Thursday, August 16, 2018

Impact of a stellar intruder on our solar system

The solar system was formed from a protoplanetary disk consisting of gas and dust. Since the cumulative mass of all objects beyond Neptune is much smaller than expected and the bodies there have mostly inclined, eccentric orbits it is likely that some process restructured the outer solar system after its formation. Susanne Pfalzner from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and her colleagues present a study showing that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density in the outer part of the solar system and excite the bodies there onto eccentric, inclined orbits. Their numerical simulations show that many additional bodies at high inclinations still await discovery, perhaps including a sometimes postulated Planet X. The findings are published in the present issue of "The Astrophysical Journal." A near catastrophe billions of years ago might have shaped the outer parts of the solar system, while leaving the inner regions basically untouched. Researchers from the Max Planck Institute for Radio Astronomy in Bonn and their collaborators found that a close fly-by of another star can explain many of the features observed in the outer solar system.


"Our group has been looking for years at what fly-bys can do to other planetary systems never considering that we actually might live right in such a system," says Susanne Pfalzner, the leading author of the project. "The beauty of this model lies in its simplicity."

The basic scenario of the formation of the solar system has long been known: our Sun was born from a collapsing cloud of gas and dust. In the process a flat disk was formed where not only large planets grew but also smaller objects like the asteroids, dwarf planets, etc. Due to the flatness of the disk one would expect that the planets orbit in a single plane unless something dramatic happened afterwards.

Looking at the solar system right to the orbit of Neptune everything seems fine: most planets move on fairly circular orbits and their orbital inclinations vary only slightly. However, beyond Neptune things become very messy. The biggest puzzle is the dwarf planet Sedna, which moves on an inclined, highly eccentric orbit and is so far outside, that it could not have been scattered by the planets there.

Just outside Neptune's orbit another strange thing happens. The cumulative mass of all the objects dramatically drops by almost three orders of magnitude. This happens at approximately the same distance where everything becomes messy. It might be coincidental, but such coincidences are rare in nature.

Susanne Pfalzner and her co-workers suggest that a star was approaching the Sun at an early stage, 'stealing' most of the outer material from the Sun's protoplanetary disk and throwing what was left over into inclined and eccentric orbits.

Performing thousands of computer simulations they checked what would happen when a star passes very close-by and perturbs the once larger disk. It turned out that the best fit for today's outer solar systems comes from a perturbing star which had the same mass as the Sun or somewhat lighter (0.5-1 solar mass) and flew past at approximately three times the distance of Neptune.

However, the most surprising thing for the researchers was that a fly-by does not only explain the strange orbits of the objects of the outer solar system, but also gives a natural explanation for several unexplained features of our solar system, including the mass ratio between Neptune and Uranus, and the existence of two distinct populations of Kuiper Belt objects.

"It is important to keep exploring all the possible avenues for explaining the structure of the outer solar system. The data are increasing but still too sparse, so theories have a lot of wiggle room to develop," says Pedro Lacerda from the Queen's University in Belfast, a co-author of the paper. "There is a certain danger that one theory crystallises as truth, not because it explains the data better but because of other pressures. Our paper shows that a lot of what we currently know can be explained by something as simple as a stellar fly-by."

The big question is the likelihood for such an event. Nowadays, fly-bys even hundreds of times more distant are luckily rare. However, stars like our Sun are typically born in large groups of stars which are much more densely packed. Therefore, close fly-bys were significantly more common in the distant past. Performing another type of simulation, the team found that there was a 20%-30% chance of experiencing a fly-by over the first billion years of the Sun's life.

This is no final proof that a stellar fly-by caused the messy features of the outer solar system, but it can reproduce many observational facts and seems relatively realistic. So far it is the simplest explanation and if simplicity is a sign for validity this model is the best candidate so far.

"In summary, our close fly-by scenario offers a realistic alternative to present models suggested to explain the unexpected features of the outer solar system," concludes Susanne Pfalzner. "It should be considered as an option for shaping the outer solar system. The strength of the fly-by hypothesis lies in the explanation of several outer solar system features by one single mechanism."

Wednesday, August 15, 2018

Iron and Titanium in the Atmosphere of an Exoplanet

Exoplanets, planets in other solar systems, can orbit very close to their host star. When, in addition to this, the host star is much hotter than our Sun, then the exoplanet becomes as hot as a star. The hottest "ultra-hot" planet was discovered last year by American astronomers. Today, an international team, led by researchers from the University of Geneva (UNIGE), who joined forces with theoreticians from the University of Bern (UNIBE), Switzerland, discovered the presence of iron and titanium vapours in the atmosphere of this planet. The detection of these heavy metals was made possible by the surface temperature of this planet, which reaches more than 4,000 degrees [Celsius]. This discovery is published in the journal Nature [www.nature.com]. KELT-9 is a star located 650 light-years from Earth in the constellation Cygnus (the Swan). With a temperature of over 10,000 degrees [Celsius], it is almost twice as hot as the Sun. This star is orbited by a giant gas planet, KELT-9b, which is 30 times closer than the Earth's distance from the Sun. Because of this proximity, the planet circles its star in 36 hours and is heated to a temperature of over 4,000 degrees. It's not as hot as the Sun, but hotter than many stars. At present, we do not yet know what an atmosphere looks like and how it can evolve under such conditions.


That is why NCCR PlanetS researchers affiliated with the University of Bern recently performed a theoretical study on the atmosphere of the planet KELT-9b.

"The results of these simulations show that most of the molecules found there should be in atomic form, because the bonds that hold them together are broken by collisions between particles that occur at these extremely high temperatures," explains Kevin Heng, professor at the UNIBE. This is a direct consequence of the extreme temperature. Their study also predicts that it should be possible to observe gaseous atomic iron, in the planet's atmosphere using current telescopes.


Light Reveals the Chemical Components of the Atmosphere
The UNIGE FOUR ACES team, which is also part of the NCCR PlanetS at the Department of Astronomy of the Faculty of Science of the UNIGE, had observed this planet precisely as it was moving in front of its host star (i.e., during a transit). During transit, a tiny fraction of the light from the star filters through the planet's atmosphere, and analysis of this filtered light can reveal the chemical composition of the atmosphere.

This is achieved with a spectrograph, an instrument that spreads white light into its component colours, called a spectrum. If present among the components of the atmosphere, iron vapour would leave a well-recognisable fingerprint in the spectrum of the planet.

Using the HARPS-North spectrograph, built in Geneva and installed on the Telescopio Nazionale Galileo in La Palma, astronomers discovered a strong signal corresponding to iron vapour in the planet's spectrum. "With the theoretical predictions in hand, it was like following a treasure map," says Jens Hoeijmakers, a researcher at the Universities of Geneva and Bern and lead author of the study, "and when we dug deeper into the data, we found even more," he adds with a smile. Indeed, the team also detected the signature of another metal in vapour form: titanium.

This discovery reveals the atmospheric properties of a new class of so-called "ultra-hot Jupiter." However, scientists believe that many exoplanets have completely evaporated in environments similar to KELT-9b. Although this planet is probably massive enough to withstand total evaporation, this new study demonstrates the strong impact of stellar radiation on the composition of the atmosphere. Indeed, these observations confirm that the high temperatures reigning on this planet break apart most molecules, including those containing iron or titanium.

In cooler giant exoplanets, these atomic species are thought to be hidden within gaseous oxides or in the form of dust particles, making them hard to detect. This is not the case on KELT-9b. "This planet is a unique laboratory to analyze how atmospheres can evolve under intense stellar radiation," concludes David Ehrenreich, principal investigator with the UNIGE's FOUR ACES team.

Tuesday, August 14, 2018

India's Second Moon Mission as "Complex" as NASA's Apollo Mission

The Indian Space Agency had planned the launch of its second moon mission for October this year, but scientists reviewing their preparedness suggested that more tests were needed before the launch. The mission is now likely to be preceded by Israel's moon mission, planned for December this year. The Indian Space Research Organisation (ISRO) has announced the postponement of its much-awaited second lunar mission - Chandrayaan 2. The mission was expected to be launched in October this year but ISRO says it will now conduct it in the first quarter of 2019. "We are aiming to launch the mission on January 3 next year, but the window to land on the lunar surface is open until March 2019. Chandrayaan-2 mission is the most complex mission attempted by ISRO so far. The mission has three components - orbiter, lander, and rover. We set up a committee of eminent scientists from across the country which studied the project and suggested changes. It is nothing less than the Apollo mission," ISRO Chairman K. Sivan told reporters in Bengaluru. Apollo was the NASA program that resulted in American astronauts' making a total of 11 spaceflights and walking on the moon. The first moon landing took place in 1969. The last moon landing was in 1972.


ISRO has increased the weight of Chandrayaan-2 by 600 kg as the space scientists had noticed during experiments that after the moon lander was ejected, the satellite would shake. So they decided that design modification was required for landing and the mass had to be increased.

The total estimated cost of the mission is about INR 8 billion ($124 million), which includes INR 2 billion ($31 million) at the cost of launching and INR 6 billion ($93 million) for the satellite.

ISRO has pointed out that the success rate of lunar landing missions is less than 50% as 27 had failed out of 47 lunar landings.

Saturday, August 11, 2018

NASA postpones for 24 hours launch of historic spaceship to Sun

NASA postponed until Sunday the launch of the first ever spacecraft to fly directly toward the Sun on a mission to plunge into our star's sizzling atmosphere and unlock its mysteries. The reason for the delay was not immediately clear, but was called for after a gaseous helium alarm was sounded in the last moments before liftoff, officials said. Engineers are taking utmost caution with the $1.5 billion Parker Solar Probe, which Thomas Zurbuchen, head of NASA's science mission directorate, described as one of the agency's most "strategically important missions." The next launch window opens at 3:31 am (0731 GMT) on Sunday, when weather conditions are 60 percent favorable for launch, NASA said. By coming closer to the Sun than any spacecraft in history, the unmanned probe's main goal is to unveil the secrets of the corona, the unusual atmosphere around the Sun. Not only is the corona about 300 times hotter than the Sun's surface, but it also hurls powerful plasma and energetic particles that can unleash geomagnetic space storms, wreaking havoc on Earth by disrupting the power grid. These solar outbursts are poorly understood, but pack the potential to wipe out power to millions of people.


- 'Full of mysteries' -

The probe is protected by an ultra-powerful heat shield that is 4.5 inches (11.43 centimeters) thick.

The shield should enable the spacecraft to survive its close shave with the fiery star, coming within 3.83 million miles (6.16 million kilometers) of the Sun's surface.

The heat shield is built to withstand radiation equivalent to up to about 500 times the Sun's radiation on Earth.

Even in a region where temperatures can reach more than a million degrees Fahrenheit, the sunlight is expected to heat the shield to just around 2,500 degrees Fahrenheit (1,371 degrees Celsius).

If all works as planned, the inside of the spacecraft should stay at just 85 degrees Fahrenheit.

"The sun is full of mysteries," said Nicky Fox, project scientist at the Johns Hopkins University Applied Physics Lab.

- 91-year-old namesake -

The tools on board will measure the expanding corona and continually flowing atmosphere known as the solar wind, which solar physicist Eugene Parker first described in 1958.

Parker, now 91, recalled that at first some people did not believe in his theory.

But then, the launch of NASA's Mariner 2 spacecraft in 1962 -- becoming the first robotic spacecraft to make a successful planetary encounter -- proved them wrong.

"It was just a matter of sitting out the deniers for four years until the Venus Mariner 2 spacecraft showed that, by golly, there was a solar wind," Parker said earlier this week.

Parker said he was "impressed" by the Parker Solar Probe, calling it "a very complex machine."

According to Zurbuchen, Parker is an "incredible hero of our scientific community."

"Life is all about these big arcs. Sometimes you just see, like how over a lifetime, things just come together and create these amazing stories, these leaps going forward."

Friday, August 10, 2018

Satellite measurements of the Earth's magnetosphere promise better space weather forecasts

Earth is constantly being hammered by charged particles emitted by the Sun that have enough power to make life on Earth almost impossible. We survive because Earth's magnetic field traps and deflects these particles, preventing the vast majority of them from ever reaching the planet's surface. The trapped particles bounce back and forth between the North and South poles in complex, ever-changing patterns that are also influenced by equally intricate and shifting electric fields. We get to enjoy the sight of those particles when the bands they move in (the Van Allen radiation belts) dip into our atmosphere near the poles creating the Northern (and Southern) lights. However, bursts of these particles can damage satellites and sensitive equipment on the ground. It is therefore vital to understand the intricacies of the radiation belts. So far, NASA have launched twin satellites to study the Van Allen belts--however, their orbits only allow them to explore the equatorial regions. This limits our ability to understand flow of particles and prevents us from predicting their effects on all satellites. To also explore regions further from the equator, the Institute of Space and Astronautical Science, a division of the Japan Aerospace Exploration Agency, launched the Arase satellite in 2016.


A Japan-based research team centered at Kanazawa University equipped the Arase satellite with multiple different sensors (termed the Plasma Wave Experiment) to probe the electric field and plasma waves in the Earth's inner magnetosphere. Now, they have collected their first set of data from their sensors, which they recently published in the Springer journal Earth, Planets and Space.

The Arase consists primarily of electric and magnetic field detectors covering a wide frequency range; it can also measure plasma/particles in a wide energy range. To improve efficiency, an on-board computer studies the correlations between the fields and the particles before sending only the most important information back to Earth.

"The Plasma Wave Experiment equipment has passed initial checks and has successfully acquired high quality data. Huge amount of burst waveform data has been taken, and we should soon know a lot more about mechanisms of wave-particle interaction occurring in the inner magnetosphere than before.

Another strength of our project is that we can also compare the satellite data with data collected simultaneously on the ground. We expect those comparisons will greatly broaden our understanding of this area of science," first author Yoshiya Kasahara says.

Understanding how electrons and other particles are hurled out of the magnetosphere onto our planet could be key to predicting such bursts and protecting against them.