Proxima b Is Surely Not “Earth-like.” But It’s A Research Magnet And Just May Be Habitable.

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Simulated comparison of a sunset on Earth and Proxima b. The red-dwarf star Proxima Centauri appears almost three times bigger than the Sun in a redder and darker sky. Red-dwarf stars appear bigger in the sky than sun-like stars, even though they are smaller. This is because they are cooler and the planets have to be closer to them to maintain temperate conditions. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. Credit: PHL @ UPR Arecibo.
A simulated comparison of a sunset on Earth and Proxima b. The images sets out to show that the red-dwarf star Proxima Centauri appears almost three times bigger than our sun in a redder and darker sky. There is value in illustrating how conditions in different solar systems would change physical conditions on the planets, but there is a real danger that the message conveyed becomes the similarities between planets such as Earth and Proxima b.  At this point, there is no evidence that Proxima b is “Earth-like” at all. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. (PHL @ UPR Arecibo))

It is often discussed within the community of exoplanet scientists that a danger lies in the description of intriguing exoplanets as “Earth-like.”

Nothing discovered so far warrants the designation, which is pretty nebulous anyway.  Size and the planet’s distance from a host star are usually what earn it the title “Earth-like,” with its inescapable expectation of inherent habitability. But residing in a habitable zone is just the beginning; factors ranging from the make-up of the planet’s host star to the presence and content of an atmosphere and whether it has a magnetic field can be equally important.

The recent announcement of the detection of a planet orbiting Proxima Centauri, the closest star to our own, set off another round of excitement about an “Earth-like” planet.  It was generally not scientists who used that phrase — or if they did, it was in the context of certain “Earth-like” conditions.  But the term nonetheless became a kind of shorthand for signalling a major discovery that just might some day even yield a finding of extraterrestrial life.

Consider, however, what is actually known about Proxima b:

  • The planet, which has a minimum mass of 1.3 Earths and a maximum of many Earths, orbits a red dwarf star.  These are the most common class of star in the galaxy, and they put out considerably less luminosity than a star like our sun — about one-tenth of one percent of the power.
  • These less powerful red dwarf stars often have planets orbiting much closer to them than what’s found in solar systems like our own.   Proxima b, for instance, circles the star in 11.3 days.
  • A consequence of this proximity is that the planet is most likely tidally locked by the gravitational forces of the star — meaning that the planet does not rotate like Earth does but rather has a daytime and nighttime side like our moon.  Some now argue that a tidally locked planet could theoretically be habitable,  but the consensus seems to be that it is an obstacle to habitability rather than a benefit.
  • The authors of the Proxima b paper make clear that evidence that the planet is rocky (as opposed to gaseous) is limited, and that’s why they label it as a “candidate terrestrial planet.”

So to describe Proxima b as “Earth-like” seems unfortunate to me, and prone to giving the public the misguided impression of a planet with blue skies, oceans, and fish swimming in them.  Proxima b may have some very broadly defined characteristics that parallel Earth, but so do many other exoplanets that are definitely not habitable.  And therein lies the really interesting part.

Before getting into that area, it should be made clear that the Proxima b detection was a game-changer, a discovery of historic proportions.  It was and will be for a long time.

The detection, after all, provides the nearest opportunity possible for beginning to understand the extraordinary complexities of what makes a planet truly habitable — something that is far from understood today.  And later, Proxima b may become a petri dish of sorts for identifying and measuring chemical signatures that could be a byproduct of some kind of life.

These are difficult tasks, to say the least, and will take an army of scientists years to come up with answers.  But the good news is that the exoplanet field has already begun publishing papers on the dynamics and possible habitability of Proxima b, and they provide beginning insights into the issues, the excitement and the untold difficulties associated with this grandest of scientific chases.

The detection of Proxima b has been met with enormous enthusiasm in the exoplanet community. Some call it the biggest discovery since the detection of 51 Pegasi a, the first exoplanet to be positively identified. Detecting a planet, however, is just the beginning of the still unsettled process of determining its history and current makeup, and whether or not it might be habitable.
The detection of Proxima b has been met with enormous enthusiasm in the exoplanet community. Some call it the biggest discovery since the detection of 51 Pegasi a, the first exoplanet to be positively identified. Detecting a planet, however, is just the beginning of the still unsettled process of determining its history and current makeup, and whether or not it might be habitable. (ESO/L.Calçada/Nick Risinger)

Two Proxima papers that appeared soon after the August 24 announcement came from the University of Washington’s Virtual Planetary Laboratory. Supported by the NASA Astrobiology Institute since 2001, it is a leader in exoplanet research, modelling and habitability  The team, directed by Victoria Meadows, received the Proxima detection paper before it was released because of longstanding relationships with the lead author, Guillem Anglada-Escude.

The two papers from the VPL team — one with Meadows in the lead and the other organized by University of Washington astronomer Rory Barnes — take broad, interdisciplinary approaches to the planet.

“Part one is what happened to this planet over time — what can we learn about its history, its evolution?” Meadows said.  “Part two is what does the history mean for the current environment right now? We need photo-chemical models of what might be present, and then we have to look at what instruments we would need to detect what is decided we should be looking for.”

Victoria Meadows is an astrobiologist and planetary astronomer whose research interests focus on acquisition and analysis of remote-sensing observations of planetary atmospheres and surfaces. In addition to studying planets within our own Solar System, she is interested in exoplanets, planetary habitability and biosignatures. Since 2000, she has been the Principal Investigator for the Virtual Planetary Laboratory Lead Team of the NASA Astrobiology Institute. Her NAI team uses models of planets, including planet-star interactions, to generate plausible planetary environments and spectra for extrasolar terrestrial planets and the early Earth. This research is being used to help define signs of habitability and life for future extrasolar terrestrial planet detection and characterization missions.
Victoria Meadows is an astrobiologist and planetary astronomer at the University of Washington. Since 2000, she has been the Principal Investigator for the Virtual Planetary Laboratory Lead Team of the NASA Astrobiology Institute. Her NAI team uses models of planets, including planet-star interactions, to generate plausible planetary environments and spectra for extrasolar terrestrial planets and the early Earth.

The Barnes paper is here and the Meadows paper is here.

The two take on different tasks, but really are one. Both start with an appreciative nod to what will quickly become the planet that scientists want to study, and then they go into the extraordinary complexity of the task ahead.

As Barnes and his team concluded, a major obstacle to habitability on Proxima b is the well documented evolution of red dwarf stars.

While their energy output is relatively low when mature, they go through early phases when they are much brighter and send out enormous solar flares that can double their brightness in a matter of minutes.  Barnes said these intense phases could easily sterilize a close-in planet, leaving it incapable of evolving into a potentially living world even if, a billion years later, conditions for life were much more favorable.

“The planet is in a habitable zone and so could have had liquid water,  but my biggest concern is the retention of that water,” Barnes said of Proxima b.  “If the planet was formed in its current orbit, then it was baked enough for 100, 200 million years to form a runaway greenhouse effect.”  A different dynamic may have resulted in the same results on Venus, which once was wet but now is super-hot and parched.

This leads to the next big question:  Could Proxima b have been formed elsewhere, and was later pushed or pulled to its current location?  Barnes said it is certainly possible that the planet spent its early years much further from Proxima Centauri, and he said that the (relatively) nearby presence of much larger star Alpha Centauri A and B certainly could have had dramatic effects on the locations and evolution of the planet.

For these reasons and many more, Barnes said, there is absolutely no way to conclude now that Proxima b either is, or is not, potentially habitable.

This artist's impression shows a view of the surface of the planet Proxima b orbiting t he red dwarf star Proxima Centauri, the closest star to the Solar System. The double star A lpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, wh ere the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser
This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red
dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB
also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than
the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature issuitable for liquid water to exist on its surface.
(ESO/M. Kornmesser)

The VPL group specializes in modeling possible planetary scenarios based on particular proposed condition.  The team gets an idea of whether a planet might be habitable based on the history and potential current atmospheric and surface characteristics introduced into the model.

In the paper she led, Meadows and her team reported:

“We used coupled climate-photochemistry models to simulate several plausible states for the current environment of Proxima Cen b, for those various evolutionary scenarios. We find several post-runaway {greenhouse} states that are uninhabitable either due to extreme water loss or inclement surface temperatures. In particular, a dense Venus-like CO2 atmosphere will result in extremely high surface temperatures at Proxima Cen’s current semi-major axis.

“However, several evolutionary scenarios may lead to possibly habitable planetary environments, including O2-rich atmospheres that retain a remnant ocean after extreme water loss.”

So the conclusion of the VPL effort is that we really have no idea now whether Proxima b might be habitable, but the joined papers move the discussion significantly further by describing scenarios where the planet definitely would not be habitable, and some where it just might be.

These are essential guidepost for astronomers to know when actually observing Proxima b, which will surely become a target for many a telescope.  To make an important discovery,  it’s definitely useful  to know what you’re looking for.

Meadows also looked into which telescopes have capacities to make the needed observations, and concluded that large ground-based telescopes have a role to play, though with current technology it will be a very challenging one.  But given the possible results, she said, “I can’t image they wouldn’t make the upgrades happen as fast a possible.”

And then there’s the James Webb Space Telescope (JWST), due to launch in 2018.  Because it observes in the infrared section of the spectrum, it is able to measure heat signatures with precision.  And that opens some exciting possibilities.

Caption: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lowe r-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani
This picture combines a view of the southern skies over the European Southern Observatory’s  3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discoveredusing the HARPS instrument on the ESO 3.6-metre telescope.
 (Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani)

Meadows and her team laid them out, and so did Harvard astronomer Laura Kreidberg, in a paper for The Astrophysical Journal with Harvard-Smithsonian Center for Astrophysics theoretical physicist and cosmologist  Abraham Loeb.

As Loeb explained in an email:  “As the planet orbits around the star, we should see a changing fraction of its day side, similarly to the phases of the Earth’s moon. The changing  color of the planet  as it orbits the star provides evidence for the temperature contrast between its day and night sides.

“This contrast has an extreme value for bare rock, but is moderated by an atmosphere or an ocean that transfers heat across the planet’s surface. Our paper shows that JWST will be able to distinguish between these cases with high significance after observing the planet for a full orbital time of 11 days.”

In other words, the temperature difference between the planet’s day side and its night side will be larger than expected if there is no atmosphere., and lower than expected if there is.

Determining that there is an atmosphere present, Loeb said, would substantially increase the chances that Proxima b is, or once was, habitable.  The first order would be to look for things like oxygen, water vapor, and methane, which could indicate habitable conditions if not active biological processes.

This is a very difficult task because it requires the ability to catch starlight as it bounces off or filters through the planet’s atmosphere.  He said that while the JWST might be able to detect a few compounds including ozone, full atmospheric analysis will have to wait for future ground-based observatories like the European Extremely Large Telescope, which is expected to see first light in the mid-2020s. Ultimately, it will take a direct imaging space telescope like the one being proposed for a launch in the 2030s to answer many of the important questions.

So the process of getting to really know Proxima b, of learning more than its promising but less-than-revealing location and mass is about to begin.

It’s exciting for sure, and not because Proxima b is “Earth-like.”  Rather, there’s the real possibility of finding a habitable planet that — except in some grand-scale structural ways — is really not so “Earth-like” at all.

 

 

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Found: Our Nearest Exoplanet Neighbor

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This artist ’ s impression shows a view of the surface of the planet Proxima b orbiting t he red dwarf star Proxima Centauri, the closest star to the Solar System. The double star A lpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, wh ere the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser
An artist impression of the surface of the candidate planet Proxima b orbiting the red
dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.
(ESO/M. Kornmesser)

No exoplanet can possibly be closer to us than the one just detected around our nearest stellar neighbor, Proxima Centauri.

The long-sought and long-imagined planet is larger than Earth, but small enough to be rocky as opposed to a gas or ice giant.  Making things even more exciting, the planet was detected inside the habitable zone of Proxima, suggesting that the planet could potentially have temperatures that allow for pooling liquid water.

Innumerable questions remain to be answered before we know if it actually is habitable (as opposed to residing in a habitable zone), and far more before we know if it might actually be inhabited.

But the very exciting news is that an exoplanet has almost definitively been found only 4 light-years from our solar system.  There’s every reason to believe it will become the focus of intense and sustained scientific scrutiny.

The detection is the culmination of a “Pale Red Dot” observing campaign that began in earnest early this year to search the regions close to Proxima for exoplanets.  Guillem Anglada-Escudé  of Queen Mary University, London, was a leader that campaign, as well as earlier efforts to dig deeper into decade-old Proxima Centauri data from other teams that hinted at a planet but were far from definitive.

“The signal that a planet orbits Proxima every 11 days is strong, so we have little doubt that it’s there,” AngladaEscude´ said.  “And because this is the closest possible planet outside our solar system, there’s a sense of finding something special, even inspirational.”

His hope is that the detection will become a global “driver,”  a discovery that is significant enough to change how people think about our world, as well as about the possibility that some day humans will explore up close a planet outside our system.

Said Anglada-Escude´:  “The search for life on Proxima b comes next….”

Caption: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lowe r-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani
This picture combines a view of the southern skies over the European Southern Observatory’s 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima b was discovered using the HARPS instrument on the ESO 3.6-metre telescope, as well as by aggressively refining previous measurements taken around Proima Centauri. (Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani)

Indeed, it already has started.  Excitement in the exoplanet world is palpable, and papers based on the finding are already on their way to the public.  Anglada-Escude´ gave early copies of the Proxima paper to a number of groups around the world so they could begin digging deeper.

One such group was the Virtual Planetary Laboratory at the University of Washington, and director Victoria Meadows and research assistant professor Rory Barnes have been looking at and modeling the possible evolutions of Proxima b and how its potential for habitability can be assessed in the years and decades ahead.

“This is a huge discovery,” said Barnes.  “Before Proxima b, we didn’t really know what planet would be of greatest interest, so we had to prepare for whatever we might find.

“Now it’s no longer a question of what is the prime target, where do we want to first focus and dig deep. Now we know exactly what we want to look at.”

Barnes’ paper will focus on models of  the possible evolutions of Proxima b, while Meadows will focus on what researchers should look for in terms of habitability on the planet and what current and future instruments would be best suited for the search.

“The planet is in the habitable zone, but that doesn’t mean it’s habitable,” Meadows said.  “What we’re focused on is what are the surface conditions now, the atmospheric make-up, whether or not there might be an ocean present.  These are very difficult questions to address, and it will definitely take time to develop the instruments we need and for the community to find answers.”

his infographic compares the orbit of the planet around P roxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than t he Sun and the planet orbits much closer to its star than Mercury. As a result it lies wel l within the habitable zone, where liquid water can exist on the planet ’ s surface. Credit: ESO/M. Kornmesser/G. Coleman
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than the sun and the planet orbits much closer to its star than Mercury does to our sun.  But because Proxima b’s host star put out so much less heat and radiation,  it lies well within the habitable zone,where liquid water can exist on the planet’s surface. (ESO/M. Kornmesser/G. Coleman)

Proxima Centauri is a red dwarf (or M dwarf) star,  a very long-lived but small and cool star compared to our sun.  Red dwarfs are the most common type of star in the galaxy, by far.  To date, however, only a few Earth-mass planets have been discovered in the temperate zones of such stars. But because there are countless billions red dwarf stars, only a small percentage need to have temperate-zone planets to make our galaxy potentially teeming with life.

When it comes to the odds of finding an exoplanet orbiting the star nearest us, I turned to Natalie Bathalia, project scientist for NASA’s Kepler Space Telescope.  Based on statistical analysis of Kepler survey data, she said, we should expect at least one potentially habitable, Earth-size planet orbiting an M dwarf  (or red dwarf)  star within 10 light-years of the solar system.

“Well, it turns out that there are only 7 M dwarfs within 10 light-years of the solar system,”  she said.  “That means we’re ‘roll-of-the-dice’ lucky, not ‘winning-the-lottery’  lucky.  Had it been the latter, I might have been more skeptical.  Instead, I’m relishing the moment.  Numbers aside, I’m certainly feeling like we’ve all just won the cosmic lottery.”

As described in a cover article in the journal Nature, Proxima b was identified by the “Doppler wobble” of its host star — the effect caused by the planet’s gravitational pull. This method was used to discover the first exoplanet around a sun-like star, 51 Pegasi, in 1995.

Guillem Anglada-Escude, leader of the Pale Red Dot campaign and a lecturer at St. Mary's College,London.
Guillem Anglada-Escude´, leader of the Pale Red Dot campaign and a lecturer at St. Mary’s College,London.

Astronomer Paul Butler of the Carnegie Institution for Science was one of the two scientists who made the essential confirmation of that first exoplanet, and he played an important supporting role in the Proxima b detection as well.

“We’ll continue finding planets, but this may well be the last big deal detection,” Butler said.  “There may be even more important planets out there, but it’s pretty hard to imagine what they might be.”

The Doppler method is particularly useful and effective in studying or red dwarf stars. Because these stars are cooler than our sun, their potentially habitable planets would orbit much close to the host star (about one-tenth of the sun–Earth distance,) and so are potentially easier to detect.

These red dwarf stars are also less massive than the sun, and so are more visibly affected by the presence of an orbiting planet.  A Doppler wobble for a red dwarf with exoplanets is large enough (about 1 meter per second) to be detected by current instruments. By comparison, Earth causes a Doppler wobble of the sun of 0.09 meters per second.

Anglada-Escudé and colleagues first detected a possible signal of their exoplanet using Doppler measurements taken by the Ultraviolet and Visual Echelle Spectrograph at the European Southern Observatory (ESO) in Chile between 2000 and 2008.

These data showed a hint — but not an entirely convincing one — of a Doppler wobble of 1.38 meters per second.  With the help of Butler, they also re-analyzed date from an earlier Proxima run using the ESO’s High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph in Chile.

The authors confirmed the signal by using many more Doppler measurements taken in 2016 with the same spectrometer and telescope. Nonetheless, to be on the safe side, Proxima b is still formally called a “candidate” exoplanet.

The European Southern Observatory's La Silla facility in Chile. The "Pale Red Dot" campaign used previous data collected at La Silla, and updated with new observations there this year. (ESO)
The European Southern Observatory’s La Silla facility in Chile’s Atacama Desert. The “Pale Red Dot” campaign used previous data collected at La Silla, and updated with new observations there this year. (ESO)

The Pale Red Dot campaign has been aimed at Proxima, but plans to continue looking for exoplanets orbiting nearby red dwarf stars.  The group includes 31 scientists from eight international and national organizations, and shares its news on this site: https://palereddot.org/.

The name of the campaign plays on the “pale blue dot” image of the Earth taken in 1990 by Voyager 1 on its way to interstellar space. The phrase was later used by Carl Sagan, who lobbied for Voyager to make the necessary maneuvers to take the images,  for his essay, “Pale Blue Dot: A Vision of the Human Future in Space.”

I spoke recently with Anglada-Escude´ about the campaign and its breakthrough Proxima b detection. Both clearly flow from his deep interests and drives.

“I’m a scientist and I’ve been excited about exoplanets for a long time.  But when I was young I read a lot of science fiction, I was a geek I guess.  Proxima and the Centauri system always seemed like the next place to go — not now, but in a few hundred years.  But to do this we had to learn all about the stars and hopefully the planets around them.  So you could say that none of this happened by chance.

“So this is all about detection and characterizing the planet and star, but about exploration as well.  It’s the nearest object outside of our solar system, and that makes it a natural destination.

“We know the planet is there, so now is the time for people to get creative about imaging it, and learning how to search for life there, and ultimately set out on interstellar expeditions in that direction.”

Proxima Centauri, it should be noted, is 266,000 times as far away as the distance from the Earth to our sun.

his image of the sky around the bright star Alpha Centauri A B also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from pictures forming part of the Digitized Sky Survey 2. The blue halo aroun d Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in co lour like the Sun. Credit: Digitized Sky Survey 2 Acknowledgement: Davide De Martin/Mahdi Zamani
While Alpha Centauri AB (very bright in the image) and Proxima Centauri (much fainter and reddish) are considered part of a single system, it remains unclear whether they are subject to the gravitational tugs of each other — a key issue in understanding the possibile evolution of the stars and detected and potential exoplanets. This image was created from pictures forming part of the Digitized Sky Survey 2. The blue halo aroun d Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in color like the sun. (Digitized Sky Survey 2/Davide De Martin/Mahdi Zamani)

 

 

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Earth: A Prematurely Inhabited Planet?

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A schematic of the history of the cosmos since the Big Bang identifies the period when planets began to form, but there's indication of when life might have started. Harvard's Avi Loeb wants to put life into this cosmological map, and foresees much more of it in the future, given certain conditions. ( NASA)
A schematic of the history of the cosmos since the Big Bang identifies the period when planets began to form, but there’s no indication of when life might have started. Harvard’s Avi Loeb wants to add life into this cosmological map, and foresees much more of it in the future, given certain conditions. ( NASA)

The study of the formation and logic of the universe (cosmology) and the study of exoplanets and their conduciveness to life do not seem to intersect much.  Scientists in one field focus on the deep physics of the cosmos while the others search for the billions upon billions of planets out there and seek to unlock their secrets.

But astrophysicist and cosmologist Avi Loeb — a prolific writer about the early universe from his position at the Harvard-Smithsonian Center for Astrophysics– sees the two fields of study as inherently connected, and has set out to be a bridge between them.  The result was a recent theoretical paper that sought to place the rise of life on Earth (and perhaps elsewhere) in cosmological terms.

His conclusion:  The Earth may well be a very early example of a living biosphere, having blossomed well before life might be expected on most planets.   And in theoretical and cosmological terms, there are good reasons to predict that life will be increasingly common in the universe as the eons pass.

By eons here, Loeb is thinking in terms that don’t generally get discussed in geological or even astronomical terms.  The universe may be an ancient 13.7 billion years old, but Loeb sees a potentially brighter future for life not billions but trillions of years from now.  Peak life in the universe, he says, may arrive several trillion years hence.

“We used the most conservative approaches to understanding the appearance of life in the universe, and our conclusion is that we are very early in the process and that it is likely to ramp up substantially in the future,” said Loeb, whose paper was published in the Journal of Cosmology and Astroparticle Physics.

“Given the factors we took into account, you could say that life on Earth is on the premature side.”

 

The Earth was formed some 4.5 billions years ago, and life that existed as long ago as 3.5 to 3.8 billion years ago has been discovered. Harvard astrophysicist Avi Loeb argues that life on Earth may well be "premature" in cosmological terms, and that many more planets will have biospheres in the far future. (xxx)
The Earth was formed some 4.5 billion years ago, and signs of life have been discovered that are 3.5 to 3.8 billion years old. Harvard astrophysicist Avi Loeb, with co-authors Rafael Batista and David Sloan of the University of Oxford, argue that life on Earth may well be “premature” in cosmological terms, and that many more planets will have biospheres in the far future.  This artist rendering of early Earth was created by NASA’s  Goddard Space Flight Center Conceptual Image Lab.

This most intriguing conclusion flows from the age of the universe, the generally understood epochs when stars and then planets and galaxies formed, and then how long it would take for a planet to cool off enough to form the chemical building blocks of life and then life itself.  Given these factors, Loeb says, we’re early.

In the long term, the authors determined, the dominant factor in terms of which planets might become habitable proved to be the lifetime of stars. The higher a star’s mass, the shorter its lifetime. Stars larger than about three times the sun’s mass will burn out well before any possible life has time to evolve.

Our sun is a relatively large and bright star, which is why its lifetime will be relatively short in cosmological terms (all together, maybe 11 billion years, with 4.5 billion already gone.)  But smaller stars, the “red dwarf,” low-mass variety, are both far more common in the universe and also much longer lived — as in trillions of years.

These smallest stars generally have less than 10 percent the mass of our sun, but they burn their fuel (hydrogen and helium) much more slowly than a larger star.  Indeed, some may glow for 10 trillion years, Loeb says, giving ample time for life to emerge on any potentially habitable planets that orbit them.  What’s more, there’s every reason to believe that the population of stars in the galaxy and cosmos will increase significantly, giving life ever more opportunity to commence.

Abraham Loeb, usually called "Avi," is the chairman of the Harvard Astronomy Department and xxxx CFA. Mac G. Schumer, Harvard Crimson Mac G. Schumer, Harvard Crimson
Abraham, or Avi, Loeb, is the chairman of the Harvard Astronomy Department and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. (Mac G. Schumer, Harvard Crimson)

As a result, the relative probability of life grows over time. In fact, chances of life are 1,000 times higher in the distant future than now.

This calculation, however, comes with a major caveat:  Scientists are sharply divided about whether or not a star much smaller than ours can actually support life.

The potential obstacles are many — an insufficient amount of heat and energy emanating from the star unless the planet is close in, the fact that red dwarf stars have powerful, luminous beginnings that could send a nearby planet into a runaway greenhouse condition that might result in permanent sterilization, and that many planets around red dwarfs would be close to the stars and consequently tidally locked.  That means that one side of the planet would always face the star and be light, while the other would continue in eternal darkness.  This was earlier considered to be a pretty sure deterrent to life.

Recent theoretical analyses of planets around these red dwarfs, however, suggests that life could indeed emerge.  It could potentially survive at the margins — where day turns into night and the temperatures would likely be stable– and also in other dayside regions were temperatures could be moderated by clouds and winds.  But no observations have been made to substantiate the theory.

Because of their relatively cool temperatures and resulting low brightness, individual red dwarfs are nearly impossible to see with the naked eye from Earth. But they’re out there.

The nearest star to our sun, Proxima Centauri, is a red dwarf, as are twenty of the next thirty nearest stars.  Data from the Kepler Space Telescope suggests that as many as 25 percent of red dwarfs have planets orbiting in their habitable zones — neither too hot nor too cold to keep liquid water from sometimes pooling on their surfaces.

 

Flares from our sun and from a red dwarf star.
Flares from our sun and from a red dwarf star.  These powerful bursts of energy and heat may be larger on the sun, but in percentage terms they are greater on many red dwarf stars. Since planets around red dwarfs are much closer than larger stars like our sun, those energy bursts might sterilize red dwarf planets. (NASA)

“I think we can and we should test these theories in the years ahead with observations,” Loeb said. “We should be able to tell if nearby low-mass stars have life around them” in the decades ahead.

And if red dwarfs can support life, then the future for life in the universe is indeed grand.

The merging of cosmological theory and astronomical observation that Loeb has in mind would indeed be unusual, but it is nonetheless consistent with the interdisciplinary nature of much of the broader search for life beyond Earth.  That effort has already brought together astrophysicists and geoscientists, astronomers and biologist.  It’s just way too big for one discipline.

An interesting sidelight to Loeb’s argument that Earth may well be among the earliest planets where life appeared and continued is that it would provide a solution to the extraterrestrial life puzzle known as Fermi’s Paradox.

It was in 1950 that renowned physicist Enrico Fermi was talking with colleagues over lunch about the predicted existence of billions of still-to-be-discovered planets beyond our solar system, and the likelihood that many had planets around them.  Fermi also was convinced that the logic of the vast numbers and of evolution made it certain that intelligent, technologically-advanced life existed on some of those planets.

It was an era of fascination with aliens, flying saucers and the like, but there actually were no confirmed reports of visitations by extraterrestrial life.  Ever, it seemed.

If intelligent life is common in the universe, Fermi famously wondered, “Then where is everybody?”

There are many potential answers to the question, including, of course, that we are alone in the universe.  The possibility that Earth might be among the very early planets with life has not been put forward before, but Loeb said that now it has been.

 

If the conclusion is correct that the Earth is most likely among the first planets to support life, then the famous Fermi Paradox could be easily resolved.
If the conclusion is correct that the Earth is most likely among the first planets to support life, then the famous Fermi Paradox could be easily resolved.

“Our view is that we’re at the very beginning of life in the universe, we’re just ramping up,” he said.  “So of course we haven’t been visited by anything extraterrestrial.”

As a congenital thinker in the very long term, Loeb also raised the issue of whether it makes sense for human life to remain on Earth and in our solar system.  The sun, after all, will run out of fuel in those remaining six billion years, will expand enormously as that occurs, and then will re-emerge as a super-dense white dwarf star.  Any biology in our solar system would have been destroyed long before that.

But Proxima Centauri, one of those very long-lived stars?

“It will be there a very long time,” he said.  “If the conditions are right, then maybe a time will come to migrate to any planets that might be around Proxima.  It’s four light years away, so it would take generations of humans to get there.  Certainly very difficult, but some day in the far future people may be faced with an alternative that’s considerably worse.”

 

 

 

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The Pale Red Dot Campaign

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Alpha and Beta Centauri are the bright stars; Proxima Centauri is the small, faint one circles in red.
Alpha Centauri A and B are the bright stars; Proxima Centauri, a red dwarf star, is the small, faint one circled in red. (NASA, Julia Figliotti)

Astronomers have been trying for decades to find a planet orbiting Proxima Centauri, the star closest to our sun and so a natural and tempting target.  Claims of an exoplanet discovery have been made before, but so far none have held up.

Now, in a novel and very public way, a group of European astronomers have initiated a focused effort to change all that with their Pale Red Dot Campaign.  Based at the La Silla Observatory in Chile, and supported by  networks of smaller telescopes around the world, they will over the next three months observe Proxima and its environs and then will spend many more months analayzing all that they find.  And in an effort to raise both knowledge and excitement, the team will tell the world what they’re doing and finding over Twitter, Facebook, blogs and other social and traditional media of all kind.

“We have reason to be hopeful about finding a planet, but we really don’t know what will happen,” said Guillem Anglada-Escudé  of Queen Mary University, London, one of the campaign organizers.  “People will have an opportunity to learn how astronomers do their work finding exoplanets, and they’ll be able to follow our progress.  If we succeed, that would be wonderful and important.  And if no planet is detected, that’s very important too.”

The Pale Blue Dot, as photographed by Voyager 1 (NASA)
The Pale Blue Dot, as photographed by Voyager 1 (NASA)

The name of the campaign is, of course, a reference to the iconic “Pale Blue Dot” image of Earth taken by the Voyager 1 spacecraft in 1990, when it was well beyond Pluto.  The image came to symbolize our tiny but precious place in the galaxy and universe.

But rather than potentially finding a pale blue dot, any planet orbiting the red dwarf star Proxima Centauri would reflect the reddish light of the the star, which lies some 4.2 light years away from our solar system.  Proxima — as well as 20 of the 30 stars in our closest  neighborhood — is reddish because it is considerably smaller and less luminous than a star like our sun.

Anglada-Escudé said he is cautiously optimistic about finding a planet because of earlier Proxima observations that he and colleagues made at the same observatory.  That data, he said, suggested the presence of a planet 1.2 to 1.5 times the size of Earth, within the habitable zone of the star.

“We did not and are not making claims in terms of having discovered a planet,”  he said.  “We’re saying that we detected signals that could mean there is a planet.  This is why we’ve planned this campaign — to see if the signal is telling us something real.”  He described the campaign as a “partnership between scientists involved in the observations and European Southern Observatory.”

Even without a previous signal, it’s a reasonable bet that Proxima does have at least one planet orbiting it.  Based on the results of the Kepler Space Telescope survey in particular, there is a consensus of sorts in the astronomy community that on average, every star has at least one planet circling it.

Alpha Centauri A and Alpha Centauri B are a binary pair, while Proxima Centauri is far away but is xxx
Alpha Centauri A, Alpha Centauri B and Proxima Centauri make up a three-star system, although Proxima Centauri is a distant .2 lightyears away rom the other two.  (Ian Morrison)

Paul Butler, a pioneer in planet hunting at the Carnegie Institution of Washington who has done extensive observing of Proxima with his team at Las Campanas Observatory in Chile, will be providing data to the Pale Red Dot campaign.  Proxima search results from the ESO’s Very Large Telescope at Paranal, Chile, will also be provided to campaign.

Butler said that in some ways Proxima “is the most exciting star in the sky.  It’s the very nearest star and so the discovery of a planet there would be huge – front page of the paper around the world.”

What’s more, he said, such a discovery could be enormously helpful in motivating Congress and taxpayers to spend the money needed for what is considered the holy grail of planet hunting — building a space-based exoplanet observatory that could directly image exoplanets.  “We have to give people a clear reasons to spend all that money and finding a potentially habitable planet around Proxima, that would be it.”

 Hubble Space Telescope image is our closest stellar neighbour: Proxima Centauri, just over four light-years from Earth. Although it looks bright through the eye of Hubble, Proxima Centauri -- with only about one eight the mass of our sun -- is not visible to the naked eye.Shining brightly in this Hubble image is our closest stellar neighbour: Proxima Centauri. Proxima Centauri lies in the constellation of Centaurus (The Centaur), just over four light-years from Earth. Although it looks bright through the eye of Hubble, as you might expect from the nearest star to the Solar System, Proxima Centauri is not visible to the naked eye. Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun. However, on occasion, its brightness increases. Proxima is what is known as a “flare star”, meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness. The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that Proxima Centauri is in for a very long life. Astronomers predict that this star will remain middle-aged — or a “main sequence” star in astronomical terms — for another four trillion years, some 300 times the age of the current Universe. These observations were taken using Hubble’s Wide Field and Planetary Camera 2 (WFPC2). Proxima Centauri is actually part of a triple star system — its two companions, Alpha Centauri A and B, lie out of frame. Although by cosmic standards it is a close neighbour, Proxima Centauri remains a point-like object even using Hubble’s eagle-eyed vision, hinting at the vast scale of the Universe around us.
A Hubble Space Telescope image of Proxima Centauri, just over four light-years from Earth. Proxima Centauri — with only about one eight the mass of our sun — is not visible to the naked eye. Its average luminosity is very low but, on occasion, its brightness increases. Proxima is what is known as a “flare star” — where convection processes within the star’s make it prone to random and dramatic changes in brightness. (NASA)

Proxima and the other Alpha Centauri stars are also an especially appealing target because they have loomed so large in science fiction.  From Robert Heinlein’s “Ophans of the Sky” stories of crews traveling to Proxima to Isaac Asimov’s “Foundation and Earth ” set around Alpha Centauri and more recently to the James Cameron’s movie “Avatar,” also set in the Centauri neighborhood, these closer-by have been a frequent and logical destination.

While Alpha Centauri B has gotten much scientific attention in recent years with a reported but still unconfirmed and now often dismissed planet candidate, Proxima Centauri has been the object of much observation, too, and that has begun to define what kinds of planets might and might not be present.

So far, the work of Butler’s team has not found any particularly promising signs of a planetary-caused Proxima wobble.  But he said nothing established so far about Proxima rules out the presence of a small planet relatively close to the sun — the very time-consuming observations needed to potentially detect that size planet just haven’t been done.

Similarly, the Very Large Telescope results ruled out the presence of Saturn-size planets with many-year orbits and Neptune-size planets with orbits less than about 40 day, and no planets more than 6 to 10 Earths in the habitable zone.  This is actually promising news, since the absence of larger planets in the habitable zone leaves the field open for smaller ones.

Two other teams are now focused on Proxima as well.  One is led by David Kipping of Columbia University  using the Canadian Microvariability & Oscillations of STars space telescope (MOST) to search for transits.  The other is led by Kailash Sahu of the Space Science Telescope Institute in Baltimore, using the Hubble Space Telescope for microlensing of the star. The stars are aligned for the microlensing event this month.

A ring of telescopes at ESO's La Silla observatory. La Silla, in  the  southern part of the Atacama desert, 600 km north of  Santiago de  Chile,  was ESO's first observation site. The telescopes are 2400 metres  above  sea level, providing excellent observing conditions. ESO  operates the 3.6-m telescope, the  New Technology Telescope (NTT), and   the 2.2-m Max-Planck-ESO telescope  at La Silla. La Silla also hosts  national telescopes, such as the 1.2-m  Swiss  Telescope and the 1.5-m  Danish Telescope.
A ring of telescopes at ESO’s La Silla observatory. La Silla, in the southern part of the Atacama desert, 600 km north of Santiago de Chile, was ESO’s first observation site. The telescopes are 2400 metres above sea level, providing excellent observing conditions. ESO operates the 3.6-m telescope, the New Technology Telescope (NTT), and the 2.2-m Max-Planck-ESO telescope at La Silla. La Silla also hosts national telescopes, such as the 1.2-m Swiss Telescope and the 1.5-m Danish Telescope. (ESO)

The Pale Red Dot observing began last week and will run for two and a half month using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory (ESO) telescope at La Silla, Chile. The observations — like those made at the Magellan and at Paranal — look for tiny wobbles in the star’s motion created by the gravitational pull of an orbiting planet. (More on how the radial velocity method works, as well as other connections to and details about the campaign can be found at:  https://palereddot.org/introduction/)

The campaign is the beneficiary of a substantial amount of HARPS observing time — 25 minutes of observing for 60 nights in a row — which is essential to confidently detect the presence of a small, Earth-sized planet.

Other robotic telescopes — including the Burst Optical Observer and Transient Exploring System,  the Las Cumbres Observatory Global Telescope Network and the Astrograph for the Southern Hemisphere II — will participate.  The role of these automated telescopes is to measure the brightness of Proxima each night, a backup that will help astronomers determine whether the wobbles of the star detected via radial velocity are the tug of an orbiting planet or activity on the surface of the star. Anglada-Escudé said that after a full analysis, the findings will offered to a peer-reviewed journal and published.

While the goal of the campaign is definitely to detect a planet orbiting our closest stellar neighbor, it is also very consciously a public outreach effort for astronomy and exoplanets.  Everything about the campaign will be made public, and often immediately via Twitter and other social media.  It will provide a window, said Anglada-Escudé, into how planet-hunting astronomy works.

Guillem Anglada-Escude
Guillem Anglada-Escudé is leading the Pale Red Dot campaign.

“We think this to be a good way to explain things that are not obvious to the public, to show them that looking for planets is not always excitement and ‘eurekas.’   We’ll show life at the observatory, how our observations are made, what happens as we analyze the data.  And if in the end we don’t find evidence of a planet, we will have shown how we search for such tiny objects so far away, and do it with a pretty amazing precision.”

Involving the public so early and often definitely brings risks, since the campaign could certainly come up empty-handed.  But in terms of real-life planet hunting, that result is hardly unusual.  An awful lot of planet-hunting campaigns end without a detection.

When red dwarf stars, also called M dwarfs, are found with orbiting planets, they tend to be much closer in than with more massive stars, and their habitable zones are also much more narrow.  Initially, red dwarfs were not considered good candidates for habitable planets because they are so relatively small — between 50 to 5 percent the mass of our sun.  Any planets orbiting close to a red dwarf would likely be tidally locked as well, with only one side ever facing the sun.  The pull of the host star causes the locking.

These issues and more earlier led scientists to dismiss red dwarf exoplanets as unlikely to be habitable. That unpromising view has changed with the creation of models for tidally locked planets that could be habitable, and with the discovery of many exoplanets orbiting around the red dwarfs.  These small suns actually  constitute more than 70 percent of the stars in the sky, although very few of the ones you can see without a telescope.

So the time seems ripe for a substantial exoplanet campaign at Proxima — one that just might find a planet and that certainly has a lot to teach the public.

Sites where you can follow the campaign:
Twitter: @Pale_red_dot #palereddot
Facebook page:  ‘Pale Red Dot’

 Artist rendering of a cold desert on a planet orbiting Proxima Centauri. (Vladimir Romanyuk, Space Engine)
Artist rendering of a cold desert on a planet orbiting Proxima Centauri. (Vladimir Romanyuk, Space Engine)
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