Most stars including the Sun generate magnetic activity that drives a fast-moving, ionized wind and also produces X-ray and ultraviolet emission (often referred to as XUV radiation). XUV radiation from a star can be absorbed in the upper atmosphere of an orbiting planet, where it is capable of heating the gas enough for it to escape from the planet’s atmosphere. M-dwarf stars, the most common type of star by far, are smaller and cooler than the Sun, and they can have very active magnetic fields. Their cool surface temperatures result in their habitable zones (HZ) being close to the star (the HZ is the range of distances within which an orbiting planet’s surface water can remain liquid). Any rocky exoplanets that orbit an M-dwarf in its HZ, because they are close to the star, are especially vulnerable to the effects of photoevaporation which can result in partial or even total removal of the atmosphere. Some theorists argue that planets with substantial hydrogen or helium envelopes might actually become more habitable if photoevaporation removes enough of the gas blanket.
The effects of XUV radiation on exoplanet atmospheres have been studied for almost twenty years, but the effects of the stellar wind on exoplanet atmospheres are only poorly understood. CfA astronomers Laura Harbach, Sofia Moschou, Jeremy Drake, Julian Alvarado-Gomez, and Federico Frascetti and their colleagues have completed simulations modeling the effects of a stellar wind on an exoplanet with a hydrogen-rich atmosphere orbiting close to an M-dwarf star. As an example, they use the exoplanet configuration in TRAPPIST-1, a cool M-dwarf star with a system of seven planets, six of which are close enough to the star to be in its HZ.
The simulations show that, depending on the details, the stellar wind can generate outflows from a planet’s atmosphere. The team finds that both the star’s and the planet’s magnetic fields play significant roles in defining many of the details of the outflow, which could be observed and studied via atomic hydrogen lines in the ultraviolet. The complex simulation results indicate that planets around M-dwarf host stars are likely to display a diverse range of atmospheric properties, and some of the physical conditions can vary over short timescales making observational interpretations of sequential exoplanet transits more complex. The simulation results highlight the need to use 3-D simulations that include magnetic effects in order to interpret observational results for planets around M-dwarf stars.
Reference: “Stellar Winds Drive Strong Variations in Exoplanet Evaporative Outflow Patterns and Transit Absorption Signatures,” Laura M. Harbach, Sofia P. Moschou, Cecilia Garraffo, Jeremy J. Drake, Julián D. Alvarado-Gómez, Ofer Cohen, and Federico Fraschetti, The Astrophysical Journal 913, 130, 2021.
When Pluto passed in front of a star on the night of August 15, 2018, a Southwest Research Institute-led team of astronomers had deployed telescopes at numerous sites in the U.S. and Mexico to observe Pluto’s atmosphere as it was briefly backlit by the well-placed star. Scientists used this occultation event to measure the overall abundance of Pluto’s tenuous atmosphere and found compelling evidence that it is beginning to disappear, refreezing back onto its surface as it moves farther away from the Sun.
The occultation took about two minutes, during which time the star faded from view as Pluto’s atmosphere and solid body passed in front of it. The rate at which the star disappeared and reappeared determined the density profile of Pluto’s atmosphere.
“Scientists have used occultations to monitor changes in Pluto’s atmosphere since 1988,” said Dr. Eliot Young, a senior program manager in SwRI’s Space Science and Engineering Division. “The New Horizons mission obtained an excellent density profile from its 2015 flyby, consistent with Pluto’s bulk atmosphere doubling every decade, but our 2018 observations do not show that trend continuing from 2015.”
Several telescopes deployed near the middle of the shadow’s path observed a phenomenon called a “central flash,” caused by Pluto’s atmosphere refracting light into a region at the very center of the shadow. When measuring an occultation around an object with an atmosphere, the light dims as it passes through the atmosphere and then gradually returns. This produces a moderate slope on either end of the U-shaped light curve. In 2018, refraction by Pluto’s atmosphere created a central flash near the center of its shadow, turning it into a W-shaped curve.
“The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation,” Young said. “The central flash gives us very accurate knowledge of Pluto’s shadow path on the Earth.”
Like Earth, Pluto’s atmosphere is predominantly nitrogen. Unlike Earth, Pluto’s atmosphere is supported by the vapor pressure of its surface ices, which means that small changes in surface ice temperatures would result in large changes in the bulk density of its atmosphere. Pluto takes 248 Earth years to complete one full orbit around the Sun, and its distance varies from its closest point, about 30 astronomical units from the Sun (1 AU is the distance from the Earth to the Sun), to 50 AU from the Sun.
For the past quarter century, Pluto has been receiving less and less sunlight as it moves farther away from the Sun, but, until 2018, its surface pressure and atmospheric density continued to increase. Scientists attributed this to a phenomenon known as thermal inertia.
“An analogy to this is the way the Sun heats up sand on a beach,” said SwRI Staff Scientist Dr. Leslie Young, who specializes in modeling the interaction between the surfaces and atmospheres of icy bodies in the outer solar system. “Sunlight is most intense at high noon, but the sand then continues soaking up the heat over course of the afternoon, so it is hottest in late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by stored heat under the surface. The new data suggests they are starting to cool.”
The largest known nitrogen reservoir is Sputnik Planitia, a bright glacier that makes up the western lobe of the heart-shaped Tombaugh Regio. The data will help atmospheric modelers improve their understanding of Pluto’s subsurface layers, particularly regarding compositions that are compatible with the observed limits on heat transfer.
Eliot Young will discuss these results at a press conference Monday, October 4, at the 53rd American Astronomical Society Division for Planetary Sciences Annual Meeting.
Featured image: When Pluto passed in front of a star on the night of August 15, 2018, a SwRI-led team of astronomers measured the abundance of Pluto’s atmosphere, shown here in New Horizons 2015 flyby data, as it was briefly backlit by the well-placed star. These data indicate that the surface pressure on Pluto is decreasing and that its nitrogen atmosphere is condensing, forming ice on its surface as the object moves away from the Sun.Courtesy of NASA/JHU-APL/SwRI
The newest known example of a rare type of object in the Solar System – a comet hidden among the main-belt asteroids – has been found and studied, according to a new paper by Planetary Science Institute Senior Scientist Henry Hsieh.
Discovered to be active on July 7, 2021, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey, asteroid (248370) 2005 QN137 is just the eighth main-belt asteroid, out of more than half a million known main-belt asteroids, confirmed to not only be active, but to have been active on more than one occasion. “This behavior strongly indicates that its activity is due to the sublimation of icy material,” said Hsieh, lead author of the paper “Physical Characterization of Main-Belt Comet (248370) 2005 QN173” that he presented at a press conference today at the 53rd annual meeting of the American Astronomical Society’s Division for Planetary Sciences. “As such, it is considered a so-called main-belt comet, and is one of just about 20 objects that have currently been confirmed or are suspected to be main-belt comets, including some that have only been observed to be active once so far.
“248370 can be thought of as both an asteroid and a comet, or more specifically, a main-belt asteroid that has just recently been recognized to also be a comet. It fits the physical definitions of a comet, in that it is likely icy and is ejecting dust into space, even though it also has the orbit of an asteroid,” Hsieh said. “This duality and blurring of the boundary between what were previously thought to be two completely separate types of objects – asteroids and comets – is a key part of what makes these objects so interesting.”
Hsieh found that size of the nucleus, the solid object at the “head” of the comet that is surrounded by a dust cloud, is 3.2 kilometers (2 miles) across, the length of the tail in July 2021 was more than 720,000 kilometers (450,000 miles) long, or three times the distance from the Earth to the Moon, and the tail at that time was just 1,400 kilometers (900 miles) wide. These dimensions mean that if the length of the tail was scaled to the length of a football field, the tail would be just 7 inches wide and the nucleus would be half a millimeter across.
“This extremely narrow tail tells us that dust particles are barely floating off of the nucleus at extremely slow speeds and that the flow of gas escaping from the comet that normally lifts dust off into space from a comet is extremely weak. Such slow speeds would normally make it difficult for dust to escape from the gravity of the nucleus itself, so this suggests that something else might be helping the dust to escape. For example, the nucleus might be spinning fast enough that it’s helping to fling dust off into space that has been partially lifted by escaping gas. Further observations will be needed to confirm the rotation speed of the nucleus though,” Hsieh said.
“Cometary activity is generally thought to be caused by sublimation – the transformation from ice to gas – of icy material in a Solar System object, which means that most comets are found to come from the cold outer Solar System, beyond the orbit of Neptune, and spend most of their time there, with their highly elongated orbits only bringing them close to the Sun and the Earth for short periods at a time,” Hsieh said. “During those times when they are close enough to the Sun, they heat up and release gas and dust as a result of ice sublimation, producing the fuzzy appearance and often spectacular tails associated with comets.”
By contrast, main-belt asteroids, which orbit between the orbits of Mars and Jupiter, are thought to have been in the warm inner Solar System where we see them today (inside the orbit of Jupiter) for the last 4.6 billion years. Any ice in these objects was expected to be long gone from being so close to the Sun for so long, meaning that cometary activity was not expected to be possible from any of these objects. However, a few rare objects that challenge this expectation called main-belt comets, first discovered as a new class of comets by Hsieh and David Jewitt in 2006, have been found over the last several years. These objects are interesting because a substantial part of Earth’s water is thought to have been delivered via impacts by asteroids from the main asteroid belt when the Earth was being formed. Given that the activity observed for these objects means they are likely to still contain ice, they offer a potential way to test that hypothesis and learn more about the origin of life on Earth by learning more about the abundance, distribution, and physical properties of icy objects in the inner Solar System.
Hsieh’s work was funded by a grant to PSI from NASA’s Solar System Observations program (Grant 80NSSC19K0869). This work also made use of observations carried out under the Las Cumbres Observatory Outbursting Objects Key Project (LOOK) and the Faulkes Telescope Project’s Comet Chasers program, and from Lowell Observatory’s Lowell Discovery Telescope and Palomar Observatory’s Hale Telescope.
Featured image:Composite image of (248370) 2005 QN173 taken with Palomar Observatory’s Hale Telescope in California on July 12, 2021. The head, or nucleus, of the comet is in the upper left corner, with the tail stretching down and to the right, getting progressively fainter farther from the nucleus. Stars in the field of view appear as short dotted lines due to the apparent motion of Solar System objects against background stars and the process of adding together multiple images to increase the visibility of the tail. Credit: Henry H. Hsieh (PSI), Jana Pittichová (NASA/JPL-Caltech).
A research team led by researchers at the Astrobiology Center and the University of Tokyo has observed low-temperature “ultra-short-period planets” with an orbital period of less than one day by observing with the Subaru Telescope’s near-infrared spectrometer IRD. It was discovered around a star and revealed that its internal composition consists mainly of iron and rock. The planets (TOI-1634b and TOI-1685b) found around the two low-temperature stars are both equivalent to Super-Earth (Note 1), which is about 1.5-2 times the size of the Earth, and in particular, TOI-1634b It is one of the terrestrial planets with the largest radius (1.8 Earth radius) and mass (10 Earth mass) among the ultra-short-period planets found so far. Planets of this size are on the border between rock and gas giants, especially around low-temperature stars, so how many planets have a “one year” less than the length of a day on Earth? It can be said that the most valuable celestial body was discovered in investigating whether it was formed.
Observations have revealed that about 1% of extrasolar planets (exoplanets) are planets with an orbital period of less than one day (ultra-short-period planets). It is thought that ultra-short-period planets formed in outer orbits may have moved to inner orbits due to interactions with other planets, etc., in order to understand the formation of various planets. , A rare and important celestial body.
Most of the ultra-short-period planets observed so far are small planets with a radius of 1.5 times or less that of the Earth, and it is known that their internal composition is similar to that of the Earth, which is mainly composed of iron and rocks. However, most of the ultra-short-period planets scrutinized in this way are known only around sun-like stars (solar stars), and there are only a few observations around low-temperature, small-mass stars. is. Low-temperature stars are known to have multiple small planets, so ultra-short-period planets may also be present. A closer look at the frequency and characteristics of ultrashort-period planets around low-temperature stars is expected to give a general understanding of the origins of ultra-short-period planets.
The research team focused on two low-temperature stars TOI-1634 and TOI-1685 with transit planet candidates (Note 2) detected by NASA’s Transiting Exoplanet Exploration Satellite “TESS”. .. The mass of these stars is only about half that of the Sun. Independent analysis of TESS data and follow-up observation of transit using the MuSCAT series of multicolor simultaneous imaging cameras (Note 3), followed by spectroscopic observation using the Subaru Telescope’s infrared spectroscope IRD (InfraRed Doppler). Did. The IRD is a spectroscope that accurately measures the radial velocity of a star (radial velocity), and is a unique observation device optimized for observing low-temperature stars that appear brighter with infrared rays than visible light.
As a result of detailed analysis of the radial velocities observed by IRD, the actual ultra-short-period planets around TOI-1634 and TOI-1685 were 0.989 days (TOI-1634b) and 0.669 days (TOI-1685b), respectively. It was confirmed that it revolves in the cycle of. Furthermore, from the amplitude of the change in radial velocity, it became clear that TOI-1634b and TOI-1685b have about 10 times and about 3.4 times the mass of the earth, respectively (Note 4). When the composition of the planet was theoretically estimated based on this planet mass and the planet radius (TOI-1634b is about 1.8 earth radius, TOI-1685b is about 1.5 earth radius) obtained from transit observation, which planet It was found that, like the earth, has an internal composition mainly centered on iron and rocks (Fig. 2). Two planetary systems have been discovered in which Super-Earth, which has a composition similar to that of the Earth, revolves in the immediate vicinity of low-temperature, small-mass stars.
TOI-1634b is one of the planets with the largest radius and mass among the ultra-short-period planets confirmed to have an internal composition similar to that of the Earth, and such planets are around stars that are much lighter than the Sun. It is very interesting to find in. From the “mass-radius” relationship (Fig. 2), it was also found that there is no thick hydrogen atmosphere on both planets. On both planets, where no protoplanetary atmosphere of gas from the protoplanetary disk is left, a secondary atmosphere of gas released by the planets may be formed. It is also an interesting observation target for studying how the atmosphere of terrestrial planets that orbit the immediate vicinity of stars evolves.
Both planetary systems are located relatively close to the Earth about 100 light-years, and are particularly bright among low-temperature stars with ultra-short-period planets, making them promising observation candidates for next-generation telescopes. The lead author of the paper, Assistant Professor Teruyuki Hirano (National Institute of Natural Sciences, Astrobiology Center / National Institute of Natural Sciences, Hawaii Observatory) said, “In the future, we will observe the planetary system found in this research with the James Webb Space Telescope (JWST). By investigating the planetary atmosphere and detailed orbits, it is expected that the origin of the still mysterious ultra-short-period planets will be elucidated. Also, the planetary candidate celestial bodies identified by TESS will be intensively tracked by IRD. The observing project is still underway and many unique planets should be identified in the IRD in a year or two, “he said.
(Note 1) “Super Earth” is a planet larger than the Earth, and refers to an exoplanet whose mass is about 10 times or less that of the Earth and whose diameter is about 2 times or less that of the Earth. Since there are no planets of such weight and size in the solar system, observations of exoplanets have revealed that such planets exist for the first time.
(Note 2) “Transit” is a phenomenon in which a star appears to be dark periodically because the planet passes in front of the star. The exoplanet system in which transit is observed is called the transit planetary system. In transit exploration such as TESS, many transit-like dimmings are detected by large-scale photometric monitor observations, including false detections by “eclipsing binaries”. It is confirmed that the “transit planet candidate” detected by TESS is a real transit planet for the first time by performing follow-up observations using other telescopes.
(Note 3) Multicolor simultaneous imaging cameras mounted on the 188 cm telescope in Okayama Prefecture, the 1.52 m telescope at the Teide Observatory in Tenerife, Spain, and the 2 m telescope at the Haleakala Observatory in Maui, USA, MuSCAT, MuSCAT2 , MuSCAT3 was used for follow-up observation of transit. For all planets, this follow-up observation accurately determined parameters such as the orbital period and planetary radius that were tentatively obtained by TESS.
(Note 4) If there are planets around the star, the star will fluctuate slightly due to the influence of the planet’s gravity. The radial velocity method captures this fluctuation as a periodic change in the radial velocity of a star. The larger the mass of the planet, the larger the amplitude of the change in radial velocity. The masses of the two planets found were determined by follow-up observations by the IRD.
Featured image: An image illustration comparing the sizes of the terrestrial planets discovered in this study. TOI-1685b is 1.5 times the diameter of the earth and TOI-1684b is 1.8 times the diameter. Both planets are surrounded by stars that are cooler than the Sun, so they are illuminated by reddish light. (Credit: National Institute of Natural Sciences Astrobiology Center)
A rogue planet is an interstellar object of planetary mass without a host planetary system. As they freely roam around space, could they be fertile nurseries for life?
A Florida Tech scientist believes it’s possible based on extensive research he has undertaken over the past several years.
In research highlighted this summer in Discover Magazine, university astrobiologist Manasvi Lingam (along with Harvard researcher Avi Loeb) studied how life might survive on a rogue planet via oceans prevalent underneath a thick layer of ice. The cold of interstellar space would be too much for the oceans to remain entirely liquid, but the researchers believe any putative biospheres would be protected from the cold via the ice layer, and the planet’s core would heat the planet from the inside. Underneath the ice would potentially exist Earth-like oceans that could support life.
The possibilities for rogue planets facilitating life are of deep interest to Lingam as more planets are being discovered. He noted that for every solar system discovered (each of which contains a handful of terrestrial planets), there are approximately 30-40 rogue planets traveling in the cold expanses of interstellar space. The nearest exoplanet to Earth is therefore expected to be one of these rogue planets.
“We normally think of planets bound to stars, such as Mars, that could support life, but in reality, these types of life-supporting planets could just be floating out there in the vast void of space with rich biospheres,” he said.
The next steps in the research are to do experiments on Earth to ascertain under what extreme conditions life could survive, such as low temperatures or low pressure. A way of doing this is to analyze microbes that would not need sunlight, thereby building on previous research that has conclusively shown that more microbes exist that don’t require sunlight than those that do. Another direction that merits future research is to look at rogue planets as they enter our solar system and research the planet’s conditions to see if it would facilitate life.
Lingam noted that technology would have to advance by only a modest amount to make traveling to these planets – if they are in our solar system – easier. He has published a paper on this subject, detailing how missions to these interstellar interlopers are feasible. And this subject is also covered in his recent graduate-level textbook on astrobiology, Life in the Cosmos, published by Harvard University Press in July 2021.
“You might be able to get to a rogue planet in a few decades, and, rather than looking for other planets around other stars, this might be the best chance to study these planets,” Lingam said. “Through a combination of gravity assists and suitable propulsion systems, you could reach the rogue planet in 20 years or so. Once you have a probe on the surface, you can beam the data back and it would probably take a few months to learn what it looks like on the surface.”
New data analysis has found that the sunlight filtering through Venus’ clouds could support Earth-like photosynthesis in the cloud layers and that chemical conditions are potentially amenable to the growth of microorganisms.
Biochemistry Professor Rakesh Mogul is the lead author of the study, Potential for Phototrophy in Venus’ Clouds, published online this weekin the journal Astrobiology’s October 2021 special issue focused on the possible suitability of Venus’ clouds for microbial life, and constraints that may prohibit life.
According to Mogul and his team, which includes Michael Pasillas (’21, M.S.), photosynthesis could occur round-the-clock in Venus’ clouds with the middle and lower clouds receiving solar energy similar to the Earth’s surface. Much like on Earth, hypothetical phototrophs in Venus’ clouds would have access to solar energy during the day.
In a fascinating twist, the team found that photosynthesis may continue through the night due to thermal or infrared energy originating from the surface and the atmosphere. In this habitat, light energy would be available from both above and below the clouds, which could provide photosynthetic microorganisms ample opportunities to diversify across the cloud layers. Both the solar and thermal radiation in Venus’ clouds possess wavelengths of light that can be absorbed by the photosynthetic pigments found on Earth.
The study also found that after filtering through the Venusian atmosphere, scattering and absorption scrubs the sunlight of much of the ultraviolet radiation (UV) that is harmful to life, providing a benefit like Earth’s ozone layer.
Yeon Joo Lee, a co-author of the study, used a radiative transfer model to show that the present-day middle and lower cloud layers above Venus receive significantly less UV, 80-90% less flux in the UV-A when compared to Earth’s surface, and are essentially depleted of radiation in the UV-B and UV-C, which represent the most harmful components of the UV.
To gauge the nighttime photosynthetic potential via Venus’ thermal energy, Mogul and his team compared the photon fluxes rising from Venus’ hot atmosphere and surface to the photon fluxes measured within low-light phototrophic habitats on Earth – hydrothermal vents in the East Pacific Rise, where geothermal emissions are reported to support phototrophy at depths of 2400 meters, and the Black Sea, where solar powered phototrophs are found at depths of 120 meters. These comparisons showed that photon fluxes from Venus’ atmosphere and surface exceed the fluxes measured in these low-light phototrophic environments on Earth.
While a recent report by Hallsworth et al. 2021, concluded that Venus’ clouds were too dry to support terrestrial life, Mogul and his team found that the chemical conditions of Venus’ clouds could be partly composed of neutralized forms of sulfuric acid, such as ammonium bisulfate. These chemical conditions would exhibit dramatically higher water activities when compared to Hallsworth’s calculations and much lower acidities when compared to current models for Venus.
“Our study provides tangible support for the potential for phototrophy and/or chemotrophy by microorganisms in Venus’ clouds,” said Mogul. “The acidity and water activity levels potentially fall within an acceptable range for microbial growth on Earth, while the constant illumination with limited UV suggests that Venus’ clouds could be hospitable for life. We believe that Venus’ clouds would make a great target for habitability or life detection missions, like those currently planned for Mars and Europa.”
Pasillas, who recently graduated with a M.S. focused on chemical education from the Chemistry & Biochemistry Department, began his work on the study in a graduate seminar course (CHM 5500) taught by Mogul. Students in the class worked on small literature projects and were offered the opportunity to continue with the research. Pasillas ultimately worked on the assessments of acidity and included the work in his thesis. He is currently teaching chemistry at Mt. San Antonio College.
The co-authors of the study are Sanjay S. Limaye (University of Wisconsin, Madison), Yeon Joo Lee (Technische Universitat Berlin, Berlin, Germany) and Pasillas (M.S., chemistry ‘21).
Featured image: Night on Venus in Infrared from Orbiting Akatsuki. Source: ISAS, JAXA
New telescope will see planetary neighbors’ atmospheres
hen the world’s most powerful telescope launches into space this year, scientists will learn whether Earth-sized planets in our ‘solar neighborhood’ have a key prerequisite for life — an atmosphere.
These planets orbit an M-dwarf, the smallest and most common type of star in the galaxy. Scientists do not currently know how common it is for Earth-like planets around this type of star to have characteristics that would make them habitable.
“As a starting place, it is important to know whether small, rocky planets orbiting M-dwarfs have atmospheres,” said Daria Pidhorodetska, a doctoral student in UC Riverside’s Department of Earth and Planetary Sciences. “If so, it opens up our search for life outside our solar system.”
To help fill this gap in understanding, Pidhorodetska and her team studied whether the soon-to-launch James Webb Space Telescope, or the currently-in-orbit Hubble Space Telescope, are capable of detecting atmospheres on these planets. They also modeled the types of atmospheres likely to be found, if they exist, and how they could be distinguished from each other. The study has now been published in the Astronomical Journal.
Study co-authors include astrobiologists Edward Schwieterman and Stephen Kane from UCR, as well as scientists from Johns Hopkins University, NASA’s Goddard Space Flight Center, Cornell University and the University of Chicago.
The star at the center of the study is an M-dwarf called L 98-59, which measures only 8% of our sun’s mass. Though small, it is only 35 light years from Earth. It’s brightness and relative closeness make it an ideal target for observation.
Shortly after they form, M-dwarfs go through a phase in which they can shine two orders of magnitude brighter than normal. Strong ultraviolet radiation during this phase has the potential to dry out their orbiting planets, evaporating any water from the surface and destroying many gases in the atmosphere.
“We wanted to know if the ablation was complete in the case of the two rocky planets, or if those terrestrial worlds were able to replenish their atmospheres,” Pidhorodetska said.
The researchers modeled four different atmospheric scenarios: one in which the L 98-59 worlds are dominated by water, one in which the atmosphere is mainly composed of hydrogen, a Venus-like carbon dioxide atmosphere, and one in which the hydrogen in the atmosphere escaped into space, leaving behind only oxygen and ozone.
They found that the two telescopes could offer complementary information using transit observations, which measure a dip in light that occurs as a planet passes in front of its star. The L 98-59 planets are much closer to their star than Earth is to the sun. They complete their orbits in less than a week, making transit observations by telescope faster and more cost effective than observing other systems in which the planets are farther from their stars.
“It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds,” Schwieterman said. “With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”
Of the four atmospheric scenarios the researchers considered, Pidhorodetska said the dried-out oxygen-dominated atmosphere is the most likely.
“The amount of radiation these planets are getting at that distance from the star is intense,” she said.
Though they may not have atmospheres that lend themselves to life today, these planets can offer an important glimpse into what might happen to Earth under different conditions, and what might be possible on Earth-like worlds elsewhere in the galaxy.
The L 98-59 system was only discovered in 2019, and Pidhorodetska said she is excited to get more information about it when Webb is launched later this year.
“We’re on the precipice of revealing the secrets of a star system that was hidden until very recently,” Pidhorodetska said.
An international team of researchers has used seasonal variations to identify likely sub-surface deposits of water ice in the temperate regions of Mars where it would be easiest for future human explorers to survive. The results are being presented this week by Dr Germán Martínez at the European Planetary Science Conference (EPSC) 2021.
Using data from NASA’s Mars Odyssey, which has spent almost 20 years orbiting the Red Planet, Martínez and his colleagues have identified two areas of particular interest: Hellas Planitia and Utopia Rupes, respectively in the southern and northern hemisphere. Seasonal variations in levels of hydrogen detected suggests that significant quantities water ice can be found in the metre or so below the surface in these regions.
Martínez, of the Lunar and Planetary Institute, said: ‘Data from Mars Odyssey’s Neutron Spectrometer showed signs of hydrogen beneath the surface Mars from mid to equatorial latitudes, but we still had the challenge of working out whether this is in the form of water ice, which can readily be used as a resource, or locked away in mineral salts or in soil grains and minerals. This is where the seasonal variation provides an important clue. As the coldest ground temperatures occur at the same time as the largest observed increase in hydrogen content, it suggests that water ice is forming in the shallow subsurface of these regions during the fall and winter seasons, and then sublimating into gas during the warm season of each hemisphere.’
Water ice in the shallow subsurface has been found in plentiful supply at the poles. However, the frigid temperatures and the limited solar light make polar regions a hostile environment for human exploration. The areas from equatorial to mid latitudes are much more hospitable for both humans and robotic rovers, but only deeper reservoirs of water ice have been detected to date, and these are hard to reach.
To survive on Mars, astronauts would need to rely on resources already available in-situ, as sending regular supplies across the 55 million kilometres between Earth and Mars at their closest point is not an option. As liquid water is not available in the cold and arid Martian environment, ice is a vital resource. Water will not only be essential for life-support of the explorers, or the growth of plants and food, but could also be broken down into oxygen and hydrogen for use as rocket fuel.
Two other regions are rich in hydrogen: Tharsis Montes and the Medusae Fossae Formation. However, these do not display seasonal variations and appear to be the less accessible forms of water.
‘Definitely, those regions too are interesting for future missions,’ added Martínez. ‘What we plan to do now for them or Hellas Planitia and Utopia Rupes, is to study their mineralogy with other instruments in the hope of spotting types of rock altered by water. Such areas would be ideal candidates for robotic missions, including sample return ones, as the ingredients for rocket fuel would be available there too.’
Featured image: Global map of Mars with overlaid topography indicating areas with significant seasonal variations in hydrogen content during northern spring (top) and fall (bottom). Green (red) represents increase (decrease) in hydrogen content. The areas highlighted in orange are Hellas Planitia in the southern hemisphere, and Utopia Rupes in the northern hemisphere. These are the only extended regions undergoing a significant variation throughout the Martian year. Credit: G. Martínez.
What doesn’t stick comes around: Using machine learning and simulations of giant impacts, researchers at the Lunar and Planetary Laboratory found that the planets residing in the inner solar system were likely born from repeated hit-and-run collisions, challenging conventional models of planet formation.
Planet formation – the process by which neat, round, distinct planets form from a roiling, swirling cloud of rugged asteroids and mini planets – was likely even messier and more complicated than most scientists would care to admit, according to new research led by researchers at the University of Arizona Lunar and Planetary Laboratory.
The findings challenge the conventional view, in which collisions between smaller building blocks cause them to stick together and, over time, repeated collisions accrete new material to the growing baby planet.
Instead, the authors propose and demonstrate evidence for a novel “hit-and-run-return” scenario, in which pre-planetary bodies spent a good part of their journey through the inner solar system crashing into and ricocheting off of each other, before running into each other again at a later time. Having been slowed down by their first collision, they would be more likely to stick together the next time. Picture a game of billiards, with the balls coming to rest, as opposed to pelting a snowman with snowballs, and you get the idea.
The research is published in two reports appearing in the Sept. 23 issue of The Planetary Science Journal, with one focusing on Venus and Earth, and the other on Earth’s moon. Central to both publications, according to the author team, which was led by planetary sciences and LPL professor Erik Asphaug, is the largely unrecognized point that giant impacts are not the efficient mergers scientists believed them to be.
“We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” Asphaug said. “To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”
One implication is that Venus and Earth would have had very different experiences in their growth as planets, despite being immediate neighbors in the inner solar system. In this paper, led by Alexandre Emsenhuber, who did this work during a postdoctoral fellowship in Asphaug’s lab and is now at Ludwig Maximilian University in Munich, the young Earth would have served to slow down interloping planetary bodies, making them ultimately more likely to collide with and stick to Venus.
“We think that during solar system formation, the early Earth acted like a vanguard for Venus,” Emsenhuber said.
The solar system is what scientists call a gravity well, the concept behind a popular attraction at science exhibits. Visitors toss a coin into a funnel-shaped gravity well, and then watch their cash complete several orbits before it drops into the center hole. The closer a planet is to the sun, the stronger the gravitation experienced by planets. That’s why the inner planets of the solar system on which these studies were focused – Mercury, Venus, Earth and Mars – orbit the sun faster than, say, Jupiter, Saturn and Neptune. As a result, the closer an object ventures to the sun, the more likely it is to stay there.
So when an interloping planet hit the Earth, it was less likely to stick to Earth, and instead more likely to end up at Venus, Asphaug explained.
“The Earth acts as a shield, providing a first stop against these impacting planets,” he said. “More likely than not, a planet that bounces off of Earth is going to hit Venus and merge with it.”
Emsenhuber uses the analogy of a ball bouncing down a staircase to illustrate the idea of what drives the vanguard effect: A body coming in from the outer solar system is like a ball bouncing down a set of stairs, with each bounce representing a collision with another body.
“Along the way, the ball loses energy, and you’ll find it will always bounce downstairs, never upstairs,” he said. “Because of that, the body cannot leave the inner solar system anymore. You generally only go downstairs, toward Venus, and an impactor that collides with Venus is pretty happy staying in the inner solar system, so at some point it is going to hit Venus again.”
Earth has no such vanguard to slow down its interloping planets. This leads to a difference between the two similar-sized planets that conventional theories cannot explain, the authors argue.
“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then,” Emsenhuber said. “But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”
To track all these planetary orbits and collisions, and ultimately their mergers, the team used machine learning to obtain predictive models from 3D simulations of giant impacts. The team then used these data to rapidly compute the orbital evolution, including hit-and-run and merging collisions, to simulate terrestrial planet formation over the course of 100 million years. In the second paper, the authors propose and demonstrate their hit-and-run-return scenario for the moon’s formation, recognizing the primary problems with the standard giant impact model.
“The standard model for the moon requires a very slow collision, relatively speaking,” Asphaug said, “and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth.”
In the team’s new scenario, a roughly Mars-sized protoplanet hits the Earth, as in the standard model, but is a bit faster so it keeps going. It returns in about 1 million years for a giant impact that looks a lot like the standard model.
“The double impact mixes things up much more than a single event,” Asphaug said, “which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place.”
The researchers think the resulting asymmetry in how the planets were put together points the way to future studies addressing the diversity of terrestrial planets. For example, we don’t understand how Earth ended up with a magnetic field that is much stronger than that of Venus, or why Venus has no moon.
Their research indicates systematic differences in dynamics and composition, according to Asphaug.
“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus,” he said. “Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus.”
This would create a bias in which, for example, protoplanets from the outer solar system, at higher velocity, would have preferentially accreted to Venus instead of Earth. In short, Venus could be composed of material that was harder for the Earth to get ahold of.
“You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus. But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material,” Asphaug said.
The co-authors on the two papers are Saverio Cambioni and Stephen R. Schwartz at the Lunar and Planetary Laboratory and Travis S. J. Gabriel at Arizona State University in Tempe, Arizona.
Featured image: Artist’s illustration of two massive objects colliding.NASA/JPL-Caltech