difference in the underlying philosophy of the two missions. The goal of Kepler was to understand the statistics of exoplanets, to conduct a census to understand the population as a whole.

Artist's rendering of a Jupiter-sized exoplanet and its host, a star slightly more massive than our sun. Image credit: ESO
Artist’s rendering of a Jupiter-sized exoplanet and its host, a star slightly more massive than our sun. Image credit: ESO

TESS, on the other hand, is about finding planets around bright, nearby stars –planets that will be well-suited to follow-up observations from both the ground and from space. On average, the stars observed by TESS will be between 30 and 100 times brighter than those observed by Kepler. These brighter targets will allow for follow-up observations that will be critical for understanding the nature of the newly discovered planets – more on that in a moment.

In raw numbers, what do we expect from TESS?

Former MIT graduate student Peter Sullivan conducted detailed simulations of the mission to make a prediction on what it might discover, and these results are incredible. With TESS, we expect to find over 1,600 new exoplanets within the postage stamp data, with about 70 of those being about the size of the Earth (within 25% of the Earth’s diameter), and almost 500 “super-Earth” planets (less than twice the diameter of Earth).

Perhaps most exciting is the likelihood that TESS will discover a handful of Earth-sized planets in the habitable zones of their host stars.

Finding an Earth-sized planet in a distant habitable zone is a top goal of TESS, and of the exoplanet community as a whole. (NASA/Chester Harman)
Finding an Earth-sized planet in a distant habitable zone is a top goal of TESS, and of the exoplanet community as a whole. (NASA/Chester Harman)

In addition, while TESS obtains the postage stamp data every two minutes, it also obtains a full-frame image – a picture of the entire observing sector – every thirty minutes.

In those data, we expect to find over 20,000 additional planets. The majority of those will be large (Jupiter-size) planets, but there will also be about 1,400 additional super-Earths discovered. The sheer number of planets that will be found is amazing, but more important than the number is the fact that all of these planets will be orbiting bright, nearby stars. This is a fantastic leap relative to where we were just 25 years ago, when not a single exoplanet was known.

One of the challenges of transit measurements is that they can produce false positives. Stellar activity can cause quasi-periodic dips in the brightness of a star. An eclipsing binary star in the background could mimic the dip from a transiting planet. With careful analysis, most of these effects can be accounted for, but it remains important to follow a transit observation with a confirmation — making a secondary measurement to ensure that what was observed is, in fact, a planet.

The most straightforward way to confirm a transiting exoplanet is with a radial velocity (RV) measurement. The RV method takes advantage of the reflex motion of the star; as a planet orbits a star, the star itself doesn’t remain stationary. In fact, both the planet and the star orbit the center of mass of the system. So, if one looks at spectral lines from the host star, it is possible to measure the Doppler shift of those lines as the star does it’s little pirouette around the center of mass.

From this data, astronomers can measure the mass and the year (orbital period) of the exoplanet. This confirms the orbital period observed from the transit data, and the combination of radius (observed from the transit) and the mass (observed from the RV) gives us the bulk density of the planet. With that, we can make inferences about the composition of the planet – is it a rock, like Earth? A water-world or a ball of ice? A gas giant?

Measurements of the TESS space telescope. (NASA)
Measurements of the TESS space telescope. (NASA)

Making the RV measurement, while straightforward, is not an easy one – less than 10% of the exoplanet candidates found by Kepler have been confirmed with RV measurements, largely because the host stars themselves are faint. For TESS, however, because the host stars are nearby and bright, it will be possible to make follow-up observations on nearly all of the stars that host small planets – the only major limitation will be due to the noise from the stars themselves (i.e. flares, starspots).

Further, because these host stars are bright, they will also be excellent targets for transit spectroscopy. Imagine, for a moment, that there is a transiting planet with a very large atmosphere, and that this atmosphere is transparent in red and blue, but completely opaque in the green. Then, if you observe the planet in red light (or blue light), only the “rock” part of the planet will block light from the star. In green light, however, the rock and the atmosphere will both block light – in the green, the planet appears to be larger than at other wavelengths.

This is the core idea behind transit spectroscopy. By measuring how the apparent size of a transiting planet varies with wavelength, we can infer the composition (and potentially the structure) of the planetary atmosphere. This technique has been used successfully on a very small number of exoplanets to date, but with the large number of planets that TESS will find, and the fact that they will all be around bright, nearby stars, it will be possible to use the James Webb Space Telescope and the next generation of large ground-based telescopes to make these observations.

TESS is expected to add 2,000 new exoplanets to the already long list of the ones alrday detected. (NASA)
TESS is expected to monitor more than 200,000 stars and add 1,500 new exoplanets to the already long list of those confirmed or awaiting confirmation.  (NASA)

For the first time, astronomers will actually be able to study not only individual exoplanets, but will be able to study enough of them to make comparisons and draw conclusions about how planets form and evolve.

For me, TESS is endlessly exciting. The sheer quantity of new exoplanets is stunning. The ability to use follow-up observations to characterize these planets will create new paths for scientific investigation. And the discoveries made will help define the science that will be pursued by future missions such as WFIRST, and perhaps more ambitious missions in the future. But, perhaps most exciting, TESS is in part about making “Exoplanets for Everyone.”

In a few years, it will be possible for everyone to go outside to a dark location, point at a star that you can see with the naked eye, and say “there is a planet around that star.” And the night sky may never feel quite the same again.

Video link: TESS Trailer — https://youtube/ZsPStvGgNuk

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