15,000 Galaxies in One Image

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Astronomers have just assembled one of the most comprehensive portraits yet of the universe’s evolutionary history, based on a broad spectrum of observations by the Hubble Space Telescope and other space and ground-based telescopes.  Each of the approximately 15,000 specks and spirals are galaxies, widely distributed in time and space. (NASA, ESA, P. Oesch of the University of Geneva, and M. Montes of the University of New South Wales)

Here’s an image to fire your imagination: Fifteen thousand galaxies in one picture — sources of light detectable today that were generated as much as 11 billion years ago.

Of those 15,000 galaxies, some 12,000 are inferred to be in the process of forming stars.  That’s hardly surprising because the period around 11 billions years ago has been determined to be the prime star-forming period in the history of the universe.  That means for the oldest galaxies in the image, we’re seeing light that left its galaxy but three billion years after the Big Bang.

This photo mosaic, put together from images taken by the Hubble Space Telescope and other space and ground-based telescopes, does not capture the earliest galaxies detected. That designation belongs to a galaxy found in 2016 that was 420 million years old at the time it sent out the photons just collected. (Photo below.)

Nor is it quite as visually dramatic as the iconic Ultra Deep Field image produced by NASA in 2014. (Photo below as well.)

But this image is one of the most comprehensive yet of the history of the evolution of the universe, presenting galaxy light coming to us over a timeline up to those 11 billion years.  The image was released last week by NASA and supports an earlier paper in The Astrophysical Journal by Pascal Oesch of Geneva University and a large team of others.

And it shows, yet again, the incomprehensible vastness of the forest in which we are a tiny leaf.

Some people apparently find our physical insignificance in the universe to be unsettling.  I find it mind-opening and thrilling — that we now have the capability to not only speculate about our place in this enormity, but to begin to understand it as well.

The Ultra-Deep field composite, which contains approximately 10,000 galaxies.  The images were collected over a nine-year period.  {NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)} 

For those unsettled by the first image, here is the 2014 Ultra Deep Field image, which is 1/14 times the area of the newest image.  More of the shapes in this photo look to our eyes like they could be galaxies, but those in the first image are essentially the same.

In both images, astronomers used the ultraviolet capabilities of the Hubble, which is now in its 28th year of operation.

Because Earth’s atmosphere filters out much ultraviolet light, the space-based Hubble has a huge advantage because it can avoid that diminishing of ultraviolet light and provide the most sensitive ultraviolet observations possible.

That capability, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, allows astronomers to assemble these ultra deep space images and to gain a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.

The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths.

These images, then,  straddle the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies which can be seen across a broad spectrum of wavelengths.

The farthest away galaxy discovered so far is called GN-z11 and is seen now as it was 13.4 billion years in the past.  That’s  just 400 million years after the Big Bang.

GN-z11 is surprisingly bright infant galaxy located in the direction of the constellation of Ursa Major. Thus NASA video explains much more:

The farthest away galaxy ever detected — GN-z11. {NASA, ESA, P. Oesch (Yale University, Geneva University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz)} 

 

Galaxy formation chronology, showing GN-z11 in context. Hubble spectroscopically confirmed the farthest away galaxy to date. {NASA, ESA, P. Oesch and B. Robertson (University of California, Santa Cruz), and A. Feild (STScI)}

In addition representing cutting-edge science — and enabling much more — these looks into the most distant cosmic past offer a taste of what the James Webb Space Telescope, now scheduled to launch in 2021, is designed to explore.  It will have greatly enhanced capabilities to explore in the infrared, which will advance ultra-deep space observing.

But putting aside the cosmic mysteries that ultra deep space and time astronomy can potentially solve, the images available today from Hubble and other telescopes are already more than enough to fire the imagination about what is out there and what might have been out there some millions or billions of years ago.

A consensus of exoplanet scientists holds that each star in the Milky Way galaxy is likely to have at least one planet circling it, and our galaxy alone has billions and billions of stars.  That makes for a lot of planets that just might orbit at the right distance from its host star to support life and potentially have atmospheric, surface and subsurface conditions that would be supportive as well.

A look these deep space images raises the question of how many of them also house stars with orbiting planets, and the answer is probably many of them.  All the exoplanets identified so far are in the Milky Way, except for one set of four so far.

Their discovery was reported earlier this year by Xinyu Dai, an astronomer at the University of Oklahoma, and his co-author, Eduardo Guerras.  They came across what they report are planets while using NASA’s Chandra X-ray Observatory to study the environment around a supermassive black hole in the center of a galaxy located 3.8 billion light-years away from Earth.

In The Astrophysical Journal Letters , the authors report the galaxy is home to a quasar, an extremely bright source of light thought to be created when a very large black hole accelerates material around it. But the researchers said the results of their study indicated the presence of planets in a galaxy that lies between Earth and the quasar.

Furthermore, the scientists said results suggest that in most galaxies there are hundreds of free-floating planets for every star, in addition to those which might orbit a star.

The takeaway for me, as someone who has long reported on astrobiology and exoplanets, is that it is highly improbable that there are no other planets out there where life occurs, or once occurred.

As these images make clear, the number of planets that exist or have existed in the universe is essentially infinite.  That no others harbor life seems near impossible.

 

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Joining the Microscope and the Telescope in the Search for Life Beyond Earth

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Niki Parenteau of NASA’s Ames Research Center is a microbiologist working in the field of exoplanet and Mars biosignatures. She adds a laboratory biology approach to a field generally known for its astronomers, astrophysicists and planetary scientists. (Marisa Mayer, Stanford University.)

 

The world of biology is filled with labs where living creatures are cultured and studied, where the dynamics of life are explored and analyzed to learn about behavior, reproduction, structure, growth and so much more.

In the field of astrobiology, however, you don’t see much lab biology — especially when it comes to the search for life beyond Earth.  The field is now largely focused on understanding the conditions under which life could exist elsewhere, modeling what chemicals would be present in the atmosphere of an exoplanet with life, or how life might begin as an organized organism from a theoretical perspective.

Yes, astrobiology includes and learns from the study of extreme forms of life on Earth, from evolutionary biology, from the research into the origins of life.

But the actual bread and butter of biologists — working with lifeforms in a lab or in the environment — plays a back seat to modeling and simulations that rely on computers rather than actual life.

Niki Parenteau with her custom-designed LED array, can reproduce the spectral features of different simulated stellar and atmospheric conditions to test on primitive microbes. (Marc Kaufman)

There are certainly exceptions, and one of the most interesting is the work of Mary “Niki” Parenteau at NASA’s Ames Research Center in the San Francisco Bay area.

A microbiologist by training, she has been active for over five years now in the field of exoplanet biosignatures — trying to determine what astronomers could and should look for in the search for extraterrestrial life.

Working in her lab with actual live bacteria in laboratory flasks, test tubes and tanks, she is conducting traditional biological experiments that have everything to do with astrobiology.

She takes primitive bacteria known to have existed in some form on the early Earth, and she blasts them with the radiation that would have hit the planet at the time to see under what conditions the organisms can survive.  She has designed ingenious experiments using different forms of ultraviolet light and a LED array that simulate the broad range of radiations that would come from different types of stars as well.

What makes this all so intriguing is that her work uses, and then moves forward, cutting edge modeling from astronomers and astrobiologists regarding thick photochemical hazes understood to have engulfed the early Earth — making the planet significantly colder but also possibly providing some protection from deadly ultraviolet radiation.

That was a time when the atmosphere held very little oxygen, and when many organisms had to make their living via carbon dioxide and sulfur-based photosynthesis that did not use water and did not produce oxygen. This kind of photosynthesis has been the norm for much of the history of life on Earth, and certainly could be common on many exoplanets orbiting other stars as well.

So anything learned about how these early organisms survived in frigid conditions with high ultraviolet radiation — and what potentially detectable byproducts they would have produced under those conditions — would be important in the search for biosignatures and extraterrestrial life.

Parenteau has spent years learning from astronomers working to find ways to characterize exoplanet biosignatures, and she has been eager to convert her own work into something useful to them.

“These are not questions that can be answered by one discipline,” she told me.  “I certainly understand that when it comes to exoplanet biosignatures and life detection, astronomy has to be in the lead.  But biologists have a role to play, especially when it comes to characterizing what life produces.”

When haze built up in the atmosphere of Archean Earth, the young planet might have looked like this artist’s interpretation – a pale orange dot. A team led by Goddard scientists thinks the haze was self-limiting, cooling the surface by about 36 degrees Fahrenheit (20 Kelvins) – not enough to cause runaway glaciation. The team’s modeling suggests that atmospheric haze might be helpful for identifying earthlike exoplanets that could be habitable. (NASA’s Goddard Space Flight Center/Francis Reddy)

Here is the back story to Parenteau’s work:

Recent work by NASA Goddard Space Flight Center astronomer and astrobiologist Giada Arney and colleagues points to the existence of a thick haze around the early Archean Earth and probably today around some, and perhaps many, exoplanets.  This haze — which is more like pollution than clouds — is produced by the interaction of strong incoming radiation and chemicals (most commonly methane and carbon dioxide) already in the atmosphere.

The haze, Arney concluded based on elaborate modeling of those radiation-chemical interactions, would be hard on any life that might exist on the planet because it would reduce surface temperatures significantly, though probably not always fatally.

Giada Arney is an astronomer and astrobiologist at NASA’s Goddard Space Flight Center.  As with Parenteau, her general approach to science was formed at the University of Washington’s pioneering Virtual Planetary Laboratory. (NASA/Goddard Space Flight Center)

On the other hand, the haze would also have the effect of blocking 84 percent of the destructive ultraviolet radiation bombarding the planet — especially the most damaging ultraviolet-C light that would otherwise destroy nucleic acids in cells and disrupt the working of DNA.  (Ultraviolet-C radiation is used as a microbial disinfectant.)

Ozone in our atmosphere now plays the role of blocking the most destructive forms of UV radiation, but ozone is formed from oxygen and on early Earth there was very little oxygen at all.

So how did organisms survive the radiation assault?  Might it have been that haze? And might there be hazes surrounding exoplanets as well?  (None have been found so far.)

It’s difficult enough to sort through the potentially protective role of a haze on early Earth.  To do it for exoplanets requires not only an understanding of the effects of a haze on ultraviolet light, but also how the dynamics of a haze would change based on the amounts and forms of radiation emitted by different types of stars.

It’s all very complicated, but the answers needn’t be theoretical, Arney concluded. They could be tested in a lab.

And that’s where Parenteau comes in, with her desire and ability to design biological experiments that might help scientists understand better how to look for life on distant exoplanets.

“I knew that (Parenteau) had been super interested in this kind of question for a long time,” Arney said.  “She one of the few people in the world with the know-how to simulate an atmosphere, and probably the only one in the world who could do the experiment.”

The 48 LEDs (light-emitting diodes) of the board designed and created by Parenteau and Ames intern Cameron Hearne. Each one is independently controlled and can be used to simulate the amount of radiation arriving on a planetary surface — taking into account the flux from the planet’s star and some aspects of its atmosphere.  A microbe is then exposed to the radiation to see whether or how it can survive. (Niki Parenteau.)

Parenteau’s experiment at first looks pretty low-tech, but in fact it’s very much custom-designed and custom-built.

The ultraviolet bulbs include the powerful, germicidal ultraviolet-C variety, some of the glass for the experiment is made of special quartz that is transparent to that ultraviolet light, the LED array has 48 tiny bulbs that can be controlled by software to provide different amounts and kinds of light as identified and provided by Arney

Before designing and making her own LED board with Ames intern Cameron Hearne, Parenteau met with solar panel specialists who might be able to provide an instrument she could use, but it turned out they were very expensive and not nearly as versatile as she wanted.  Having grown up on a farm in northern Idaho, Parenteau is comfortable with making things from scratch, and her experiments reflect that comfort and talent.

How would Parenteau determine whether the haze does indeed protect the microbial cells after exposing them to the various radiation regimes?  This is how she explained the process, which measures the number of cells living or dead given a simulated UV and stellar bombardment:

“Imagine the cells as soap bubbles in a clear glass.  If you look through the glass, the soap bubbles prevent you from seeing through and the glass has a higher ‘optical density.’ However, if you pop or lyse the soap bubbles, suddenly you can see through the glass and the optical density decreases. 

“The latter represents dead ‘popped’ cells that were killed by the UV irradiation.   I predict that by simulating the spectral qualities of the haze, which decreases the UV flux by 84%, more cells will survive.”

The Parenteau-Arney collaboration is being funded through a National Astrobiology Institute grant to the University of Washington’s famously-interdisciplinary Virtual Planetary Laboratory.

The microbes-and-haze experiment is one of many that Parenteau is working on in the general field of biosignatures.  While the haze experiment is primarily designed to determine if microbes could survive a UV bombardment if a haze was present, she is also working on the central question of what might constitute a biosignature.

With that in mind, she is also measuring the gases produced by microbes under different radiation and atmospheric conditions, and that is directly applicable to searching for extraterrestrial life.

A densely-packed community of microbes, including oxygen-producing cyanobacteria as well as anoxygenic purple and green bacteria, being studied with Parenteau’s LED array. A central question involves what gases are emitted and might be detectable on a distant planet. (Niki Parenteau)

 

Parenteau’s lab glove box with green, purple and other bacteria that is regularly exposed to radiation conditions believed to have existed on early Earth when a photochemical haze is believed to have been present.  (Marc Kaufman)

If and when she does find particularly interesting results in the gas measurements inside the anaerobic glove box, she says, she knows where to go.

“I would hand the results to an astronomer.  We could say that if a particular kind of exoplanet with a particular atmosphere had microbial life, this is the suite of gases we would expect to be emitted.”

Those gases, Parenteau says, may be photochemically altered as they as they rise through the planet’s atmosphere to the upper levels where they could be detected by the telescopes of the future. But in the challenging and complex world of biosignatures, every bit of hard-won data is most valuable since it could some day lead to a discovery for the ages.

 

 

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

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

 

 

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

 

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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 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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