For almost a decade, astronomers have tried to explain why so many pairs of planets outside our solar system have an odd configuration—their orbits seem to have been pushed apart by a powerful unknown mechanism. Yale researchers say they’ve found a possible answer, and it implies that the planets’ poles are majorly tilted.
The finding could have a big impact on how researchers estimate the structure, climate, and habitability of exoplanets as they try to identify planets that are similar to Earth. The research appears in the March 4 online edition of the journal Nature Astronomy.
NASA’s Kepler mission revealed that about 30% of stars similar to our Sun harbor “Super-Earths.” Their sizes are somewhere between that of Earth and Neptune; they have nearly circular and coplanar orbits; and it takes them fewer than 100 days to go around their star. Yet curiously, a great number of these planets exist in pairs with orbits that lie just outside natural points of stability.
That’s where obliquity—the amount of tilting between a planet’s axis and its orbit—comes in, according to Yale astronomers Sarah Millholland and Gregory Laughlin.
“When planets such as these have large axial tilts, as opposed to little or no tilt, their tides are exceedingly more efficient at draining orbital energy into heat in the planets,” said first author Millholland, a graduate student at Yale. “This vigorous tidal dissipation pries the orbits apart.”
A similar, but not identical, situation exists between Earth and its moon. The moon’s orbit is slowly growing due to dissipation from tides, but Earth’s day is gradually lengthening.
Laughlin, who is a professor of astronomy at Yale, said there is a direct connection between the over-tilting of these exoplanets and their physical characteristics. “It impacts several of their physical features, such as their climate, weather, and global circulations,” Laughlin said. “The seasons on a planet with a large axial tilt are much more extreme than those on a well-aligned planet, and their weather patterns are probably non-trivial.”
Millholland said she and Laughlin already have started work on a follow-up study that will examine how these exoplanets’ structures respond to large obliquities over time.
Fundamental work on RNA is intended to help assist with probing life’s origins. IMAGE CREDIT: NASA/JENNY MOTTAR.
Just like the mythical creation stories that depict the formation of the world as the story of order from chaos, the early Earth was home to a chaotic clutter of organic molecules from which, somehow, more complex biological structures such as RNA and DNA emerged.
There was no guiding hand to dictate how the molecules within that prebiotic clutter should interact to form life. Yet, had those molecules just interacted randomly then, in all likelihood, that they would never have chanced upon the right interactions to ultimately lead to life.
“The question is, out of all the random possibilities, are there any rules that govern these interactions?” asks Ramanarayanan Krishnamurthy, an organic chemist at the Scripps Research Institute in California.
These rules would be selective, inevitably leading to the right interactions for assembling life’s building blocks. To unlock the secrets of these rules and how the prebiotic clutter transitioned to the biologically ordered world of life, Krishnamurthy utilizes a discipline called “systems chemistry,” and published a paper concerning the topic in the journal Accounts of Chemical Research that explores this relatively new way of understanding how life came from non-life.
Nobel prize-winner and geneticist Jack Szostak of Harvard Medical School describes systems chemistry as: “one of the news ways of thinking about the problems of prebiotic chemistry.” To understand how systems chemistry works, think of a flask full of chemical A, to which another chemical, B, is added and which reacts with A to produce two more chemicals, C and D. Since no process is 100 percent efficient, the flask now contains chemicals A, B, C and D. “So now you have a system,” explains Krishnamurthy. Systems chemistry considers the system as a whole and explores the rules within that system that govern how each chemical interacts with the others, and in different conditions.
The Krishnamurthy Lab at the Scripps Research Institute. IMAGE CREDIT: RAMANARAYANAN KRISHNAMURTHY/SCRIPPS.
Yet, systems chemistry is about more than just dealing with systems containing many chemicals, says Szostak. “It’s a matter of thinking about what chemicals or conditions are likely to be available and likely to be helpful.” He cites the example of phosphate, which is automatically present in biochemical systems because of its existence in biology’s nucleotide-building blocks, and therefore is on hand to play multiple roles in the story of life, such as acting as a catalyst and protecting cells from pH changes.
Of course, unravelling the chemistry of the prebiotic clutter is a far cry from explaining the interactions of four chemicals in a flask. The computing and analytical power required to simulate such a complex system was beyond reach just a decade or two ago. Instead, the majority of research into the origin of life previously had focused on individual classes of biomolecules, the most promising being RNA (ribonucleic acid).
A chicken and egg scenario
The RNA world theory, which is the idea that RNA existed before cells did, faces a paradox. RNA makes proteins, but proteins also make up RNA. “Biologists took modern biology and for the sake of parsimony ran it backwards, but they then ran into the problem of what came first, proteins or RNA?” says Krishnamurthy
When the University of Colorado’s Thomas Cech discovered in 1981 that RNA can catalyze reactions within itself, the problem appeared to have been solved. Overnight, RNA’s importance to life was transformed. By being catalytic, RNA could kickstart other biochemistry including the formation of proteins and therefore had to come first. The subsequent discovery that it is the RNA molecule in a ribosome that is responsible for protein synthesis gave further credence to the “RNA world” hypothesis.
The RNA world has, however, come in for much criticism lately, which Krishnamurthy believes is deserved. RNA is able to transfer genetic information in organisms and is made of chains of ribonucleotides. But there’s a catch.
“Nucleotides don’t just pop up from chemical mixtures, they have to be made in a very defined manner,” he says. “There has to be a certain order to the reaction sequence. It’s not like Stanley Miller’s spark discharge experiment where he put all these gases together, pressed a switch and ‘Voila!‘”
Systems chemistry depicts the development of RNA as a chain of events driven by selective interactions and catalysis. Ribonucleotides are formed from ribonucleosides linked to phosphate. A nucleoside consists of a nucleobase, which is a nitrogen-bearing compound, bonded to a monosaccharide, which is a sugar containing five carbon atoms, called pentoses. Among the population of monosaccharides are four pentoses, among them ribose, which is somehow selectively converted into ribonucleoside instead of the other three pentoses.
Members of the Krishnamurthy Lab at the Scripps Research Institute. IMAGE CREDIT: RAMANARAYANAN KRISHNAMURTHY/SCRIPPS.
Although Szostak agrees that systems chemistry has the power to support the RNA world theory, or at least explain the origin of RNA, he points out that a disproportionate amount of work has been put into understanding how nucleotides form, and not enough into what happens after that. “There are still missing steps in understanding how RNA could be made,” he says. So, the challenge now for systems chemistry is to show how and why each of these stages occur.
“Just synthesizing a monomer of RNA like a nucleoside or a nucleotide isn’t enough to say you’ve found the origin of RNA,” says Krishnamurthy. “How do you put those monomers together in a meaningful manner that is self-sustainable?”
The selection effect could be taking place at a multitude of levels in the creation of RNA. Perhaps the selection rules are what determines why ribose, rather than the other three pentoses — xylose, lyxose or arabinose — is converted into the nucleosides used by RNA. Maybe the selection effect comes when explaining why phosphate prefers to bond with ribonucleosides, rather than any other nucleosides. Or, possibly it is the ribonucleotides themselves that are selected by being more efficient than other nucleotides at forming chains. We don’t know what the answer is yet, but Krishnamurthy believes that systems chemistry is the best tool for finding out.
We find selection rules driving interactions in chemistry as a result of environmental conditions; or emergent properties such as catalytic activity, self-assembly and self-replication; or even as a result of the specifics of chemical reactions.
Cyanide, for example, takes the form of non-toxic nitriles in biochemistry, linking with carbon-based molecules to form more complex organic molecules. It’s also a pretty handy reactant. Add cyanide to two specific organic compounds containing ketone and carboxylic acid, called keto acids and keto alcohols, and it produces cyanohydrins that are important precursors to some amino acids. However, in water cyanohydrins can undergo hydrolysis and break down, but whether they do or don’t depends on the pH of that water. In a paper published in Chemistry: A European Journal, Krishnamurthy, Scripps colleague Jayasudhan Yerabolu, and Georgia Institute of Technology chemist Charles Liotta found that hydrolysis takes place at a pH of less than 7 for cyanohydrins formed from keto acids, and a pH greater than 7 for cyanohydrins formed from keto alcohols. Therefore, the longer-term survival of cyanohydrins is selective dependent on the acidity or alkalinity of the surrounding environment.
Charles Liotta, Regents Professor Emeritus in the School of Chemistry and Biochemistry at the Georgia Institute of Technology. IMAGE CREDIT: GEORGIA INSTITUTE OF TECHNOLOGY.
Another example encompassing cyanide-reactivity involves molecules of oxaloacetate and alpha-ketoglutarate, which play a role in the citric acid cycle (a series of energy-releasing chemical reactions utilized by oxygen-breathing life). In the presence of cyanide, oxaloacetate is selectively transformed instead of alpha-ketoglutarate, to form a hydroxy-succinic acid derivative.
“In a mixture where you can find both oxaloacetate and alpha-ketoglutarate, by adding cyanide you can selectively transform one but not the other,” says Krishnamurthy.
These examples demonstrate what Krishnamurthy describes as the transition from heterogeneous heterogeneity (diverse interactions in a system of many molecules) to homogeneous heterogeneity (selecting from diverse interactions between relatively few molecules forming the backbone of life’s systems, such as RNA). In other words, it is the emergence from the prebiotic clutter of an orderly proto-biochemistry.
“The solution seems to be to move from the heterogeneous mixture to what I call the homogeneous heterogeneity,” says Krishnamurthy. “This is what our lab is trying to demonstrate as a proof of principle.”
There is a long way to go yet and Krishnamurthy recommends that progress will be best made with baby steps as scientists develop this bottom-up approach to the origin of life from the heterogeneous prebiotic clutter. By discovering reactions and catalysis that select the right interactions between organic compounds, the aim is to build up our understanding of how the basic building blocks assembled — how, for example, RNA emerged from the chaos.
Professor Ramanarayanan Krishnamurthy speaks at a Story Collider event at the San Diego Festival of Science and Engineering. IMAGE CREDIT: PHOTOS COURTESY OF CHRIS PARSONS (CENTER FOR CHEMICAL EVOLUTION, GEORGIA TECH).
Ultimately the wish is to build an experimental simulation that includes the entire heterogeneous heterogeneity of the prebiotic clutter in a replica of Earth’s early environment, and then to run that simulation over and over again to see which selective interactions are most common and whether they can repeat the origin of life.
“I’m optimistic that we will be able to work out reasonable pathways for making all the building blocks of biology, and for assembling these components into simple, primitive cells,” says Szostak. “However, there is a lot to be learned before we can accomplish this ambitious goal.”
Just like the flask that ended up containing chemicals A, B, C and D, the end products of these selective reactions could start interacting with their source chemicals, something that doesn’t happen in the clean, isolated RNA world that is studied in the laboratory. What new and previously overlooked solutions await to be discovered and how quickly will the baby steps get us to them?
Illustration of a high-mass X-ray binary system made up of a compact, incredibly dense neutron star paired with a massive ‘normal’ supergiant star. CREDIT: NASA/CXC/M.Weiss
Data recorded by NASA’s Chandra X-ray Observatory of a neutron star as it passed through a dense patch of stellar wind emanating from its massive companion star provide valuable insight about the structure and composition of stellar winds and about the environment of the neutron star itself. A paper describing the research, led by Penn State astronomers, appears January 15, 2019, in the journal, Monthly Notices of the Royal Astronomical Society.
“Stellar winds are the fast-flowing material–composed of protons, electrons, and metal atoms–ejected from stars,” said Pragati Pradhan, a postdoctoral researcher in astronomy and astrophysics at Penn State and the lead author of the paper. “This material enriches the star’s surroundings with metals, kinetic energy, and ionizing radiation. It is the source material for star formation. Until the last decade, it was thought that stellar winds were homogenous, but these Chandra data provide direct evidence that stellar winds are populated with dense clumps.”
The neutron star observed is part of a high-mass X-ray binary system–the compact, incredibly dense neutron star paired with a massive ‘normal’ supergiant star. Neutron stars in binary systems produce X-rays when material from the companion star falls toward the neutron star and is accelerated to high velocities. As a result of this acceleration, X-rays are produced that can inturn interact with the materials of the stellar wind to produce secondary X-rays of signature energies at various distances from the neutron star. Neutral–uncharged–iron atoms, for example, produce fluorescence X-rays with energies of 6.4 kilo-electron volts (keV), roughly 3000 times the energy of visible light. Astronomers use spectrometers, like the instrument on Chandra, to capture these X-rays and separate them based on their energy to learn about the compositions of stars.
“Neutral iron atoms are a more common component of stars so we usually see a large peak at 6.4 keV in the data from our spectrometers when looking at X-rays from most neutron stars in a high-mass X-ray binary system,” said Pradhan. “When we looked at X-ray data from the high-mass X-ray binary system known as OAO 1657-415 we saw that this peak at 6.4 keV had an unusual feature. The peak had a broad extension down to 6.3 keV. This extension is referred to as a ‘Compton shoulder’ and indicates that the X-rays from neutral iron are being back scattered by dense matter surrounding the star. This is only the second high-mass X-ray binary system where such a feature has been detected.”
The researchers also used the Chandra’s state-of-the-art engineering to identify a lower limit on the distance from the neutron star that the X-rays from neutral iron are formed. Their spectral analysis showed that neutral iron is ionized at least 2.5 light-seconds, a distance of approximately 750 million meters or nearly 500,000 miles, from the neutron star to produce X-rays.
“In this work, we see a dimming of the X-rays from the neutron star and a prominent line from neutral iron in the X-ray spectrum–two signatures supporting the clumpy nature of stellar winds,” said Pradhan. “Furthermore, the detection of Compton shoulder has also allowed us to map the environment around this neutron star. We expect to be able to improve our understanding of these phenomenon with the upcoming launch of spacecrafts like Lynx and Athena, which will have improved X-ray spectral resolution.”
For Pradhan’s post-doctoral work at Penn State under the supervision of Professor of Astronomy and Astrophysics David Burrows, Associate Research Professor of Astronomy and Astrophysics Jamie Kennea, and Research Professor of Astronomy and Astrophysics Abe Falcone, she is majorly involved in writing algorithms for on-board detection of X-rays from transient astronomical events such as those seen from these high-mass X-ray binary systems for instruments that will be on the Athena spacecraft.
Pradhan and her team also have a follow-up campaign looking at the same high-mass X-ray binary with another NASA satellite–NuSTAR, which will cover a broader spectrum of X-rays from this source ranging in energies from ~ 3 to 70 keV–in May 2019.
“We are excited about the upcoming NuSTAR observation too,” said Pradhan. “Such observations in hard X-rays will add another dimension to our understanding of the physics of this system and we will have an opportunity to estimate the magnetic field of the neutron star in OAO 1657-415, which is likely a million times stronger than strongest magnetic field on Earth.”
Las Cruces Mine, Spain. Photo courtesy Fernando Tornos.
One of the major issues when studying ore deposits formed in surficial or near-surface environments is the relationship between ore-forming processes and bacteria. At a first glance, these environments appear to be a preferred place for the growth of microbial ecosystems because they potentially have large amounts of nutrients. However, studies have been restricted because of the low likelihood of microbe fossilization and because biomarkers are not always definitive.
This contribution to Geology by Fernando Tornos and colleagues tries to solve the long-standing debate on the control of microbes on secondary sulfide formation. They predict that future multidisciplinary studies will prove that microbes have a key control on the precipitation of metals in these shallow environments.
Their case study is based on the unusual Las Cruces deposit in southwest Iberia, where a significant part of the high-grade copper ore occurs as thick, massive veins of copper sulfides. Tornos and colleagues have direct evidence that the mineralization is currently being formed there in relationship with active aquifers and in an area isolated from the surface by a thick layer of marl. Thus, the place is ideal for tracking for anaerobic microbes.
With the help of the mining company, First Quantum, the team was able to extract pristine samples that had never been in contact with the atmosphere.
Different microbiological techniques and detailed electron microscope studies have shown that copper sulfides are precipitating today in relationship with colonies of sulfate-reducing microbes. The nanometer-sized crystals of covellite are embedded in the polymeric compounds that encapsulate bacteria. These crystals coalesce, later forming the big veins. However, much more work is needed in order to know to which extent these processes are global and if microbes control most of the formation of the secondary copper deposits.
This is an artist’s impression of the newly discovered object. CREDIT: Ko Arimatsu
For the first time ever, astronomers have detected a 1.3 km radius body at the edge of the Solar System. Kilometer sized bodies like the one discovered have been predicted to exist for more than 70 years. These objects acted as an important step in the planet formation process between small initial amalgamations of dust and ice and the planets we see today.
The Edgeworth-Kuiper Belt is a collection of small celestial bodies located beyond Neptune’s orbit. The most famous Edgeworth-Kuiper Belt Object is Pluto. Edgeworth-Kuiper Belt Objects are believed to be remnants left over from the formation of the Solar System. While small bodies like asteroids in the inner Solar System have been altered by solar radiation, collisions, and the gravity of the planets over time; objects in the cold, dark, lonely Edgeworth-Kuiper Belt preserve the pristine conditions of the early Solar System. Thus astronomers study them to learn about the beginning of the planet formation process.
Edgeworth-Kuiper Belt Objects with radii from 1 kilometer to several kilometers have been predicted to exist, but they are too distant, small, and dim for even world-leading telescopes, like the Subaru Telescope, to observe directly. So a research team led by Ko Arimatsu at the National Astronomical Observatory of Japan used a technique known as occultation: monitoring a large number of stars and watching for the shadow of an object passing in front of one of the stars. The OASES (Organized Autotelescopes for Serendipitous Event Survey) team placed two small (28 cm) telescopes on the roof of the Miyako open-air school on Miyako Island, Miyakojima-shi, Okinawa Prefecture, Japan, and monitored approximately 2000 stars for a total of 60 hours.
Analyzing the data, the team found an event consistent with a star appearing to dim as it is occulted by a 1.3 km radius Edgeworth-Kuiper Belt Object. This detection indicates that kilometer sized Edgeworth-Kuiper Belt Objects are more numerous than previously thought. This supports models where planetesimals first grow slowly into kilometer sized objects before runaway growth causes them to merge into planets.
Arimatsu explains, “This is a real victory for little projects. Our team had less than 0.3% of the budget of large international projects. We didn’t even have enough money to build a second dome to protect our second telescope! Yet we still managed to make a discovery that is impossible for the big projects. Now that we know our system works, we will investigate the Edgeworth-Kuiper Belt in more detail. We also have our sights set on the still undiscovered Oort Cloud out beyond that.”
A Goldstone 111.5-foot (34-meter) beam-waveguide antenna tracks a spacecraft as it comes into view. The Goldstone Deep Space Communications Complex is located in the Mojave Desert in California. Engineers at NASA’s Jet Propulsion Laboratory in Pasadena, California, will use antennas like this one to transmit a new set of commands to the Opportunity rover in an attempt to compel the 15-year-old Martian explorer to contact Earth. Credits: NASA/JPL-Caltech
Engineers at NASA’s Jet Propulsion Laboratory in Pasadena, California, have begun transmitting a new set of commands to the Opportunity rover in an attempt to compel the 15-year-old Martian explorer to contact Earth. The new commands, which will be beamed to the rover during the next several weeks, address low-likelihood events that could have occurred aboard Opportunity, preventing it from transmitting.
The rover’s last communication with Earth was received June 10, 2018, as a planet-wide dust storm blanketed the solar-powered rover’s location on Mars.
“We have and will continue to use multiple techniques in our attempts to contact the rover,” said John Callas, project manager for Opportunity at JPL. “These new command strategies are in addition to the ‘sweep and beep’ commands we have been transmitting up to the rover since September.” With “sweep and beep,” instead of just listening for Opportunity, the project sends commands to the rover to respond back with a beep.
The new transmission strategies are expected to go on for several weeks. They address three possible scenarios: that the rover’s primary X-band radio — which Opportunity uses to communicate with Earth — has failed; that both its primary and secondary X-band radios have failed; or that the rover’s internal clock, which provides a timeframe for its computer brain, is offset. A series of unlikely events would need to have transpired for any one of these faults to occur. The potential remedies being beamed up to address these unlikely events include a command for the rover to switch to its backup X-band radio and commands directed to reset the clock and respond via UHF.
“Over the past seven months we have attempted to contact Opportunity over 600 times,” said Callas. “While we have not heard back from the rover and the probability that we ever will is decreasing each day, we plan to continue to pursue every logical solution that could put us back in touch.”
Time is of the essence for the Opportunity team. The “dust-clearing season” — the time of year on Mars when increased winds could clear the rover’s solar panels of dust that might be preventing it from charging its batteries — is drawing to a close. Meanwhile, Mars is heading into southern winter, which brings with it extremely low temperatures that are likely to cause irreparable harm to an unpowered rover’s batteries, internal wiring and/or computer systems.
If either these additional transmission strategies or “sweep and beep” generates a response from the rover, engineers could attempt a recovery. If Opportunity does not respond, the project team would again consult with the Mars Program Office at JPL and NASA Headquarters to determine the path forward.
The BIOMEX experiment, performed by DLR, being attached by astronauts to the exterior of the International Space Station. Image credit: ESA.
Each of NASA’s international astrobiology partners take a different tack in looking for the answer to the question of whether there is life elsewhere in the Universe. A creative, multi-pronged investigation is necessary with such a complicated problem – the answer will draw on a collaborative approach among biologists, geologists, chemists and many others.
In the case of the German Aerospace Center (DLR)’s Institute of Planetary Research, there are two areas on which they focus their attention.
DLR specializes in developing technology for space missions, including photometric technology, radiometers, laser altimeters, thermal probes and spectrometers, and contributes to NASA projects including Cassini, InSightand Dawn, plus European Space Agency (ESA) missions such as CoRoT, Rosetta and ExoMars and the forthcoming JUICE (JUpiter ICy moons Explorer) spacecraft. In particular, cameras are a speciality.
“For example, we built a high-resolution stereo camera for Mars Express, which is the oldest camera on a European Space Agency mission still in operation,” says Professor Heike Rauer, the new Director of the DLR Institute of Planetary Research. “It’s been running for 15 years, and takes 3D images.”
Those high-resolution, color images have revealed details about Mars’ geologic and climate history, including evidence of ancient water flows that have led to evidence-based discussions of human habitability and settlement on the red planet.
Heike Rauer (left), the Director of the DLR Institute of Planetary Research, and Tilam Spohn, the former Director of the Helmholtz Alliance. Image credit: DLR.
In addition, DLR has performed astrobiological experiments, for example BIOMEX (BIOlogy and Mars Experiment) on board the International Space Station, which tests the extent to which extremophiles can survive in particular space environments. Furthermore, Rauer is head of a consortium developing an instrument for the planet-finding PLATO mission that will detect and characterize Earth-like planets in the habitable zone of Sun-like stars. This ties in with their second focus, which is to understand the evolution of planets, both in ourSolar System and around other stars.By understanding the planetary processes that make life possible, the search for life elsewhere can be concentrated on the places where it’s most likely to have evolved.
This aspect of DLR’s work began with the Helmholtz Alliance‘Planetary Evolution and Life’ project. The Helmholtz Alliance is a science-focused programof the German government designed to solve “the grand challenges of science, society and industry.” Helmholtz gives out five-year grants to scientists who work in German institutions and elsewhere to come together on collaborative projects that especially aim to involve young people and promote equal opportunity.
DLR’s planetary research work was funded in 2008 by Helmholtz and continued through 2015, having received an extension on the work in order to use up all the funds.
In the framework of the Helmholtz Alliance, DLR became an affiliated partner of the NASA Astrobiology Institute (NAI) in early 2013. The Helmholtz program was only meant to be a one-time ‘jump-start’ for a research area, which is exactly what was accomplished with the $5 million euro per annum fund that made Germany one of the leading nations in planetary research. The planetary evolution work at DLR is now aregular research program with a long-term funding perspective, says the Alliance’s former director, Professor Tilman Spohn. While funding isn’t quite as robust as it once was under Helmholtz, it still stands as an independent program at the DLR.
During the six years that planetary evolution research was a Helmholtz program, “We did some exoplanetary research, but we had a strong focus on Mars,” says Spohn. “We made major contributions using the data from Mars Express to look into the various [potentially] habitable provinces on Mars to find where life could have originated and could still be present. It was good to start something new and interesting and then make it sustainable [under the aegis of DLR].”
A perspective view of an ancient river channel in the Libya Montes region of Mars, created by the DLR high-resolution stereo camera on board Mars Express. Image: ESA/DLR/FU Berlin.
Where to look for life
The big question that the planetary research program is currently attempting to answer is the same as before: how can we figure out which of the many planets outside our Solar System might harbor life? Scientists need to set defined parameters in order to make smart guesses about where to look. So they look for what life might leave behind, or signs that might reveal indirect evidence for life. Life might exist now, but may not be obvious, so looking for coincident or non-obvious signs of life is important. Elsewhere in the Solar System, life is more likely to have existed in the past than in the present, so what might it have left behind?
“We are looking for a better understanding of habitability and of biosignatures,” says Rauer. “In one case –our Solar System –we can go and look, but with extrasolar planets we cannot go there, which means the only way we can detect life is by studying the atmospheres of exoplanets.”
That’s why DLR is looking closely at “the link between interiors, surface and atmosphere,” of planets, says Rauer. Understanding how each of those planetary regions affects the others enables scientists to see what might be produced by normal geologic or chemical processes, for example – and what might be anomalous.
DLR is looking at some big questions that could apply to a wide variety of types of life, from single-celled to multicellular. “How does life leave imprints on the atmosphere? That’s important for places where we can’t send rovers,” says Rauer. She says that knowing what signs to look for could enable future researchers to scan for life simply by looking at the atmosphere of a planet. Of course, it’s also important for astrobiologists to study how life has “interactions with the surface and could leave its impact there.”
Other related questions include how life might affect the evolution of an entire planet over time. “This is a novel look at planetary geophysics – how do tectonics and interior structures influence the development of lifeforms?” asks Rauer. Since Earth has developed in tandem with life, and life has been affected by the geophysics of the Earth, we know that both of these things have happened at least once, here. So, looking for those signs and asking those questions elsewhere makes sense.
To that end, DLR works on modeling planet formation and tectonics, the inner structures of planets, how magnetic fields originate, and how meteor impacts affect all of the above. They also engage, along with their partners, in laboratory investigations of extremophiles in conditions similar to Mars or space, and how water behaves in different environments. And, of course, they are figuring out how to detect organisms on the surface of a planet.
One of the goals of the DLR’s Institute of Planetary Research is to characterize exoplanet atmospheres in search of biosignatures. This image shows an artist’s impression of the planet WASP-19b, in which DLR scientists participated in the detection of titanium oxide layer in its atmosphere. Image credit: ESO/M. Kornmesser.
All of these questions fit within six specific areas that DLR’s Planetary Evolution and Life program tackles, often interdependently:
Biosphere–Atmosphere–Surface Interaction and Development
Planet–Interior Atmosphere Interaction
Magnetic Field and Planetary Evolution
Impacts and Planetary Evolution; Geological Context of Life
Physics and Biology of Interface Water
Strategies and Realizations of Missions for Exploration of planetary habitability.
The Planetary Evolution and Life program started, as many great projects do, with the feeling that there was an understudied area that needed attention. Spohn says that he has long looked at the evolution of planets, including Earth, Mars, Venus and others. “But we never looked at the potential effect of life on these planets. I thought to myself that maybe we should include the interaction of life with planetary processes in our modeling. Nobody in the previous astrobiology community had really looked into a combination of geophysical tools and modeling together with the effects of life.”
Under the Helmholtz Alliance, the Planetary Evolution program worked with – and plans to continue working with, as part of DLR – international partners across Europe and beyond, including ESA, NASA Ames, NASA’s Jet Propulsion Laboratory, the Johns Hopkins University Applied Physics Laboratory, the Japan Aerospace Exploration Agency (JAXA), the French Centre national de la recherche scientifique (CNRS) and Centre national d’études spatiales (CNES), and many other institutions and universities around the world.
An important part of DLR’s work under Helmholtz was supporting and involving grad students and early-career scientists in both the questions and the work the institute undertook. “Much of the work has been done by students, young grad students and post-docs,” says Spohn. “We let students in on many aspects of the work, and they also looked into missions – how they are devised and managed, and put into space, so they see the whole process.”
This aspect of the program is likely to continue, as young scientists are drawn to the still-unanswered question: “Are we alone in the Universe?”
Illustration of a white dwarf, the dead remnant of a star like our Sun, with a crystallised, solid core. University of Warwick/Mark Garlick
Data captured by ESA’s galaxy-mapping spacecraft Gaia has revealed for the first time how white dwarfs, the dead remnants of stars like our Sun, turn into solid spheres as the hot gas inside them cools down.
This process of solidification, or crystallisation, of the material inside white dwarfs was predicted 50 years ago but it wasn’t until the arrival of Gaia that astronomers were able to observe enough of these objects with such a precision to see the pattern revealing this process.
“Previously, we had distances for only a few hundreds of white dwarfs and many of them were in clusters, where they all have the same age,” says Pier-Emmanuel Tremblay from the University of Warwick, UK, lead author of the paper describing the results, published today in Nature.
“With Gaia we now have the distance, brightness and colour of hundreds of thousands of white dwarfs for a sizeable sample in the outer disc of the Milky Way, spanning a range of initial masses and all kinds of ages.”
It is in the precise estimate of the distance to these stars that Gaia makes a breakthrough, allowing astronomers to gauge their true brightness with unprecedented accuracy.
White dwarfs are the remains of medium-sized stars similar to our Sun. Once these stars have burnt all the nuclear fuel in their core, they shed their outer layers, leaving behind a hot core that starts cooling down.
These ultra-dense remnants still emit thermal radiation as they cool, and are visible to astronomers as rather faint objects. It is estimated that up to 97 per cent of stars in the Milky Way will eventually turn into white dwarfs, while the most massive of stars will end up as neutron stars or black holes.
Artist impression of some possible evolutionary pathways for stars of different initial masses. ESA
The cooling of white dwarfs lasts billions of years. Once they reach a certain temperature, the originally hot matter inside the star’s core starts crystallising, becoming solid. The process is similar to liquid water turning into ice on Earth at zero degrees Celsius, except that the temperature at which this solidification happens in white dwarfs is extremely high – about 10 million degrees Celsius.
In this study, the astronomers analysed more than 15 000 stellar remnant candidates within 300 light years of Earth as observed by Gaia and were able to see these crystallising white dwarfs as a rather distinct group.
“We saw a pile-up of white dwarfs of certain colours and luminosities that were otherwise not linked together in terms of their evolution,” says Pier-Emmanuel.
“We realised that this was not a distinct population of white dwarfs, but the effect of the cooling and crystallisation predicted 50 years ago.”
The heat released during this crystallisation process, which lasts several billion years, seemingly slows down the evolution of the white dwarfs: the dead stars stop dimming and, as a result, appear up to two billion years younger than they actually are. That, in turn, has an impact on our understanding of the stellar groupings these white dwarfs are a part of.
“White dwarfs are traditionally used for age-dating of stellar populations such as clusters of stars, the outer disc, and the halo in our Milky Way,” explains Pier-Emmanuel.
“We will now have to develop better crystallisation models to get more accurate estimates of the ages of these systems.”
Not all white dwarfs crystallise at the same pace. More massive stars cool down more rapidly and will reach the temperature at which crystallisation happens in about one billion years. White dwarfs with lower masses, closer to the expected end stage of the Sun, cool in a slower fashion, requiring up to six billion years to turn into dead solid spheres.
The Sun still has about five billion years before it becomes a white dwarf, and the astronomers estimate that it will take another five billion years after that to eventually cool down to a crystal sphere.
“This result highlights the versatility of Gaia and its numerous applications,” says Timo Prusti, Gaia project scientist at ESA.
“It’s exciting how scanning stars across the sky and measuring their properties can lead to evidence of plasma phenomena in matter so dense that cannot be tested in the laboratory.”
Artist’s concept of a hypothetical planet orbiting far from the Sun. Credit: Caltech/R. Hurt (IPAC)
The strange orbits of some objects in the farthest reaches of our solar system, hypothesised by some astronomers to be shaped by an unknown ninth planet, can instead be explained by the combined gravitational force of small objects orbiting the Sun beyond Neptune, say researchers.
The alternative explanation to the so-called ‘Planet Nine’ hypothesis, put forward by researchers at the University of Cambridge and the American University of Beirut, proposes a disc made up of small icy bodies with a combined mass as much as ten times that of Earth. When combined with a simplified model of the solar system, the gravitational forces of the hypothesised disc can account for the unusual orbital architecture exhibited by some objects at the outer reaches of the solar system.
While the new theory is not the first to propose that the gravitational forces of a massive disc made of small objects could avoid the need for a ninth planet, it is the first such theory which is able to explain the significant features of the observed orbits while accounting for the mass and gravity of the other eight planets in our solar system. The results are reported in the Astronomical Journal.
Beyond the orbit of Neptune lies the Kuiper Belt, which is made up of small bodies left over from the formation of the solar system. Neptune and the other giant planets gravitationally influence the objects in the Kuiper Belt and beyond, collectively known as trans-Neptunian Objects (TNOs), which encircle the Sun on nearly-circular paths from almost all directions.
However, astronomers have discovered some mysterious outliers. Since 2003, around 30 TNOs on highly elliptical orbits have been spotted: they stand out from the rest of the TNOs by sharing, on average, the same spatial orientation. This type of clustering cannot be explained by our existing eight-planet solar system architecture and has led to some astronomers hypothesising that the unusual orbits could be influenced by the existence of an as-yet-unknown ninth planet.
The ‘Planet Nine’ hypothesis suggests that to account for the unusual orbits of these TNOs, there would have to be another planet, believed to be about ten times more massive than Earth, lurking in the distant reaches of the solar system and ‘shepherding’ the TNOs in the same direction through the combined effect of its gravity and that of the rest of the solar system.
“The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection,” said co-author Antranik Sefilian, a PhD student in Cambridge’s Department of Applied Mathematics and Theoretical Physics. “We wanted to see whether there could be another, less dramatic and perhaps more natural, cause for the unusual orbits we see in some TNOs. We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not simply account for the gravity of small objects constituting a disc beyond the orbit of Neptune and see what it does for us?”
Professor Jihad Touma, from the American University of Beirut, and his former student Sefilian modelled the full spatial dynamics of TNOs with the combined action of the giant outer planets and a massive, extended disc beyond Neptune. The duo’s calculations, which grew out of a seminar at the American University of Beirut, revealed that such a model can explain the perplexing spatially clustered orbits of some TNOs. In the process, they were able to identify ranges in the disc’s mass, its ’roundness’ (or eccentricity), and forced gradual shifts in its orientations (or precession rate), which faithfully reproduced the outlier TNO orbits.
“If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs,” said Sefilian, who is a Gates Cambridge Scholar and a member of Darwin College.
Earlier attempts to estimate the total mass of objects beyond Neptune have only added up to around one-tenth the mass of the Earth. However, in order for the TNOs to have the observed orbits and for there to be no Planet Nine, the model put forward by Sefilian and Touma requires the combined mass of the Kuiper Belt to be between a few to ten times the mass of the Earth.
“When observing other systems, we often study the disc surrounding the host star to infer the properties of any planets in orbit around it,” said Sefilian. “The problem is when you’re observing the disc from inside the system, it’s almost impossible to see the whole thing at once. While we don’t have direct observational evidence for the disc, neither do we have it for Planet Nine, which is why we’re investigating other possibilities. Nevertheless, it is interesting to note that observations of Kuiper belt analogues around other stars, as well as planet formation models, reveal massive remnant populations of debris.
“It’s also possible that both things could be true – there could be a massive disc and a ninth planet. With the discovery of each new TNO, we gather more evidence that might help explain their behaviour.”
Thermus scotoductus cells are visible at 1,000x magnification using a technique called phase-contrast microscopy. Scientists found the heat-loving microbes living in water heaters in homes across the United States. Credit: Zhidan Zhang / Penn State
Microbes that thrive in some of the most extreme places on Earth have discovered another cozy place to live — inside homes across the United States.
Extremophiles like those found in hot springs and thermal vents are also common in residential water heaters, according to a nationwide study that sheds new light on the extent of extremophile colonization in homes and provides insight about how the heat-loving microbes spread.
“Extremophiles are a huge part of the biosphere,” said Regina Wilpiszeski, a postdoctoral researcher at Oak Ridge National Laboratory and a recent Penn State doctoral recipient. “If we want to understand what life on Earth is doing, we need to understand it across the board.”
The researchers analyzed samples from water heaters in all 50 states, as well as Washington D.C. and Puerto Rico, and found evidence of microbes in about half of the homes. They reported their findings online in the journal Extremophiles.
Despite their presence, the microbes pose no health concerns for humans and water from the systems remains safe to drink.
A single species, Thermus scotoductus, dominated in all the positive samples in the study, even in locations near natural hot springs that host other similar but distinct strains.
“That included houses located near Yellowstone National Park, where we would expect other extreme organisms to live in the natural hot spring environment,” Wilpiszeski said. “That was a surprise.”
Thermus scotoductus has been found in nature, at hot springs in Iceland, hydrothermal waters off the coast of Hawaii and even deep in a gold mine in South Africa.
“Most of the time when you are thinking about extreme environments, you are thinking about going out into nature and into these weird and inaccessible places,” Wilpiszeski said. “But the truth is we spend the vast majority of our time indoors. And we really just don’t necessarily know what is living there.”
Water heaters offer high temperatures and low levels of organic matter, ideal conditions for extremophile microbe colonies, the researchers said.
“Water heaters are unique because they are isolated from each other by cooler temperatures in water lines,” said Christopher House, professor of geosciences and director of the Penn State Astrobiology Research Center and the NASA Pennsylvania Space Grant Consortium. “This study helps us learn more about how microbes are spreading and how they are inhabiting these environments we have inadvertently created for them.”
Cooler temperatures would not kill the microbes, meaning they could survive being transported between water heaters, scientists said. However, further study is needed to determine the exact paths extremophiles take to enter our homes.
Citizen scientists submitted filters and water samples from their homes as part of a collaboration between the NASA Astrobiology Institute and the NASA National Space Grant Fellowship and Scholarship Program.
Researchers analyzed DNA sequences found on the collected filters and grew cultures from the water samples. Together, these different data sets provided a novel view of the thermophilic microbes inhabiting water heaters across the nation. Analyses of the data included comparing genetic differences between samples to the physical distance between their water heater locations.
Heather Nelson, formerly of Penn State, oversaw the citizen science portion of the project. Zhidan Zhang, research technician in the Department of Geosciences, also contributed to the study.
The NASA Astrobiology Institute and National Space Grant Consortium funded this research.