Category: Planetary and Solar System Characteristics (page 1 of 4)

The Interiors of Exoplanets May Well Hold the Key to Their Habitability

Scientists have had a working — and evolving — understanding of the interior of the Earth for only a century or so.  But determining whether a distant planet is truly habitable may require an understanding of its inner dynamics — which will for sure be a challenge to achieve. (Harvard-Smithsonian Center for Astrophysics)

The quest to find habitable — and perhaps inhabited — planets and moons beyond Earth focuses largely on their location in a solar system and the nature of its host star,  the eccentricity of its orbit, its size and rockiness, and the chemical composition of its atmosphere, assuming that it has one.

Astronomy, astrophysics, cosmochemistry and many other disciplines have made significant progress in characterizing at least some of the billions of exoplanets out there, although measuring the chemical makeup of atmospheres remains a immature field.

But what if these basic characteristics aren’t sufficient to answer necessary questions about whether a planet is habitable?  What if more information — and even more difficult to collect information — is needed?

That’s the position of many planetary scientists who argue that the dynamics of a planet’s interior are essential to understand its habitability.

With our existing capabilities, observing an exoplanet’s atmospheric composition will clearly be the first way to search for signatures of life elsewhere.   But four scientists at the Carnegie Institution of Science — Anat Shahar, Peter Driscoll, Alycia Weinberger, and George Cody — argued in a recent perspective article in Science that a true picture of planetary habitability must consider how a planet’s atmosphere is linked to and shaped by what’s happening in its interior.

They argue that on Earth, for instance, plate tectonics are crucial for maintaining a surface climate where life can fill every niche. And without the cycling of material between the planet’s surface and interior, the convection that drives the Earth’s magnetic field would not be possible and without a magnetic field, we would be bombarded by cosmic radiation.

What makes a planet potentially habitable and what are signs that it is not. This graphic from the Carnegie paper illustrates the differences (Shahar et al.)

 

“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” said lead author Shahar, who is trained in geological sciences.

“This can be used to guide the search for exoplanets and star systems where life could thrive, signatures of which could be detected by telescopes.”

It all starts with the formation process.

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Exoplanets With Complex Life May Be Very Rare, Even in Their “Habitable Zones”

The term “habitable zone” can be a misleading one, since it describes a limited number of conditions on a planet to make it hospitable to life. (NASA)

 

For years now, finding planets in the habitable zones of their host stars has been a global astrophysical quest and something of a holy grail.  That distance from a star where temperatures could allow H20 to remain liquid some of the time has been deemed the “Goldilocks” zone where life could potentially emerge and survive.

The term is valuable for sure, but many in the field worry that it can be as misleading or confusing as it is helpful.

Because while the habitable zone is a function of the physics and architecture of a solar system, so much more is needed to make a planet actually potentially habitable.  Does it have an atmosphere?  Does it have a magnetic field. Does it orbit on an elliptical path that takes it too far (and too close) to the sun?  Was it sterilized during the birth of the host star and orbiting planets?  What kind of star does it orbit, and how old and luminous is that star?

And then there’s the sometimes confused understanding that many habitable zones may well support complex, even technologically-advanced life.  They are, after all, habitable.

But as a new paper in the Astrophysical Journal makes clear, the likelihood of a habitable zone planet being able to support complex life — anything beyond a microbe — is significantly limited by the amount of toxic chemicals such as carbon monoxide and excesses of carbon dioxide.

Eddie Schwieterman, a NASA postdoc at the University of California, Riverside and lead author of the article, told me that the odds for complex life on most exoplanets in their habitable zones weren’t great.

“A rough estimate is between 10-20% of habitable zone planets are truly suitable for analogs to humans and animals.” he said. “Of course, being located in this part of the habitable zone isn’t enough by itself – you still need the build-up of oxygen via the evolution of oxygenic photosynthesis and certain planetary biogeochemical cycles.”

 

A rendering of the exoplanet Kepler 442 b, compared in size to  Earth.  Kepler 442 b was detected using the Kepler Space Telescope and is 0ne of a handful of planets found so far deemed to be most likely to be habitable. But it’s 1200 light-years away, so learning its secrets will be challenging.

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A New and Revelatory Window Into Evolution on Earth

A Leanchoilia fossil from at the Qingjiang site in China. A very early arthropod  found with sharply defined appendages is an arthropod and  one of the prime examples of early Cambrian life (D Fu et al., Science 363:1338 (2019)

Virtually every definition of the word “life” includes the capability to undergo Darwinian evolution as a necessary characteristic.  This is true of life on Earth and of thinking about what would constitute life beyond Earth.  If it can’t change, the thinking goes, then it cannot be truly alive.

In addition, evolutionary selection and change occurs within the context of broad planetary systems — the chemical makeup of the atmosphere, the climactic conditions, the geochemistry and more.  If an environment is changing, then the lifeforms that can best adapt to the new conditions are the ones that will survive and prosper.

So evolution is very much part of the landscape that Many Worlds explores — the search for life beyond Earth and effort to understand how life emerged on Earth.  Evolution happens in the context of broad conditions on Earth (and perhaps elsewhere), and finding potential life elsewhere involves understanding the conditions on distant planets and determining if they are compatible with life.

This all came to mind as I read about the discovery of a remarkable collection of fossils alongside a river in China, fossils of soft-bodied creatures that lived a half billion years ago in the later phase of what is termed the the Cambrian explosion.  They are of being compared already with the iconic “Burgess Shale” fossil find in Canada of decades ago, and may well shed equally revelatory light on a crucial time in the evolution of life on Earth.

Artist rendering of Qingjiang life showing characteristics of different early Cambrian taxonomical groups.  More than 50 percent had never been identified before. (ZH Yao and DJ Fu)

The new discovery is reported in the journal Science in a paper authored by Dongjing Fu and a team largely from the Northwest University in Xi’an.  The paper reports on a zoo of Cambrian-era creatures, with more than half of them never identified before in the rock record.

The animals are soft-bodied — making it all the more remarkable that they were preserved — and some bear little resemblance to anything that followed.   Like the Burgess Shale fossils, the Qingjiang discovery is of an entire ecosystem that largely disappeared as more fit (and predatory) animals emerged.… 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

Does Proxima Centauri Create an Environment Too Horrifying for Life?

Artist’s impression of the exoplanet Proxima Centauri b. (ESO/M. Kornmesser)

 

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.

 

Comparison of the orbit of Proxima Centauri  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. As a result it lies well within the habitable zone. (ESO/M. Kornmesser/G. Coleman.)

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.… Read more

Barnard’s Star, The "Great White Whale" of Planet Hunting, Has Surrendered Its Secret

Barnard’s Star is the closest single star to our sun, and the most fast moving. It has long been attractive to planet hunters because it is so close and so bright, especially in the infared section of the spectrum. But until now, the exoplanets of this “great white whale” have avoided detection.

 

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.

 

An artist’s rendering of the Barnard’s star planet at sunset. (Martin Kornmesser/ESO)

 

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.… Read more

What Would Happen If Mars And Venus Swapped Places?

Venus, Earth and Mars (ESA).

 

What would happen if you switched the orbits of Mars and Venus? Would our solar system have more habitable worlds?

It was a question raised at the “Comparative Climatology of Terrestrial Planets III”; a meeting held in Houston at the end of August. It brought together scientists from disciplines that included astronomers, climate science, geophysics and biology to build a picture of what affects the environment on rocky worlds in our solar system and far beyond.

The question regarding Venus and Mars was proposed as a gedankenexperiment or “thought experiment”; a favorite of Albert Einstein to conceptually understand a topic. Dropping such a problem before the interdisciplinary group in Houston was meat before lions: the elements of this question were about to be ripped apart.

The Earth’s orbit is sandwiched between that of Venus and Mars, with Venus orbiting closer to the sun and Mars orbiting further out. While both our neighbors are rocky worlds, neither are top picks for holiday destinations.

Mars has a mass of just one-tenth that of Earth, with a thin atmosphere that is being stripped by the solar wind; a stream of high energy particles that flows from the sun. Without a significant blanket of gases to trap heat, temperatures on the Martian surface average at -80°F (-60°C). Notably, Mars orbits within the boundaries of the classical habitable zone (where an Earth-like planet could maintain surface water)  but the tiny planet is not able to regulate its temperature as well as the Earth might in the same location.

 

The classical habitable zone around our sun marks where an Earth-like planet could support liquid water on the surface (Cornell University).

 

Unlike Mars, Venus has nearly the same mass as the Earth. However, the planet is suffocated by a thick atmosphere consisting principally of carbon dioxide. The heat-trapping abilities of these gases soar surface temperatures to above a lead-melting 860°F (460°C).

But what if we could switch the orbits of these planets to put Mars on a warmer path and Venus on a cooler one? Would we find that we were no longer the only habitable world in the solar system?

“Modern Mars at Venus’s orbit would be fairly toasty by Earth standards,” suggests Chris Colose, a climate scientist based at the NASA Goddard Institute for Space Studies and who proposed the topic for discussion.

Dragging the current Mars into Venus’s orbit would increase the amount of sunlight hitting the red planet.… Read more

Large Reservoir of Liquid Water Found Deep Below the Surface of Mars

Artist impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed on a radar cross section of the southern polar layered deposits. The leftmost white line is the radar echo from the Martian surface, while the light blue spots are highlighted radar echoes along the bottom of the ice.  Those highlighted areas measure very high reflectivity, interpreted as being caused by the presence of water. (ESA, INAF. Graphic rendering by Davide Coero Borga )

Far beneath the frigid surface of the South Pole of Mars is probably the last place where you might expect the first large body of Martian liquid water would be found.  It’s -170 F on the surface, there are no known geothermal sources that could warm the subterranean ice to make a meltwater lake, and the liquid water is calculated to be more than a mile below the surface.

Yet signs of that liquid water are what a team of Italian scientists detected — a finding that they say strongly suggests that there are other underground lakes and streams below the surface of Mars.  In a Science journal article released today, the scientists described the subterranean lake they found as being about 20 kilometers in diameter.

The detection adds significantly to the long-studied and long-debated question of how much surface water was once on Mars, a subject that has major implications for the question of whether life ever existed on the planet.

Finding the subterranean lake points to not only a wetter early Mars, said co-author Enrico Flamini of the Italian space agency, but also to a Mars that had a water cycle that collected and delivered the liquid water.  That would mean the presence of clouds, rain, evaporation, rivers, lakes and water to seep through surface cracks and pool underground.

Scientists have found many fossil waterways on Mars, minerals that can only be formed in the presence of water, and what might be the site of an ancient ocean.

But in terms of liquid water now on the planet, the record is thin.  Drops of water collected on the leg of NASA’s Phoenix Lander after it touched down in 2008, and what some have described as briny water appears to be flowing down some steep slopes in summertime.  Called recurrent slope lineae or RSLs, they appear at numerous locations when the temperatures rise and disappear when they drop.

This lake is different, however, and its detection is a major step forward in understanding the history of Mars.… Read more

Asteroid Remains Around Dead Stars Reveal the Likely Fate of Our Solar System

Artist concept of an asteroid breaking up. (NASA/JPL-Caltech)

June 30th has been designated “Asteroid Day” to promote awareness of these small members of our solar system. But while asteroids are often discussed in the context of the risk they might pose to the Earth, their chewed up remains around other stars may also reveal the fate of our solar system.

It is 6.5 billion years into our future. The sun has fused hydrogen into a core of heavier helium. Compressed by its own gravity, the helium core releases heat and the sun begins to swell. It is the end of our star’s life, but what will happen to the solar system?

While very massive stars end their element-fusing days in a colossal explosion known as a supernovae, the majority of stars in our galaxy will take a less dramatic exit.

Our sun’s helium core will fuse to form carbon but there is not enough mass to achieve the crushing compression needed for the creation of heavier elements. Instead, the outer layers of the dying star will be blown away to leave a dense remnant with half the mass of our current sun, but squeezed down to the size of the Earth. This is a white dwarf; the most common of all stellar ends.

The life cycle of our sun

The white dwarf rapidly cools to become a dim twinkle in the sky. Within a few million years, our white dwarf will be less luminous that the sun today. Within 100 million years, it will be dimmer by a factor of 100. But examination of white dwarfs in our galaxy reveals this gentle dimming of the lights is not as peaceful as first appears.

The remnants of stars too light to fuse carbon, white dwarfs have atmospheres that should be thin shells of residue hydrogen and helium. Instead, observations have detected 20 different heavy elements in this envelope of gases that include rock-forming elements such as silicon and iron and volatiles such as carbon and nitrogen.

Infrared observations of over forty white dwarfs have additionally revealed compact dusty discs circling the dead stars. Sitting within the radius of a regular star, these could not have formed before the star shrank into a white dwarf. These must be the remains of what occurred as the star morphed from a regular fusion burner into a white dwarf.

This grizzly tale begins with the star’s expansion.… Read more

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

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

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

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

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

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

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

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

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


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

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

In 2014, a journal paper authored by Fabienne Bastien from Vanderbilt University suggested that nearly half of the brightest stars observed by the Kepler Space Telescope are not regular stars like our sun, but actually are distant and much larger sub-giant stars.… Read more

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