Category: Planetary and Solar System Characteristics (page 1 of 6)

Why Does Our Solar System Have No Super-Earths, and Other Questions for Comparative Planetology

An artist’s impression of the exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth. Using the European Southern Observatory’s telescope at La Silla, Chile, and other telescopes around the world, an international team of astronomers discovered this super-Earth orbiting in the habitable zone around the faint star LHS 1140. This world is a little larger and much more massive than the Earth. (ESO)

Before the explosion in discovery of extrasolar planets, the field of comparative planetology was pretty limited  — confined to examining the differences between planets in our solar system and how they may have come to pass.

But over the past quarter century, comparative planetology and the demographics of planets came to mean something quite different.  With so many planets now identified in so many solar systems, the comparisons became not just between one planet and another but also between one solar system and another.

And the big questions for scientists became the likes of:  How and why are the planetary makeups of distant solar systems often so different from our own and from each other; what does the presence  or absence of large planets in a solar system do to the distribution of smaller planets;  how large can a rocky planet can get before it turns to a gas giant planet; and on a more specific subject, why do some solar systems have hot Jupiters close to the host star and others have cold Jupiters much further out like our own

Another especially compelling question involves our own solar system, though as something of an outlier rather than a prototype.

That question involves the absence in our solar system of anything in the category of a “super-Earth” — a rocky or gaseous extrasolar planet with a mass greater than Earth’s but substantially below those of our solar system’s planets next in mass,  Uranus and Neptune.

The term “super-Earth” refers only to the mass and radii of the planet, and so does not imply anything about the surface conditions or habitability. But in the world of comparative planetology “super-Earths” are very important because they are among the most common sized exoplanets found so far and some do seem to have planetary characteristics associated with habitability.

Yet they do not exist in our solar system.  Why is that?

Artist rendition of Earth in comparison to one of the many super-Earth planets. (NASA)

In a recent article in The Astrophysical Journal Letters,  planetary demographer Gijs D.… Read more

Many Planets Form in a Soup of Life-Friendly Organic Compounds

Artist’s depiction of a protoplanetary disk with young planets forming around a star. The right-side panel zooms in to show various organic molecules that are accreting onto a planet. (M.Weiss/Center for Astrophysics | Harvard & Smithsonian)

One of the more persuasive arguments in favor of the potential existence of life beyond Earth is that the well-known chemical building blocks of that life are found throughout the galaxy.  These chemical components aren’t all present in all examined solar systems and planets, but they are common and behave in ways familiar to scientists here.

And when it comes elements and compounds found on distant planets but not found here, there just aren’t many. That doesn’t mean they don’t exist — some unstable compounds in interstellar space, for instance — but rather that the cosmos holds many surprises but none have involved extraterrestrial elements or compounds near planets or stars.

This is in large part the result of how elements are formed in the universe.  Other than hydrogen and helium, all other elements are forged in the thermonuclear explosion of stars that have exhausted their supply of fuel.  These massive explosions (supernovae) then shoot the newly-formed elements out into space where they can and do collect in gas and dust clouds that will form other new stars.  They are spread throughout the disks that form around new stars and over time they become components of new planets in formation.

This galactic evolution includes the bonding together of carbon-based organic compounds — the building blocks of life as we know it.  They are an essential component to any theory of a planet’s habitability and,  while their presence in space and star nurseries has been known for some time,  they have remained a subject of great interest but limited detailed knowledge.

That is why an international team from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. set out to intensively study five disks forming around young stars to determine more precisely what organic compounds were present and available for objects developing into planets.

And the results are striking:  The abundance of organic material detected was 10 to 100 times more than expected.

“These planet-forming disks are teeming with organic molecules, some of which are implicated in the origins of life here on Earth,” said team leader Karin Öberg. “This is really exciting; the chemicals in each disk will ultimately affect the type of planets that form and determine whether or not the planets can host life.”… Read more

Findings Suggest that Red Dwarf Stars May Not Sterilize Many Exoplanets As Feared

An illustration of a red dwarf star with orbiting exoplanet. The question of whether this very common type of star can support habitable planets is a much debated one. (NASA)

Red dwarf suns are the most common in the universe, and many of the exoplanets officially discovered so far orbit this type of “cool” star.  Red dwarfs are much smaller and less powerful than the G type stars such as our own sun, and it is easier to detect exoplanets orbiting them because of their reduced size and energy.

As a result, a number of relatively nearby red dwarf stars — in the Trappist-1 system, Proxima Centauri and Barnard’s star, for instance — are avidly studied for their potential habitability.  The exoplanets of red dwarfs tend to orbit much closer than around other larger stars, but the suns have that lower radiative power and so some are considered habitable candidates.  And if they are indeed habitable, they could be for a very long time because red dwarfs live much longer than most other stars.

But there have been two (at least) problems with the habitable red dwarf exoplanet scenario.  The first is that many of the planets so close to their star are tidally locked, meaning that only one side ever faces the sun.  Some have argued a tidally locked planet can still be habitable, but it would not be easy.

More crucial, however, is that red dwarf stars are known for sending out many, many powerful solar flares, especially during their solar infancy and childhood.  These high radiation and particle flares could and would potentially kill any life emerging on a dwarf exoplanet, and the stellar flares could even sterilize the planets’ atmosphere for all time.  Although direct observations have not shown this deadly scenario to be inevitable or even present, the red dwarf flaring is well documented.  And so potentially the flares have seemed to rule out, or make improbable, life on an estimated 75 percent of the stars in our galaxy.

This is why there is interest in the astrobiology world about a new paper that addresses a particular kind of stellar flare that would hit red dwarf exoplanets.  Such studies of how the behavior of a star effects orbiting planets is one of the less well studied aspects of the exoplanet field, and so the paper is especially welcomed.

And the results suggests that the red dwarf flares would strike orbiting exoplanets from an angle rather than straight on, and therefore would land in a way that would theoretically minimize damage to potential atmospheres and life.… Read more

A Young Planet Found That May Well Be Making Moons

An image made by the Very Large Telescope in Chile shows a forming planet, the bright spot at right. The overpowering light of the host star is blocked out by a coronagraph inside the telescope. (ESO/A. Müller et al.)

Astronomers have many theories about how planets are formed within the gas, dust, pebbles and gradually rocks of the circumstellar disks that encircle a star after it has been born.  While the general outlines of this remarkable process are pretty well established, many questions large and small remain unanswered.

One is how and when exomoons are formed around these planets, with the assumption that the process that forms planets must also give birth moons.  But the potential moons have been far too small for the current generation of space and ground telescopes to identify.

Now astronomers have detected something almost as significant:  a circumplanetary disk surrounding a young planet that appears to be in the process of making moons.  The moon itself has not been detected, but a forming planet has been found with a ring of dust and gas that surrounds it.  And within that circumplanetary disk, astronomers infer, a moon is possibly being formed.

“Our work presents a clear detection of a disk in which satellites could be forming,” said Dr. Myriam Benisty, an astronomer at the University of Grenoble and the University of Chile.

“The new … observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with {the exoplanet} and we are able to constrain its size for the first time,” she said in a release.

While the first detection of the planet was made via the European Southern Observatory’s Very Large Telescope in Chile, the more granular observation of the forming planet and its moon-forming disk was made with the Atacama Large Millimeter/submillimeter Array (ALMA), also in Chile.

This ALMA image shows the young PDS 70 planetary system. The system features a star at its center and at least two planets orbiting it, PDS 70b (not visible in the image) and PDS 70c, surrounded by a circumplanetary disk (the dot to the right of the star). Image credit: ALMA / ESO / NAOJ / NRAO / Benisty et al.)

The finding, published in the Astrophysical Journal Letters, came via direct imaging — in effect through extremely high power photography rather through the indirect methods much more common in exoplanet astronomy.… Read more

And Then There Were Three: ESA Follows NASA in Selecting a Mission to Venus

Artist illustration of the EnVision orbiter at Venus (ESA/VR2Planets/DamiaBouic)

It was quite a week for Venus scientists. Just seven days after NASA announced the selection of two Venus missions, DAVINCI+ and VERITAS, the European Space Agency (ESA) revealed that a third Venus mission had been chosen for the agency’s medium-class mission category.

(See last week’s post here on Many Worlds about DAVINCI+ and VERITAS)

The new mission is named EnVision, and will be ESA’s second Venus mission following Venus Express (2005 – 2014), which investigated the Venusian climate. While EnVision is an orbiter like Venus Express and VERITAS, its focus is the planet’s geological circulation system that links the atmosphere, surface and interior.

In case you are starting to get your Venus missions in a tangle, the set can be broadly divided up as follows:

Venus Express (ESA: 2005 – 2014) and Akatsuki (JAXA: 2015 – current) are both Venus orbiters focussed on the planet’s climate, returning information about the rapidly rotating upper atmosphere and acidic cloud deck of Venus.

DAVINCI+ (NASA: est. 2029 launch) is an orbiter and descending probe that will dive through the Venusian atmosphere to return top-to-bottom data on the planet’s stifling gases.

VERITAS (NASA: est. 2028 launch) is an orbiter focussed on Venus’s surface and the deep interior. VERITAS will bring us global maps in three-dimensions at a resolution of 30m. This will knock the socks off our current images from NASA’s Magellan orbiter (1989 – 1994), which had a resolution of around 200m.

EnVision (ESA: early 2030s) is the mission focused on how these environments are linked together. Equipped with an instrument suite that covers the top of the atmosphere through to below the planet surface, EnVision will probe how the different regions influence one another to create the planet’s internal systems.

“EnVision has a holistic approach,” explained Jörn Helbert who is a member of the EnVision team. “The larger and more complex payload studies Venus from the top of the atmosphere all the way to the subsurface, with a focus on understanding how the coupled system on Venus works.”

Artist illustration of the EnVision spacecraft, reflecting the goal of understanding why Venus and Earth are so different (NASA / JAXA / ISAS / DARTS / Damia Bouic / VR2Planets).

The coupled system is at the heart of how habitability can develop on rocky planets. A major player in the Earth’s environment is the ability to cycle carbon between the atmosphere, surface and planet mantle.… Read more

Return to Hell: NASA Selects Two Missions to Venus to Explore the Pathway to Habitability

Artists’ renderings show the VERITAS spacecraft (left) and DAVINCI+ probe (right) as they arrive at Venus (Lockheed Martin).

For NASA scientists, Venus missions must feel like buses. You wait thirty years for one, and then two come along at once.

Last week, NASA selected two Venus missions for the space agency’s Discovery Program; solar system exploration missions that can tuck under a lower cost cap than candidates for NASA’s New Horizons or Flagship categories. The first of these is DAVINCI+, which is an orbiter equipped with a descending probe that will take a big whiff of Venus’s stifling atmosphere. The second is the VERITAS orbiter that plans to peer through the clouds to scrutinise the Venusian surface.

While Europe and Japan have both visited Venus more recently than NASA (in fact, the Japanese orbiter is still there), there is little doubt that our inner neighbor is dramatically under-explored compared to Mars. But why the past neglect, and why go twice now?

The answer to the first question is perhaps the easiest.

Venus is hell.

The planet is wrapped in a thick atmosphere consisting of carbon dioxide and clouds of sulfuric acid that beat down on the Venusian surface with pressures nearly one hundred times higher than on Earth and create temperatures sufficient to melt lead.

These conditions have made it difficult to follow the usual pattern of planetary exploration from fly-bys and orbiters to landers and rovers. The Venusian surface is so inhospitable that a rover like NASA’s Mars Perseverance would become rover goop. Although recent engineering combined with high-temperature electronics means that the surface is no longer impossible, it does greatly add to the challenge (and therefore cost) of a lander mission.

Professor Stephen Kane, University of California, Riverside.

Hell-scape conditions have also resulted in Venus being overlooked for any astrobiological studies compared to (the still rather nasty but at least you can stand a rover on the surface) Mars. This makes the urgency to explore Venus now particularly surprising. The missions are a quest to understand habitability. The bottom line is that the hell world of Venus is essential to understanding how a planet becomes habitable and to discovering other habitable worlds outside our solar system.

“Imagine you live in a small town full of life,” explains Professor Stephen Kane from the DAVINCI+ team. “The nearest town is the same size and seems it was once identical. But now, it’s burned to the ground with no sign of life.… Read more

The Faint Young Sun Paradox and Mars

This NASA image of Mars at sunset taken by the Spirit  rover, evokes the conditions on early Mars when the planet received only 70 percent of the of the solar energy that it does now.  (NASA/JPL/Texas A&M/Cornell)

When our sun was young, it was significantly less luminous and sent out significantly less warming energy than it does now.  Scientists estimate that 4 million years ago, when the sun and our solar system were 500 million years old, the energy that the sun produced and dispersed was about 75 percent of what it is today.

The paradox arises because during this time of the faint young sun Earth had liquid water on its surface and — as has been conclusively proven in recent years — so did Mars, which is 61 million miles further into space.  However difficult it is to explain the faint young sun problem as it relates to early Earth, it is far more difficult to explain for far more frigid Mars.

Yet many have tried.  And because the data is both limited and innately puzzling, the subject has been vigorously debated from a variety of different perspectives.  In 2018, the journal Nature Geoscience published an editorial on the state of that dispute titled “Mars at War.”

There are numerous point of (strenuous) disagreement, with the main ones involving whether early Mars was significantly more wet and warm than previously inferred, or whether it was essentially cold and arid with only brief interludes of warming.  The differences in interpretation also require different models for how the warming occurred.

Was there a greenhouse warming  effect produced by heat-retaining molecules in the atmosphere?  Was long-term volcanic activity the cause? Or perhaps meteor strikes?  Or heat from the interior of the planet?

All of these explanations are plausible and all may have played a role.  But that begs the question that has so energized Mars scientists since Mars orbiters and the Curiosity rover conclusively proved that surface water created early rivers and valley networks, lakes and perhaps an ocean.  To solve the “faint young sun” paradox as it played out on Mars,  a climate driver (or drivers) that produces significant amounts of heat is required.

Could the necessary warming be the result of radioactive elements in the Martian crust and mantle that decay and give off impressive amounts of heat when they do?

A team led by Lujendra Ojha, an assistant professor at Rutgers University, proposes in Science Advances that may well be the answer, or at least part of the answer.… Read more

Cores, Planets and The Mission to Psyche

The asteroid Psyche will be the first metal-rich celestial body to be visited by a spacecraft.  The NASA mission launches in 2022 and is expected to arrive at the asteroid in late 2026.  A central question to be answered is whether Psyche is the exposed  core of a protoplanet that was stripped of its rocky mantle. (NASA)

Deep inside the rocky planets of our solar system, as well as some solar system moons,  is an iron-based core.

Some, such as Earth’s core,  have an inner solid phase and outer molten phase, but the solar system cores studied so far are of significantly varied sizes and contain a pretty wide variety of elements alongside the iron.  Mercury, for instance, is 85 percent core by volume and made up largely of iron, while our moon’s core is thought to be 20 percent of its volume and is mostly iron with some sulfur and nickel.

Iron cores like our own play a central role in creating a magnetic field around the planet, which in turn holds in the atmosphere and may well be essential to make a planet habitable.  They are also key to understanding how planets form after a star is forged and remaining dense gases and dust are kicked out to form a protoplanetary disk, where planets are assembled.

So cores are central to planetary science, and yet they are obviously hard to study.  The Earth’s core starts about 1,800 miles below the surface, and the cores of gas giants such as Jupiter are much further inward, and even their elemental makeups are not fully understood.

All this helps explains why the upcoming NASA mission to the asteroid Psyche is being eagerly anticipated, especially by scientists who focus on planetary formation.

Scheduled to launch in 2022, the spacecraft will travel to the main asteroid belt between Mars and Jupiter and home in on what has been described as an unusual “metal body,”  which is also one of the largest asteroids orbiting the sun.

While some uncertainty remains,  it appears that Psyche is the  exposed nickel-iron core of a long-ago emerging rocky protoplanet, with the rest of the planet stripped away by collisions billions of years ago.

An artist’s impression of solar system formation, and the formation of a protoplanetary disk filled with gases and dust that over time clump together and smash into each other to form larger and larger bodies. (Gemini Observatory/AURA artwork by Lynette Cook )

That makes Psyche a most interesting place to visit.… Read more

How Many Habitable Zone Planets Can Orbit a Host Star?

This representation of the Trappist-1 system shows which planets could potentially have temperature conditions which would allow for the presence of liquid water, seen generally as essential for life.  The inner three planets are likely too hot, and the outer planet is probably too cold, but the middle three planets might be just right. (NASA / JPL-Caltech)

Our solar system has but one planet orbiting in what is commonly known as the habitable zone — at a distance from the host star where water could be liquid at times rather than always ice or gas.  That planet, of course, is Earth.

But from a theoretical, dynamical perspective, does this always have to be the case?  The answer to that question is no because a number of stars are known to have more than one habitable zone planet.

Now a team from the University of California, Riverside has produced a study that concludes as many as seven Earth-sized, habitable zone planets could orbit a single star — if there were no large Jupiter-sized planets in the system and if the star was of a particular type.

The article, published in the Astronomical Journal, concluded that seven habitable zone planets was the maximum for a star, but a sun such as ours could potentially support six planets with sometimes liquid water — a condition considered essential for life.

Study leader Stephen Kane, an astrobiologist who focuses on potentially habitable exoplanets, said he had been studying the nearby solar system Trappist-1, which has three Earth-like planets in its habitable zone and seven planets all together.

“This made me wonder about the maximum number of habitable planets it’s possible for a star to have, and why our star only has one,” Kane said.

With the discovery of an eighth planet, the Kepler-90 system is the first to tie with our solar system in number of planets. Artist’s concept. Credit: NASA/Ames Research Center/Wendy Stenzel

His conclusion:

“Even though (our solar system) only has one planet in the habitable zone, it’s not necessarily the typical situation. A far more typical scenario may be to have many planets in the habitable zone, depending on the presence of a giant planet.”

More later about the destabilizing effects of giant planet, but the Kane (and others) say that looking for solar systems without Jupiter-size planets has become increasingly important because of this effect on other terrestrial planets.

To determine how many habitable zone planets might be possible in a solar system, his team created a model system in which they simulated planets of various sizes orbiting their stars.

Read more

Sample Return in the Time of Coronavirus

 

Sample return from Mars. Artist rendering of a Mars sample return mission. The mission would use robotic systems and a Mars ascent rocket to collect and send samples of Martian rocks, soils and atmosphere to Earth for detailed chemical and physical analysis.  No rocket has ever taken off from Mars and this NASA and European Space Agency (ESA) project is in early planning stages. Still, blue-ribbon science panels have recommended efforts to begin preparing the public for an eventual Mars sample return. ( Wickman Spacecraft & Propulsion)

For space scientists of all stripes, few goals are as crucial as bringing pieces of Mars, of asteroids, of other planets and moons back to Earth for the kind of intensive study only possible here.  Space missions can, and have, told us many truths about the solar system,  but having a piece of Mars or Europa or an asteroid to study in a lab on Earth is considered the gold standard for learning about the actual composition of other bodies, their histories and whether they could — or once did — harbor life.

In keeping with this ambition, the last National Research Council Decadal Survey listed a Mars “sample return” as the top science priority for large Flagship missions.  And the Perseverance rover that NASA is scheduled to send to Mars next month will — among many other tasks — identify compelling rock samples, collect and cache them so a subsequent mission can pick them up and fly them to Earth.

Two asteroid sample return missions are also in progress, the NASA’s OSIRIS-REx mission to Bennu and the Japan Aerospace Exploration Agency (JAXA’s)  Hayabusa2 mission to the Ryugu.  Both spacecraft are at or have already left their intended targets now and are expected to return with rock samples later this decade, with Hayabusa2 scheduled to complete its round trip later this year.

An illustration of the coronavirus. (Centers of Disease Control)

So sample return is in our future.  And in the case of Mars the samples will not with 100 percent certainty be lifeless — a major difference from the samples brought back from the moon during the Apollo missions and the samples coming from asteroids.

This possibility of a spacecraft bringing back something biological — as in the 1969 book “The Andromeda Strain” — has always been viewed as a very low probability but high risk hazard, and much thinking has already gone into how to bring samples back safely.… Read more

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