Category: How Do They Form (page 1 of 2)

The Gale Winds of Venus Suggest How Locked Exoplanets Could Escape a Fate of Extreme Heat and Brutal Cold

Two images of the nightside of Venus captured by the IR2 camera on the Akatsuki orbiter in September 2016 (JAXA).

 

More than two decades before the first exoplanet was discovered, an experiment was performed using a moving flame and liquid mercury that could hold the key to habitability on tidally locked worlds.

The paper was published in a 1969 edition of the international journal, Science, by researchers Schubert and Whitehead. The pair reported that when a Bunsen flame was rotated beneath a cylindrical container of mercury, the liquid began to flow around the container in the opposite direction at speeds up to four times greater than the rotation of the flame. The scientists speculated that such a phenomenon might explain the rapid winds on Venus.

On the Earth, the warm equator and cool poles set up a pressure difference that creates our global winds. These winds are deflected westward by the rotation of the planet (the so-called Coriolis force) promoting a zonal (east-west) air flow around the globe. But what would happen if our planet’s rotation slowed? Would our winds just cycle north and south between the equator and poles?

The Moon is tidally locked to the Earth, so only one hemisphere is visible from our planet (Smurrayinchester / wikipedia commons).

Such a slow-rotating scenario may be the lot of almost all rocky exoplanets discovered to date. Planets such as the TRAPPIST-1 system and Proxima Centauri-b all orbit much closer to their star than Mercury, making their faint presence easier to detect but likely resulting in tidal lock. Like the moon orbiting the Earth, planets in tidal lock have one side permanently facing the star, creating a day that is equal to the planet’s year.

The dim stars orbited by these planets can mean they receive a similar level of radiation as the Earth, placing them within the so-called “habitable zone.” However, tidal lock comes with the risk of horrific atmospheric collapse. On the planet side perpetually facing away from the star, temperatures can drop low enough to freeze an Earth-like atmosphere. The air from the dayside would then rush around the planet to fill the void, freezing in turn and causing the planet to lose its atmosphere even within the habitable zone.

The only way this could be prevented is if winds circulating around the planet could redistribute the heat sufficiently to prevent freeze-out. But without a strong Coriolis force from the planet’s rotation, can such winds exist?… Read more

Weird Planets

 

 

Artist rendering of an “eyeball world,” where one side of a tidally locked planet is always hot on the sun-facing side and the back side is frozen cold.  Definitely a tough environment, but  might some of the the planets be habitable at the edges?  Or might winds carry sufficient heat from the front to the back?  (NASA/JPL-Caltech)

The very first planet detected outside our solar system powerfully made clear that our prior understanding of what planets and solar systems could be like was sorely mistaken.

51 Pegasi was a Jupiter-like massive gas planet, but it was burning hot rather than freezing cold because it orbited close to its host star — circling in 4.23 days.  Given the understandings of the time, its existence was essentially impossible. 

Yet there it was, introducing us to what would become a large and growing menagerie of weird planets.

Hot Jupiters, water worlds, Tatooine planets orbiting binary stars, diamond worlds (later downgraded to carbon worlds), seven-planet solar systems with planets that all orbit closer than Mercury orbits our sun.  And this is really only a brief peak at what’s out there — almost 4,000 exoplanets confirmed but billions upon billions more to find and hopefully characterize.

I thought it might be useful — and fun — to take a look at some of the unusual planets found to learn what they tell us about planet formation, solar systems and the cosmos.

 


Artist’s conception of a hot Jupiter, CoRoT-2a. The first planet discovered beyond our solar system was a hot Jupiter similar to this, and this surprised astronomers and led to the view that many hot Jupiters may exist. That hypothesis has been revised as the Kepler Space Telescope found very few distant hot Jupiters and now astronomers estimate that only about 1 percent of planets are hot Jupiters. (NASA/Ames/JPL-Caltech)

 

Let’s start with the seven Trappist-1 planets.  The first three were detected two decades ago, circling a”ultra-cool” red dwarf star a close-by 40 light years away.  Observations via the Hubble Space Telescope led astronomers conclude that two of the planets did not have hydrogen-helium envelopes around them, which means the probability increased that the planets are rocky (rather than gaseous) and could potentially hold water on their surfaces.

Then in 2016 a Belgian team, using  the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, found three more planets, and the solar system got named Trappist-1. Read more

A National Strategy for Finding and Understanding Exoplanets (and Possibly Extraterrestrial Life)

The National Academies of Science, Engineering and Medicine took an in-depth look at what NASA, the astronomy community and the nation need to grow the burgeoning science of exoplanets — planets outside our solar system that orbit a star. (NAS)

 

An extensive, congressionally-directed study of what NASA needs to effectively learn how exoplanets form and whether some may support life was released today, and it calls for major investments in next-generation space and ground telescopes.  It also calls for the adoption of an increasingly multidisciplinary approach for addressing the innumerable questions that remain unanswered.

While the recommendations were many, the top line calls were for a sophisticated new space-based telescope for the 2030s that could directly image exoplanets, for approval and funding of the long-delayed and debated WFIRST space telescope, and for the National Science Foundation and to help fund two of the very large ground-based telescopes now under development.

The study of exoplanets has seen remarkable discoveries in the past two decades.  But the in-depth study from the private, non-profit National Academies of Sciences, Engineering and Medicine concludes that there is much more that we don’t understand than that we do, that our understandings are “substantially incomplete.”

So the two overarching goals for future exoplanet science are described as these:

 

  • To understand the formation and evolution of planetary systems as products of star formation and characterize the diversity of their architectures, composition, and environments.
  • To learn enough about exoplanets to identify potentially habitable environments and search for scientific evidence of life on worlds orbiting other stars.

 

Given the challenge, significance and complexity of these science goals, it’s no wonder that young researchers are flocking to the many fields included in exoplanet science.  And reflecting that, it is perhaps no surprise that the NAS survey of key scientific questions, goals, techniques, instruments and opportunities runs over 200 pages. (A webcast of a 1:00 pm NAS talk on the report can be accessed here.)

 


Artist’s concept showing a young sun-like star surrounded by a planet-forming disk of gas and dust.
(NASA/JPL-Caltech/T. Pyle)

These ambitious goals and recommendations will now be forwarded to the arm of the National Academies putting together 2020 Astronomy and Astrophysics Decadal Survey — a community-informed blueprint of priorities that NASA usually follows.

This priority-setting is probably most crucial for the two exoplanet direct imaging missions now being studied as possible Great Observatories for the 2030s — the paradigm-changing space telescopes NASA has launched almost every decade since the 1970s.

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The Architecture of Solar Systems

The architecture of planetary systems is an increasingly important factor to exoplanet scientists.  This illustration shows the Kepler-11 system where the planets are all roughly the same size and their orbits spaced at roughly the same distances from each other.  The the planets are, in the view of scientists involved with the study, “peas in a pod.” (NASA)

Before the discovery of the first exoplanet that orbits a star like ours, 51 Pegasi b, the assumption of solar system scientists was that others planetary systems that might exist were likely to be like ours.  Small rocky planets in the inner solar system, big gas giants like Jupiter, Saturn and Neptune beyond and, back then, Pluto bringing up the rear

But 51 Peg b broke every solar system rule imaginable.  It was a giant and hot Jupiter-size planet, and it was so close to its star that it orbited in a little over four days.  Our Jupiter takes twelve years to complete an orbit.

This was the “everything we knew about solar systems is wrong” period, and twenty years later thinking about the nature and logic of solar system architecture remains very much in flux.

But progress is being made, even if the results are sometimes quite confounding. The umbrella idea is no longer that solar, or planetary, systems are pretty much like ours, but rather that the galaxy is filled with a wild diversity of both planets and planetary systems.

Detecting and trying to understand planetary systems is today an important focus 0f  exoplanet study, especially now that the Kepler Space Telescope mission has made clear that multi-planet systems are common.

As of early July, 632 multi planet systems have been detected and 2,841 stars are known to have at least one exoplanets.  Many of those stars with a singular planet may well have others yet to be found.

An intriguing newcomer to the diversity story came recently from University of Montreal astronomer Lauren Weiss, who with colleagues expanded on and studied some collected Kepler data.

What she found has been deemed the “peas in a pod” addition to the solar system menagerie.

Weiss was working with the California-Kepler Survey, which included a team of scientists pouring over, elaborating on and looking for patterns in, among other things, solar system architectures.

Weiss is part of the California-Kepler Survey team, which used the Keck Observatory to obtain high-resolution spectra of 1305 stars hosting 2025 transiting planets originally discovered by Kepler.… Read more

Exoplanet Science Flying High

An artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech

 

Early this spring, the organizers of an exoplanet science gathering at Cambridge University put out the word that they would host a major meeting this summer.  Within a week, the 300 allotted slots had been filled by scientists aspiring and veteran, and within a short time the waiting list was up to 150 more.

Not the kind of reaction you might expect for a hardcore, topic-specific meeting, but exoplanet science is now in a phase of enormous growth and excitement.  With so many discoveries already made and waiting to be made, so many new (and long-standing) questions to be worked on, so much data coming in to be analyzed and turned into findings,  the field has something of a golden shine.

What’s more, it has more than a little of the feel of the Wild West.

Planet hunters Didier Queloz and Michel Mayor at the European Southern Observatory’s La Silla site. (L. Weinstein/Ciel et Espace Photos)

Didier Queloz, a professor now at Cambridge but in the mid 1990s half of the team that identified the first exoplanet, is the organizer of the conference.

“It sometimes seems like there’s not much exploration to be done on Earth, and the opposite is the case with exoplanets,” he told me outside the Cambridge gathering.

“I think a lot of young scientists are attracted to the excitement of exoplanets, to a field where there’s so much that isn’t known or understood.”

Michel Mayor of the Observatory of Geneva — and the senior half of the team that detected the first exoplanet orbiting a star like our sun, 51 Pegasi b– had opened the gathering with a history of the search for extra-solar planets.

That search had some conceptual success prior to the actual 1995 announcement of an exoplanet discovery, but several claims of having actually found an exoplanet had been made and shown to be wanting.  Except for the relative handful of scientists personally involved, the field was something of a sideshow.

“At the time we made our first discovery, I basically knew everyone in the field.  We were on our own.”

Now there are thousands of people, many of them young people, studying exoplanets.  And the young people, they have to be smarter, more clever, because the questions are harder.”

And enormous progress is being made.… Read more

Planets Still Forming Detected in a Protoplanetary Disk

An artist rendering of infant star HD 163296 with three protoplanets forming in its disk  The planets were discovered using a new mode of detection — identifying unusual patterns in the flow of gas within a protoplanetary disk. (NRAO/AUI/NSF; S. Dagnello)

Just as the number of planets discovered outside our solar system is large and growing — more than 3,700 confirmed at last count — so too is the number of ingenious ways to find exoplanets ever on the rise.

The first exoplanets were found by measuring the “wobble” in their host stars caused by the gravitational pull of the planets, then came the transit technique that measured dips in the light from stars as planets passed in front of them, followed by the direct imaging of moving objects deemed to be planets, and numerous more.

A new technique can now be added to the toolkit, one that is useful only in specific galactic circumstances but is nonetheless ingenious and intriguing.

By detecting unusual patterns in the flow of gas within the protoplanetary disk of a young star, two teams of astronomers have confirmed the distinct, telltale hallmarks of newly formed planets orbiting the infant star.

In other words, the astronomers found planets in the process of being formed, circling a star very early in its life cycle.

These results came thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), and are presented in a pair of papers appearing in the Astrophysical Journal Letters.

Richard Teague, an astronomer at the University of Michigan and principal author on one of the papers, said that his team looked at “the localized, small-scale motion of gas in a star’s protoplanetary disk. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images coming from ALMA.”

ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. ( ALMA: ESO/NAOJ/NRAO; A. Isella; B. Saxton NRAO/AUI/NSF.

To make their respective discoveries, each team analyzed the data from various ALMA observations of the young star HD 163296, which is about 4 million years old and located about 330 light-years from Earth in the direction of the constellation Sagittarius.

Rather than focusing on the dust within the disk, which was clearly imaged in an earlier ALMA observation, the astronomers instead studied the distribution and motion of carbon monoxide (CO) gas throughout the disk.

As explained in a release from the National Radio Astronomy Observatory, which manages the American operations of the multi-national ALMA, molecules of carbon monoxide naturally emit a very distinctive millimeter-wavelength light that ALMA can observe.

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Can You Overwater a Planet?

Water worlds, especially if they have no land on them, are unlikely to be home to life, or at least life we can detect.  Some of the basic atmospheric and mineral cycles that make a planet habitable will be absent. Cool animation of such a world. (NASA)

Wherever we find water on Earth, we find life. It is a connection that extends to the most inhospitable locations, such as the acidic pools of Yellowstone, the black smokers on the ocean floor or the cracks in frozen glaciers. This intimate relationship led to the NASA maxim, “Follow the Water”, when searching for life on other planets.

Yet it turns out you can have too much of a good thing. In the November NExSS Habitable Worlds workshop in Wyoming, researchers discussed what would happen if you over-watered a planet. The conclusions were grim.

Despite oceans covering over 70% of our planet’s surface, the Earth is relatively water-poor, with water only making up approximately 0.1% of the Earth’s mass. This deficit is due to our location in the Solar System, which was too warm to incorporate frozen ices into the forming Earth. Instead, it is widely — though not exclusively — theorized that the Earth formed dry and water was later delivered by impacts from icy meteorites. It is a theory that two asteroid missions, NASA’s OSIRIS-REx and JAXA’s Hayabusa2, will test when they reach their destinations next year.

But not all planets orbit where they were formed. Around other stars, planets frequently show evidence of having migrated to their present orbit from a birth location elsewhere in the planetary system.

One example are the seven planets orbiting the star, TRAPPIST-1. Discovered in February this year, these Earth-sized worlds orbit in resonance, meaning that their orbital times are nearly exact integer ratios. Such a pattern is thought to occur in systems of planets that formed further away from the star and migrated inwards.

Trappist-1 and some of its seven orbiting planets.  They would have been sterilized by high levels of radiation in the early eons of that solar system — unless they were formed far out and then migrated in.  That scenario would also allow for the planets to contain substantial amounts of water. (NASA)

The TRAPPIST-1 worlds currently orbit in a temperate region where the levels of radiation from the star are similar to that received by our terrestrial worlds.… Read more

Exoplanet Clouds; Friend and Foe

Different colotd pedicted of Hot Jupiters based on their temperatures and the compounds in their atmospheres.

An illustration representing how hot Jupiters of different temperatures and different cloud compositions might appear to a person flying over the day side of these planets on a spaceship, based on computer modeling.  (NASA/JPL-Caltech/University of Arizona/V. Parmentier)

 

Understanding the make-up and dynamics of atmospheric clouds is crucial to our interpretations of how weather and climate behave on Earth, and so it should come as no surprise that clouds are similarly essential to learning the nature and behavior of exoplanets.

On many exoplanets, thick clouds and related, though different, hazes have been impediments to learning what lies in the atmospheres and on surfaces below.  Current technologies simply can’t pierce many of these coverings, and scientists have struggled to find new approaches to the problem.

One class of exoplanets that has been a focus of cloud studies has been, perhaps unexpectedly, hot Jupiters — those massive and initially most surprising gas balls that orbit very close to their suns.

Because of their size and locations, the first exoplanets detected were hot Jupiters.  But later work by astronomers, and especially the Kepler Space Telescope, has established that they are not especially common in the cosmos.

Due to their locations close to suns,  however, they have been useful targets of study as the exoplanet community moves from largely detecting new objects to trying to characterize them, to understanding their basic features.  And clouds are a pathway to that characterization.

For some time now, scientists have understood that the night sides of the tidally-locked hot Jupiters generally do have clouds, as do the transition zones between day and night.  But more recently, some clouds on the super-hot day sides — where temperatures can reach 2400 degrees Fahrenheit –have been identified as well.

Vivien Parmentier, a Sagan Fellow at the University of Arizona, Tucson, as well as planetary scientist Jonathan Fortney of the University of California at Santa Cruz have been studying those day side hot Jupiter clouds to see what they might be made of, and how and why they behave as they do.

“Cloud composition changes with planet temperature,” said Parmentier, who used a 3D General Circulation Model (GCM) to track where clouds form in hot Jupiter atmospheres, and what impact they have on the light emitted and reflected by the planets.  “The offsetting light curves tell the tale of cloud composition. It’s super interesting, because cloud composition is very hard to get otherwise.”

The paper by Parmentier, Fortney and others was published in The Astrophysical Journal.… Read more

Rethinking The Snow Line

This image of the planet-forming disc around the young star V883 Orionis was obtained by ALMA in long-baseline mode. This star is currently in outburst, which has pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form.

This planet-forming disc around the young star V883 Orionis was obtained by the European Southern Observatory’s  Atacama Large Millimeter/submillimeter Array (ALMA), a prime site for radio astronomy. The star is a state of “outburst,” which has pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form.  ALMA (ESO/NAOJ/NRAO)/L. Cieza

In every planet-forming disk there’s a point where the heat from a host star needed to keep H2O molecules as vapor peters out, and the H2O be becomes a solid crystal.  This is the snow line, and it looms large in most theories of planet formation.

Most broadly, planets formed inside the snow line will generally be rocky and small — a function of the miniscule dust grains that begin the planet forming process.  But outside the snow line the grains get coated by the icy H2O and so are much bigger, leading to gas and ice giant planets.

The existence of water snow lines (and for other molecules, too) is nothing new, but an image of a water snow line would be.  And now an international team led by Lucas Cieza of Universidad Diego Portales in Santiago, Chile, has found the water vapor/ice line around a very young star 1,350 light-years away. The results were published in 2016 journal Nature.

Using a high-precision radio astronomy array in Chile’s Atacama Desert, the team had been looking into whether the massive bursts of young stars might be caused by a theorized collapse into them of fragments of the disk.  But instead they detected and imaged the water snow line instead.

The image itself is an achievement, but what makes the finding especially intriguing is that the snow line was found at an entirely unexpected and enormous distance from the star — more than 42 astronomical units, or forty-two times the distance from our sun to Earth.

That would it was warm enough for H2O to remain a vapor roughly as far out as the orbit of the dwarf planet Pluto around the sun.  A more typical early star snow line is expected to be around 3 AU, an region between the orbits of Mars and Jupiter.

Brenda Matthews, an astronomer at the National Research Council of Canada not involved in the study, wrote in an accompanying column that the snow line finding challenges some traditional models of planet formation.… Read more

Big Bangs

Collisions between planets, planetesimals and other objects are common in the galaxies and essential for planet formation. Researchers are focusing on these collisions for clues into which exoplanets have greater or lesser potentials habitability. (NASA)

Collisions between planets, planetesimals and other objects are common in the galaxies and essential for planet formation. Researchers are focusing on these collisions for clues about which exoplanets have greater or lesser potential habitability. (NASA)

What can get the imagination into super-drive more quickly than the crashing of really huge objects?

Like when a Mars-sized planet did a head-on into the Earth and, the scientific consensus says, created the moon.  Or when a potentially dinosaur-exterminating asteroid heads towards Earth, or when what are now called  “near-Earth objects” seems to be on a collision course.  (There actually aren’t any now, as far as I can tell from reports.)

But for scientists, collisions across the galaxies are not so much a doomsday waiting to happen, but rather an essential commonplace and a significant and growing field of study.

The planet-forming centrality of collisions — those every-day crashes of objects from grain-sized to planet-sized within protoplanetary disks — has been understood for some time; that’s how rocky planets come to be.  In today’s era of exoplanets, however, they have taken on new importance: as an avenue into understanding other solar systems, to understanding the composition and atmospheres of exoplanets, and to get some insight into their potential habitability.

And collision models, it now seems likely, can play a not insignificant role in future decision-making about which planetary systems will get a long look from the high-demand, high-cost space telescopes that will launch and begin observing in the years ahead.

“We’re learning that these impacts have a lot of implications for habitability,” said Elisa Quintana, a NASA Ames Research Center and SETI Institute research scientist who has been modeling space collisions.  Her paper was published in 2016 in the Astrophysical Journal, and took the modeling into new realms.

“When you think of what we know about impacts in general, we know they can effect a planet’s spin rate and rotation and consequently its weather,  they can bring water and gases to a planet or they can destroy an atmosphere and let the volatiles escape.  They effect the relationship between the planet’s core and mantle, and they determine the compositions of the planets.  These are all factors in increasing or decreasing a planet’s potential for habitability.”

 

An artist rendering of a protoplanetart disk around a newly-formed star. Tiny grains of dust grow over millions of years into planets through collisions and the accretion of matter. (NASA)

An artist rendering of a protoplanetary disk around a newly-formed star. Tiny grains of dust grow over millions of years into planetesimals and planets through collisions and the accretion of matter.

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