In 2016, the La Silla Observatory in Chile spotted evidence of possibly the most eagerly anticipated exoplanet in the Galaxy. It was a world orbiting the nearest star to the sun, Proxima Centauri, making this our closest possible exoplanet neighbour. Moreover, the planet might even be rocky and temperate.
Proxima Centauri b had been discovered by discerning a periodic wobble in the motion of the star. This revealed a planet with a minimum mass 30% larger than the Earth and an orbital period of 11.2 days. Around our sun, this would be a baking hot world.
But Proxima Centauri is a dim red dwarf star and bathes its closely orbiting planet in a level of radiation similar to that received by the Earth. If the true mass of the planet was close to the measured minimum mass, this meant Proxima Centauri b would likely be a rocky world orbiting within the habitable zone.
Sitting 4.2 light years from our sun, a journey to Proxima Centauri b is still prohibitively long.
But as our nearest neighbor, the exoplanet is a prime target for the upcoming generation of telescopes that will attempt to directly image small worlds. Its existence was also inspiration for privately funded projects to develop faster space travel for interstellar distances.
Yet observations taken around the same time as the La Silla Observatory discovery were painting a very different picture of Proxima Centauri. It was a star with issues.
This set of observations were taken with Evryscope; an array of small telescopes that was watching stars in the southern hemisphere. What Evryscope spotted was a flare from Proxima Centauri that was so bright that the dim red dwarf star became briefly visible to the naked eye.
Flares are the sudden brightening in the atmosphere of a star that release a strong burst of energy. They are often accompanied by a large expulsion of plasma from the star known as a “coronal mass ejection”. Flares from the sun are typically between 1027 – 1032 erg of energy, released in a few tens of minutes.
For comparison, a hydrogen bomb releases the equivalent of about 10 megatons of TNT or a mere 4 x 1023 erg. Hitting the Earth, energy from solar flares and coronal mass ejections can disrupt communication equipment and create a spectacular aurora.
But the Proxima super-flare spotted by Evryscope was well beyond a regular stellar flare.
On March 18 in 2016, this tiny red dwarf emitted an energy belch of 1033.5 erg. The flare consisted of one major event and three weaker ones and lasted approximately one hour, during which time Proxima Centauri became 68 times brighter.
A sudden, colossal increase in the brightness of a star does not bode well for any closely orbiting planets.
However, such a major flare might well be rare. If the star was normally fairly quiet, perhaps a planet could recover from a single very disruptive flare in the same way the Earth has survived mass extinction events.
Led by graduate student Ward Howard at the University of North Carolina, Chapel Hill, the discovering team used Evryscope to monitor Proxima Centauri for flares for a total of 1344 hours between January 2016 and March 2018. What they found was a horrifying environment, as reported in The Astrophysical Journal Letters.
While an event on the scale of the Proxima super-flare was only seen once, 24 large eruptions were spotted from the red dwarf, with energies from 1030.5 to 1032.4 erg. Allowing for the fact the star had only been observed for a small part of the year, this pattern of energy outbursts meant that a massive super-flare (1033 erg) was likely to occur at least five times annually.
But how important is this for the planet?
The Earth is protected from flares from our sun by our atmosphere. The ozone layer absorbs harmful ultraviolet radiation with wavelengths between about 2400 – 2800 Angstroms (10-10 m), preventing it reaching the surface. So what if Proxima Centauri b had a similar protective layer of gases as the Earth?
To answer this question, Howard and his team ran simulations of an Earth-like atmosphere on Proxima Centauri b.
As is the case for the sun, the team assumed that large flares would be frequently accompanied by a coronal mass ejection. Radiation and stellar material then flooded over an Earth-like Proxima Centauri b at the observed rate. And the atmosphere crumbled.
High energy particles in the coronal mass ejections split the nitrogen molecules (N2) in the atmosphere, which reacted with the ozone (O3) to form nitrogen oxide (NO2). After just 5 years, 90% of the ozone in the atmosphere was lost and the amount was still decreasing.
Without ozone, the surface of Proxima Centauri b would be stripped of its protection from UV radiation. During the Proxima super-flare, the radiation dose without the protective ozone would be 65 times larger than that needed to kill 90% of one of the most UV-resilient organisms on Earth.
“Life would have to undergo extreme adaptation to UV or exist underground or underwater,” Howard notes. “Only the most resistant organisms could survive on the surface in this environment.”
The simulation does assume that Proxima Centauri b does not have a magnetic field. Such a shield could channel the particles from the coronal mass ejection to the poles, forming the aurora as on Earth and reducing the damage to the atmosphere.
However, orbiting so close to the star, Proxima Centauri b is likely to be in tidal lock as the moon is to the Earth. This is expected to weaken the magnetic field, as the slower rotation makes it harder to create a magnetic dynamo within the planet.
So if the protective shields are lowered on Proxima Centauri b, is our nearest planet a world populated by highly resistant UV organisms? Or have we seen evidence that rather than warming the planet to allow life to exist, this star has snuffed it out?
Astronomers have found that Barnard’s star — a very close, fast-moving, and long studied red dwarf — has a super-Earth sized planet orbiting just beyond its habitable zone.
The discovery relied on data collected over many years using the tried-and-true radial velocity method, which searches for wobbles in the movement of the host star.
But this detection was something big for radial velocity astronomers because Barnard-b was among the smallest planet ever found using the technique, and it was the furthest out from its host star as well — orbiting its star every 233 days.
For more than a century, astronomers have studied Barnard’s star as the most likely place to find an extrasolar planet.
Ultimately, said Ignasi Rablis of Spain’s Institute of Space Studies of Catalonia, lead author of the paper in journal Nature, the discovery was the result of 771 observations, an extremely high number.
And now, he said, “after a very careful analysis, we are over 99 percent confident the planet is there.”
The planet is at least 3.2 times the size of Earth and orbits near the snowline of the system, where water cannot be expected to ever be liquid. That means is it a frozen world (an estimated -150 degrees Celsius) and highly unlikely to support life.
But Rablis and others on the large team say it also an extremely good candidate for future direct imaging and next-generation observing.
Thousands of exoplanets have been identified by now, and hundreds using the radial velocity method. But this one is different.
“Barnard’s star is the ‘great white whale’ of planet hunting,” said Paul Butler, senior scientist at the Carnegie Institution, a radial velocity pioneer, and one of the numerous authors of the paper.
Because the star is so close (but 6 light-years away) and as a result so tempting, it has been the subject of exoplanet searches for 100 years, Butler said. But until the radial velocity breakthroughs of the mid 1990s, the techniques used could not find a planet.
Nonetheless, an early exoplanet hunter, the Dutch-American astronomer Peter van de Kamp of Swarthmore College, thought that he had indeed found two gas giant planets around Barnard’s star in the 1960s. He used a different technique based on the movement of the host star, and the findings even made it into some textbooks. But later the detection was found to be incorrect.
Even after the modern exoplanet era began Barnard’s star kept its planetary secret close.
As Butler explained it, the combination of the planet’s size and distance from the star ultimately pushed the technology (and astronomers) to the very limit — requiring a measurement of 1.2 meters per second of “wobble.”
In contrast, the first planets were found by radial velocity that would detect 70 meter per second of wobble caused by the gravitational pull of a planet, and 30 years ago the best instruments could detect only 300 meters per second.
The detected planet (which remains a “candidate” until further confirmed) was ultimately found following concerted effort by a large team of astronomers around the world. It was co-led and organized by Guillem Anglada-Escudé of the Queen Mary University of London. The young astronomer had made a major splash in 2016 with the detection of a planet orbiting Proxima Centauri, the closest star to our own.
That discovery was part of the “Pale Red Dot” campaign, which had the goal of detecting rocky planets around red dwarf stars. After the Proxima discovery Barnard’s star went to the top of Anglada-Escudé list with the renamed “Red Dots” collaboration — which is supported by the European Southern Observatory and universities in Chile, the United Kingdom, Spain and Germany.
By 2015, there was already almost 18 years of modern data collected regarding a possible planet orbiting the star, and a faint but clearly present signal had been detected. But more was needed to confidently report a discovery, and the Red Dots effort took up the challenge.
To see if the result could be confirmed, astronomers regularly monitored Barnard’s star with high precision spectrometers such as the CARMENES (Calar Alto Observatory in Spain), and also the HARPS (High Accuracy Radial velocity Planet Searcher.)
Ultimately, the team used observations from seven different instruments taken over 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies.
“We all have worked very hard on this result,” said Anglada-Escudé. “This is the result of a large collaboration organized in the context of the Red Dots project, which is why it has contributions from teams all over the world including semi-professional astronomers.”
Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía and co-author of the paper, said of the significance of the finding grow over decades.
“This discovery means a boost to continue on searching for exoplanets around our closest stellar neighbors, in the hope that eventually we will come upon one that has the right conditions to host life,” she said.
The next pr0ject for the Red Dots campaign is to study the star Ross 154, at 9.69 light-years away another of the closest stars to us.
The dramatically increased (and increasing) precision in radial velocity measurements is expected to continue with the next generation of ground-based telescopes and spectrometers.
Butler, for instance, said that Carnegie is in the process of upgrading its Planet Finding Spectrograph at the Las Campanas Observatory in Chile to reach a 0.5-meters-per-second measurement. Other groups including the European Southern Observatory and American teams based at Pennsylvania State and Yale Universities have similar efforts under way.
If they succeed, Butler said, it may well be possible to find potentially habitable planets around sun-like and other categories of stars using the radial velocity method.
Barnard’s a very-low-mass red dwarf star in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Celestial hemisphere.
Despite its proximity, the star is too faint to be seen with the unaided eye, though it is quite visible with an amateur 8-inch telescope. It is much brighter in the infrared than in visible light. Although Barnard’s Star is an ancient star, it still experiences star flare events, one being observed in 1998.
The star is named after the American astronomer E. E. Barnard. He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion –the apparent angular motion of a star across the sky with respect to more distant stars — as 10.3 arcseconds per year relative to the sun.
This is likely to be the fastest star in terms of proper motion, as its proximity to the sun, as well as its high velocity, make it unlikely any faster object will be discovered.
Barnard’s Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard’s Star has focused on measuring its stellar characteristics and its astrometry — which involves precise measurements of the positions and movements of stars and other celestial bodies on the plane of the sky.
When planet hunters use astrometry, they look for a minute but regular wobble in a star’s position as seen in images. Van de Kamp, for instance, used astrometry to study Barnard’s star and (incorrectly) detected those two gas giants around it.
In contrast, radial (or Doppler) velocities look for the wobble of the star perpendicular to the plane sky, and astronomers have regularly, and now once again, made history with that method.
The Kepler Space Telescope is dead. Long live the Kepler.
NASA officials announced on Tuesday that the pioneering exoplanet survey telescope — which had led to the identification of almost 2,700 exoplanets — had finally reached its end, having essentially run out of fuel. This is after nine years of observing, after a malfunctioning steering system required a complex fix and change of plants, and after the hydrazine fuel levels reached empty.
While the sheer number of exoplanets discovered is impressive the telescope did substantially more: it proved once and for all that the galaxy is filled with planets orbiting distant stars. Before Kepler this was speculated, but now it is firmly established thanks to the Kepler run.
It also provided data for thousands of papers exploring the logic and characteristics of exoplanets. And that’s why the Kepler will indeed live long in the world of space science.
“As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.
“Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.”
Kepler was initially the unlikely brainchild of William Borucki, its founding principal investigator who is now retired from NASA’s Ames Research Center in California’s Silicon Valley.
When he began thinking of designing and proposing a space telescope that could potentially tell us how common distant exoplanets were — and especially smaller terrestrial exoplanets like Earth – the science of extra solar planets was at a very different stage.
“When we started conceiving this mission 35 years ago we didn’t know of a single planet outside our solar system,” Borucki said. “Now that we know planets are everywhere, Kepler has set us on a new course that’s full of promise for future generations to explore our galaxy.”
The space telescope was launched in 2009. While Kepler did not find the first exoplanets — that required the work of astronomers using a different technique of observing based on the “wobble” of stars caused by orbiting planets — it did change the exoplanet paradigm substantially.
Not only did it prove that exoplanets are common, it found that planets outnumber stars in our galaxy (which has hundreds of billions of those stars.)
In addition it found that small, terrestrial-size planets are common as well, with some 20 to 50 percent of stars likely to have planets of that size and type. And what menagerie of planets it found out there.
Among the greatest surprises: The Kepler mission provided data showing that the most common sized planets in the galaxy fall somewhere between Earth and Neptune, a type of planet that isn’t present in our solar system.
It found solar systems of all sizes as well, including some with many planets (as many as eight) orbiting close to their host star.
The discovery of these compact systems, generally orbiting a red dwarf star, raised questions about how solar systems form: Are these planets “born” close to their parent star, or do they form farther out and migrate in?
So far, more than 2,500 peer-reviewed papers have been published using Kepler data, with substantial amounts of that data still unmined.
Natalie Batalha was the project and mission scientist for Kepler for much of its run, and I asked her about its legacy.
“When I think of Kepler’s influence across all of astrophysics, I’m amazed at what such a simple experiment accomplished,” she wrote in an email. “You’d be hard-pressed to come up with a more boring mandate — to unblinkingly measure the brightnesses of the same stars for years on end. No beautiful images. No fancy spectra. No landscapes. Just dots in a scatter plot.
“And yet time-domain astronomy exploded. We’d never looked at the Universe quite this way before. We saw lava worlds and water worlds and disintegrating planets and heart-beat stars and supernova shock waves and the spinning cores of stars and planets the age of the galaxy itself… all from those dots.”
While Kepler provided remarkable answers to questions about the overall planetary makeup of our galaxy, it did not identify smaller planets that will be directly imaged, the evolving gold standard for characterizing exoplanets. The 150,000 stars that the telescope was observing were very distant, in the range of a few hundred to a few thousand light-years away. One light year is about 6 trillion (6,000,000,000,000) miles.
Nonetheless, Kepler was able to detect the presence of a handful of Earth-sized planets in the habitable zones of their stars. The Kepler-62 system held one of them, and it is 1200 light-years away. In contrast, the four Earth-sized planets in the habitable zone of the much-studied Trappist-1 system are 39 light-years away.
Kepler made its observations using the the transit technique, which looks for tiny dips in the amount of light coming from a star caused by the presence of a planet passing in front of the star. While the inference that exoplanets are ubiquitous came from Kepler results, the telescope was actually observing but a small bit of the sky. It has been estimated that it would require around 400 space telescopes like Kepler to cover the whole sky.
What’s more, only planets whose orbits are seen edge-on from Earth can be detected via the transit method, and that rules out a vast number of exoplanets.
The bulk of the stars that were selected for close Kepler observation were more or less sun-like, but a sampling of other stars occurred as well. One of the most important factors was brightness. Detecting minuscule changes in brightness caused by transiting planet is impossible if the star is too dim.
Four years into the mission, after the primary mission objectives had been met, mechanical failures temporarily halted observations. The mission team was able to devise a fix, switching the spacecraft’s field of view roughly every three months. This enabled an extended mission for the spacecraft, dubbed K2, which lasted as long as the first mission and bumped Kepler’s count of surveyed stars up to more than 500,000.
But it was inevitable that the mission would come to an end sooner rather than later because of that dwindling fuel supply, needed to keep the telescope properly pointed.
Kepler cannot be refueled because NASA decided to place the telescope in an orbit around the sun that is well beyond the influence of the Earth and moon — to simplify operations and ensure an extremely quiet, stable environment for scientific observations. So Kepler was beyond the reach of any refueling vessel. The Kepler team compensated by flying considerably more fuel than was necessary to meet the mission objectives.
The video below explains what will happen to the Kepler capsule once it is decommissioned. But a NASA release explains that the final commands “will be to turn off the spacecraft transmitters and disable the onboard fault protection that would turn them back on. While the spacecraft is a long way from Earth and requires enormous antennas to communicate with it, it is good practice to turn off transmitters when they are no longer being used, and not pollute the airwaves with potential interference.”
And so Kepler will actually continue orbiting for many decades, just as its legacy will continue long after operations cease.
Kepler’s follow-on exoplanet surveyor — the Transiting Exoplanet Survey Satellite or TESS — was launched this year and has begun sending back data. Its primary mission objective is to survey the brightest stars near the Earth for transiting exoplanets. The TESS satellite uses an array of wide-field cameras to survey some 85% of the sky, and is planned to last for two years.
The word “SETI” pretty much brings to mind the search for radio signals come from distant planets, the movie “Contact,” Jill Tarter, Frank Drake and perhaps the SETI Institute, where the effort lives and breathes.
But there was a time when SETI — the Search for Extraterrestrial Intelligence — was a significantly broader concept, that brought in other ways to look for intelligent life beyond Earth.
In the late 1950s and early 1960s — a time of great interest in UFOs, flying saucers and the like — scientists not only came up with the idea of searching for distant intelligent life via unnatural radio signals, but also by looking for signs of unexpectedly elevated heat signatures and for optical anomalies in the night sky.
The history of this search has seen many sharp turns, with radio SETI at one time embraced by NASA, subsequently de-funded because of congressional opposition, and then developed into a privately and philanthropically funded project of rigor and breadth at the SETI Institute. The other modes of SETI went pretty much underground and SETI became synonymous with radio searches for ET life.
But this history may be about to take another sharp turn as some in Congress and NASA have become increasingly interested in what are now called “technosignatures,” potentially detectable signatures and signals of the presence of distant advanced civilizations. Technosignatures are a subset of the larger and far more mature search for biosignatures — evidence of microbial or other primitive life that might exist on some of the billions of exoplanets we now know exist.
And as a sign of this renewed interest, a technosignatures conference was scheduled by NASA at the request of Congress (and especially retiring Republican Rep. Lamar Smith of Texas.) The conference took place in Houston late last month, and it was most interesting in terms of the new and increasingly sophisticated ideas being explored by scientists involved with broad-based SETI.
“There has been no SETI conference this big and this good in a very long time,” said Jason Wright, an astrophysicist and professor at Pennsylvania State University and chair of the conference’s science organizing committee. “We’re trying to rebuild the larger SETI community, and this was a good start.”
During the three day meeting in Houston, scientists and interested private and philanthropic reps. heard talks that ranged from the trials and possibilities of traditional radio SETI to quasi philosophical discussions about what potentially detectable planetary transformations and by-products might be signs of an advanced civilization. (An agenda and videos of the talks are here.)
The subjects ranged from surveying the sky for potential millisecond infrared emissions from distant planets that could be purposeful signals, to how the presence of certain unnatural, pollutant chemicals in an exoplanet atmosphere that could be a sign of civilization. From the search for thermal signatures coming from megacities or other by-products of technological activity, to the possible presence of “megastructures” built to collect a star’s energy by highly evolved beings.
All but the near infrared SETI are for the distant future — or perhaps are on the science fiction side — but astronomy and the search for distant life do tend to move forward slowly. Theory and inference most often coming well before observation and detection.
So thinking about the basic questions about what scientists might be looking for, Wright said, is an essential part of the process.
Indeed, it is precisely what Michael New, Deputy Associate Administrator for Research within NASA’s Science Mission Directorate, told the conference.
He said that he, NASA and Congress wanted the broad sweep of ideas and research out there regarding technosignatures, from the current state of the field to potential near-term findings, and known limitations and possibilities.
“The time is really ripe scientifically for revisiting the ideas of technosignatures and how to search for them,” he said.
He offered the promise of NASA help (admittedly depending to some extent on what Congress and the administration decide) for research into new surveys, new technologies, data-mining algorithms, theories and modelling to advance the hunt for technosignatures.
Among the several dozen scientists who discussed potential signals to search for were the astronomer Jill Tarter, former director of the Center for SETI Research, Planetary Science Institute astrobiologist David Grinspoon and University of Rochester astrophysicist Adam Frank. They all looked at the big picture, what artifacts in atmospheres, on surfaces and perhaps in space that advanced civilizations would likely produce by dint of their being “advanced.”
All spoke of the harvesting of energy to perform work as a defining feature of a technological planet, with that “work” describing transportation, construction, manufacturing and more.
Beings that have reached the high level of, in Frank’s words, exo-civilization produce heat, pollutants, changes to their planets and surroundings in the process of doing that work. And so a detection of highly unusual atmospheric, thermal, surface and orbital conditions could be a signal.
One example mentioned by several speakers is the family of chemical chloroflourocarbons (CFCs,) which are used as commercial refrigerants, propellants and solvents.
These CFCs are a hazardous and unnatural pollutant on Earth because they destroy the ozone layer, and they could be doing something similar on an exoplanet. And as described in the conference, the James Webb Space Telescope — once it’s launch and working — could most likely detect such an atmospheric compound if it’s in high concentration and the project was given sufficient telescope time.
A similar single finding described by Tarter that could be revolutionary is the radioactive isotope tritium, which is a by-product of the nuclear fusion process. It has a short half-life and so any distant discovery would point to a recent use of nuclear energy (as long as it’s not associated with a recent supernova event, which can also produce tritium.)
But there many other less precise ideas put forward.
Glints on the surface of planets could be the product of technology, as might be weather on an exoplanet that has been extremely well stabilized, modified planetary orbits and chemical disequilibriums in the atmosphere based on the by-products of life and work. (These disequilibriums are a well-established feature of biosignature research, but Frank presented the idea of a technosphere which would process energy and create by-products at a greater level than its supporting biosphere.)
Another unlikely but most interesting example of a possible technosignature put forward by Tarter and Grinspoon involved the seven planets of the Trappist-1 solar system, all tidally locked and so lit on only one side. She said that they could potentially be found to be remarkably similar in their basic structure, alignment and dynamics. As Tarter suggested, this could be a sign of highly advanced solar engineering.
Grinspoon seconded that notion about Trappist-1, but in a somewhat different context.
He has worked a great deal on the question of today’s anthropocene era — when humans actively change the planet — and he expanded on his thinking about Earth into the galaxies.
Grinspoon said that he had just come back from Japan, where he had visited Hiroshima and its atomic bomb sites, and came away with doubts that we were the “intelligent” civilization we often describe ourselves in SETI terms. A civilization that may well self destruct — a fate he sees as potentially common throughout the cosmos — might be considered “proto-intelligent,” but not smart enough to keep the civilization going over a long time.
Projecting that into the cosmos, Grinspoon argued that there may well be many such doomed civilizations, and then perhaps a far smaller number of those civilizations that make it through the biological-technological bottleneck that we seem to be facing in the centuries ahead.
These civilizations, which he calls semi-immortal, would develop inherently sustainable methods of continuing, including modifying major climate cycles, developing highly sophisticated radars and other tools for mitigating risks, terraforming nearby planets, and even finding ways to evolve the planet as its place in the habitable zone of its host star becomes threatened by the brightening or dulling of that star.
The trick to trying to find such truly evolved civilizations, he said, would be to look for technosignatures that reflect anomalous stability and not rampant growth. In the larger sense, these civilizations would have integrated themselves into the functioning of the planet, just as oxygen, first primitive and then complex life integrated themselves into the essential systems of Earth.
And returning to the technological civilizations that don’t survive, they could produce physical artifacts that now permeate the galaxy.
While the conference focused on technosignature theory, models, and distant possibilities, news was also shared about two concrete developments involving research today.
The first involved the radio telescope array in South Africa now called MeerKAT, a prototype of sorts that will eventually become the gigantic Square Kilometer Array.
Breakthrough Listen, the global initiative to seek signs of intelligent life in the universe, would soon announce the commencement of a major new program with the MeerKAT telescope, in partnership with the South African Radio Astronomy Observatory (SARAO).
Breakthrough Listen’s MeerKAT survey will examine a million individual stars – 1,000 times the number of targets in any previous search – in the quietest part of the radio spectrum, monitoring for signs of extraterrestrial technology. With the addition of MeerKAT’s observations to its existing surveys, Listen will operate 24 hours a day, seven days a week, in parallel with other surveys.
This clearly has the possibility of greatly expanded the amount of SETI listening being done. The SETI Institute, with its radio astronomy array in northern California and various partners, have been listening for almost 60 years, without detecting a signal from our galaxy.
That might seem like a disappointing intimation that nothing or nobody else is out there, but not if you listen to Tarter explain how much listening has actually been done. Almost ten years ago, she calculated that if the Milky Way galaxy and everything in it was an ocean, then SETI would have listened to a cup full of water from that ocean. Jason Wright and his students did an updated calculation recently, and now the radio listening amounts to a small swimming pool within that enormous ocean.
The other news came from Shelley Wright of the University of California, San Diego, who has been working on an optical SETI instrument for the Lick Observatory and beyond.
She has developed a Near-Infrared Optical SETI (NIROSETI) instrument designed to search for signals from extraterrestrials at near-infrared wavelengths — a first. The near-infrared is an excellent spectral region to search for signals from extraterrestrials, since it offers a unique window for interstellar communication. NIROSETI is now operating 8 to 12 nights per month, overseen by students at a remote location.
In addition, Wright and Harvard University’s Paul Horowitz have been working on a novel instrument for searching the full sky all the time for very short pulses of light — an idea that came out of a Breakthrough Listen meeting in 2016. The pulses they are searching for are nanosecond to one second bursts which, could only come from technological civilizations.
This PANOSETI (Pulsed All-sky Near-infrared Optical SETI) uses a most unusual light-collection method that features some 100 compact, wide-viewing Fresnel lenses mounted on two small geodesic domes, and connected to the telescope at the Lick Observatory. I
Jason Wright of Penn State was especially impressed by the project, which he said in the future can look at much of the sky at once and was put together with on very limited budget.
Wright, who teaches a course on SETI at Penn State and is a co-author of a recent paper trying to formalize SETI terminology, said his own take-away from the conference is that it may well represent an important and positive moment in the history of technosignatures.
“Without NASA support, the whole field has lacked the normal structure by which astronomy advances,” he said. “No teaching of the subject, no standard terms, no textbook to formalize findings and understandings.
“The SETI Institute carried us through the dark times, and they did that outside of normal, formal structures. The Institute remains essential, but hopefully that reflex identification will start to change.”
The more exoplanet scientists learn about the billions and billions of celestial bodies out there, the more the question of unusual planets — those with characteristics quite different from those in our solar system — has come into play.
Hot Jupiters, super-Earths, planets orbiting much smaller red dwarf stars — they are all grist for the exoplanet mill, for scientists trying to understand the planetary world that has exploded with possibilities and puzzles over the past two decades.
Another important category of planets unlike those we know are the loosely called “water worlds” (with very deep oceans) and their “aquaplanet” cousins (with a covering of water and continents) but orbiting stars very much unlike our sun.
Two recent papers address the central question of habitability in terms of these kind of planets — one with oceans and ice hundreds of miles deep, and one particular and compelling planet (Proxima Centauri b, the exoplanet closest to us) hypothesized to have water on its surface as it orbits a red dwarf star.
The question the papers address is whether these watery worlds might be habitable. The conclusions are based on modelling rather than observations, and they are both compelling and surprising.
In both cases — a planet with liquid H20 and ice many miles down, and another that probably faces its red dwarf sun all or most of the time — the answers from modelers is that yes, the planets could be habitable. That is very different from saying they are or even might be inhabited. Rather, the conclusions are based on computer models that take into account myriad conditions and come out with simulations about what kind of planets they might be.
This finding of potential watery-world habitability is no small matter because predictions of how planets form point to an abundance of water and ice in the planetesimals that grow into planets.
As described by Eric Ford, co-author of one of the papers and a professor of astrophysics at Pennsylvania State University, “Many scientists anticipate that planets with oceans much deeper than Earths could be a common outcome of planet formation. Indeed, one of the puzzling properties of Earth is that it has oceans that are just skin deep” compared to the radius of the planet.
“While some planets very close to their star might loose all their water, it would take a delicate balancing act to remove many ocean’s worth of water and to leave a planet with oceans as shallow as those on Earth.”
An interesting place to start.
It should first be said that many scientists are dubious that extreme water worlds can support life or can support detectable life. My colleague Elizabeth Tasker wrote a column — Can You Overwater a Planet? — focused on this view last year.
The first of the two new exoplanets/ocean papers involves planets with very deep oceans. Written by Edwin Kite of the University of Chicago and Ford of Penn State, the paper in the Astrophysical Journal concludes that even a planet with such super deep oceans could — under certain conditions — provide habitable conditions.
This finding is at odds with previous simulations, and Kite says that is part of its significance. The scientific community has largely assumed that planets covered in a deep ocean would not support the cycling of minerals and gases that keeps the climate stable on Earth, and thus wouldn’t be friendly to life.
But the Kite and Ford study found that ocean planets (with 10 to 1000 times as much water as Earth) could remain habitable much longer than previously assumed. The authors performed more than a thousand simulations to reach that conclusion.
“This really pushes back against the idea you need an Earth clone—that is, a planet with some land and a shallow ocean,” said Edwin Kite, assistant professor of geophysical sciences at the University of Chicago and lead author of the study.
Because life needs an extended period to evolve — and because the light and heat on planets can change as their stars age — scientists usually look for planets that have both some water and some way to keep their climates stable over time. The method for achieving this steady state that we know is, of course, how it works on Earth. Over eons, our planet has cooled itself by drawing down atmospheric greenhouse gases into minerals and warms itself up by releasing them via volcanoes.
But this model doesn’t work on a water world, with deep water covering the rock and suppressing volcanoes.
Kite and Ford wanted to know if there was another way to achieve a balance. They set up a simulation with thousands of randomly generated planets, and tracked the evolution of their climates over billions of years.
“The surprise was that many of them stay stable for more than a billion years, just by luck of the draw,” Kite said. “Our best guess is that it’s on the order of 10 percent of them.”
These planets sit in the right location around their stars. They happened to have the right amount of carbon present, and they don’t have too many minerals and elements from the crust dissolved in the oceans that would pull carbon out of the atmosphere. They have enough water from the start, and they cycle carbon between the atmosphere and ocean only, which in the right concentrations is sufficient to keep things stable.
None of this means that such a planet exists — our ability to detect oceans worlds is in its infancy. The issue is rather that Kite and Ford conclude that a deep ocean planet could potentially be habitable if other conditions were met.
Anthony Del Genio and his team of modelers at NASA’s Goddard Institute for Space Studies in New York used their state-of-the-art climate simulations to look at another aspect of the exoplanet water story, and they chose Proxima Centauri b as their subject. The roughly Earth-sized planet was discovered in 2016 and is the closest exoplanet to Earth.
Scientists determined early on that it is a rocky (as opposed to gaseous) planet and that it orbits its host star every 11 days. If that star was as powerful as our sun, there would be no talk of possible habitability on close-in Proxima b. But the star is a red dwarf and puts out only a fraction of the radiation coming from a host star like our sun.
Still, the case for habitability on Proxima b was initially considered to be weak, in part because the planet is tidally locked by its closeness to the host star. In other words, it would most likely not spin to create days and nights as it orbits, but rather would have a sun-facing side and a space-facing side — making the temperature differences great.
Our ability to characterize a small planet like Proxima b remains very limited, and so it is unknown whether the planet has water or whether it has an atmosphere. So those two essential components of the habitability question are missing.
But Del Genio’s team decided to model the dynamics of Proxima b with a presumed ocean, though not one that is many miles deep. In Earth science parlance, what Del Genio referred to as an “aquaplanet.” And using their sophisticated models, they would simulate “dynamic” oceans with currents like our own, rather than the stationary oceans modeled earlier on exoplanets.
And rather to their surprise, they reported in the journal Astrobiology that their model of ocean behavior showed that the planet would not have only small areas of potential habitability — the earlier proposed habitable “eyeball” scenario — but rather much of the planet could be habitable. That could include some of the normally space-facing side.
“Our group said let’s hook up an atmosphere to a dynamic ocean rather than a static one,” Del Genio said. “That way you get ocean currents like those on our coasts, and they move water of different temperatures around.
“If you have a real and dynamic ocean in your model, then we found that the eyeball goes away. Usually the currents go west to east and they carry warmer water even to the night side.”
So using this more sophisticated model, not insignificant areas of Proxima b, or any other planet like it orbiting a red dwarf star, could be habitable, they concluded. But again, that is assuming some pretty big “ifs” — the presence of an ocean and an atmosphere.
And then the team added variables such as a thick nitrogen and carbon atmosphere or a thin one, fresh water or salty water, a planet that is firmly locked and never rotating, or one that rotates a modest amount — giving the dark side some light. Del Genio said that with all these added factors, a substantial portion of the surface of Proxima b, or a planet like it, would have liquid water and potentially habitable conditions.
This focus on watery worlds — including those that would be extreme compared with Earth today — makes sense in the context of the history of Earth.
While there is no direct evidence of this, many scientists think that the very early Earth was covered for a period of time with water with little or no land.
And then after land appeared, it still took some three billion years for any life form — bacteria, early planets — to colonize the land, and another half billion years for animals to come ashore. Yet the oceans were long habitable and inhabited, as early a 3.8 billion years ago.
So until astronomy and exoplanet science develop the needed instruments and scientists acquire the observed knowledge of conditions on water worlds, progress will come largely from modelling that tells us what might be possible.
This blog is being hosted by Knowinnovation Inc. and is supported by the Lunar and Planetary Institute (LPI). LPI is operated by the Universities Space Research Association (USRA) under a cooperative agreement with NASA. The purpose of this blog is to communicate the work of the Nexus for Exoplanet Systems Science (NExSS). Any opinions, findings, and conclusions or recommendations expressed on this blog or its comments are those of the author(s) and do not necessarily reflect the views of NASA.