Sunday, August 19, 2018

Study of material surrounding distant stars shows Earth's ingredients 'pretty normal'

The Earth's building blocks seem to be built from 'pretty normal' ingredients, according to researchers working with the world's most powerful telescopes. Scientists have measured the compositions of 18 different planetary systems from up to 456 light years away and compared them to ours, and found that many elements are present in similar proportions to those found on Earth. This is amongst the largest examinations to measure the general composition of materials in other planetary systems, and begins to allow scientists to draw more general conclusions on how they are forged, and what this might mean for finding Earth-like bodies elsewhere. "Most of the building blocks we have looked at in other planetary systems have a composition broadly similar to that of the Earth", said researcher Dr Siyi Xu of the Gemini Observatory in Hawaii, who was presenting the work at the Goldschmidt conference in Boston. The first planets orbiting other stars were only found in 1992 (this was orbiting a pulsar), since then scientists have been trying to understand whether some of these stars and planets are similar to our own solar system. "It is difficult to examine these remote bodies directly. Because of the huge distances involved, their nearby star tends to drown out any electromagnetic signal, such as light or radio waves" said Siyi Xu. "So we needed to look at other methods".


Because of this, the team decided to look at how the planetary building blocks affect signals from white dwarf stars. These are stars which have burnt off most of their hydrogen and helium, and shrunk to be very small and dense - it is anticipated that our Sun will become a white dwarf in around 5 billion years.

Dr Xu continued, "White dwarfs' atmospheres are composed of either hydrogen or helium, which give out a pretty clear and clean spectroscopic signal. However, as the star cools, it begins to pull in material from the planets, asteroids, comets and so on which had been orbiting it, with some forming a dust disk, a little like the rings of Saturn.

"As this material approaches the star, it changes how we see the star. This change is measurable because it influences the star's spectroscopic signal, and allows us to identify the type and even the quantity of material surrounding the white dwarf. These measurements can be extremely sensitive, allowing bodies as small as an asteroid to be detected".

The team took measurements using spectrographs on the Keck telescope in Hawaii, the world's largest optical and infrared telescope, and on the Hubble Space Telescope.

Siyi Xu continued, "In this study, we have focused on the sample of white dwarfs with dust disks. We have been able to measure calcium, magnesium, and silicon content in most of these stars, and a few more elements in some stars. We may also have found water in one of the systems, but we have not yet quantified it: it's likely that there will be a lot of water in some of these worlds. For example, we've previously identified one star system, 170 light years away in the constellation Bootes, which was rich in carbon, nitrogen and water, giving a composition similar to that of Halley's Comet. In general though, their composition looks very similar to bulk Earth.

This would mean that the chemical elements, the building blocks of earth are common in other planetary systems. From what we can see, in terms of the presence and proportion of these elements, we're normal, pretty normal. And that means that we can probably expect to find Earth-like planets elsewhere in our Galaxy".

Dr Xu continued "This work is still on-going and the recent data release from the Gaia satellite, which so far has characterized 1.7 billion stars, has revolutionized the field. This means we will understand the white dwarfs a lot better. We hope to determine the chemical compositions of extrasolar planetary material to a much higher precision"

Professor Sara Seager, Professor of Planetary Science at Massachusetts Institute of Technology, is also the deputy science director of the recently-launched TESS (Transiting Exoplanet Survey Satellite) mission, which will search for exoplanets. She said:

"It's astonishing to me that the best way to study exoplanet interiors is by planets ripped apart and absorbed by their white dwarf host star. It is great to see progress in this research area and to have solid evidence that planets with Earth-like compositions are common--fueling our confidence that an Earth-like planet around a very nearby normal star is out there waiting to be found".

Friday, August 17, 2018

China unveils Chang'e-4 rover to explore Moon's far side

China's moon lander and rover for the Chang'e-4 lunar probe, which is expected to land on the far side of the moon this year, was unveiled Wednesday. Images displayed at Wednesday's press conference showed the rover was a rectangular box with two foldable solar panels and six wheels. It is 1.5 meters long, 1 meter wide and 1.1 meters high.Wu Weiren, the chief designer of China's lunar probe program, said the Chang'e-4 rover largely kept the shape and conditions of its predecessor, Yutu (Jade Rabbit), China's first lunar rover for the Chang'e-3 lunar probe in 2013. However, it also has adaptable parts and an adjustable payload configuration to deal with the complex terrain on the far side of the moon, the demand of relay communication, and the actual needs of the scientific objectives, according to space scientists. Like Yutu, the rover will be equipped with four scientific payloads, including a panoramic camera, infrared imaging spectrometer and radar measurement devices, to obtain images of moon's surface and detect lunar soil and structure. It will also endure vacuum, intense radiation and extremes of temperature. The moon has a large temperature difference between day and night, which can reach more than 300 degrees Celsius.


Both the lander and rover will carry international payloads for other countries.

The Chang'e-4 lunar probe will land on the Aitken Basin of the lunar south pole region on the far side of the moon, which is a hot spot for scientific and space exploration.

Direct communication with the far side of the moon, however, is not possible, which is one of the many challenges for the Chang'e-4 lunar probe mission.

China launched a relay satellite, named Queqiao, in May, to set up a communication link between the Earth and Chang'e-4 lunar probe.

The global public will have a chance to name the rover, according to State Administration of Science, Technology and Industry for National Defense.

Participants can submit their proposed names for the rover through the internet from Aug. 15 to Sept. 5, and the official name will be announced in October after several selection rounds.

Winners will be rewarded at most 3,000 yuan and invited to watch the lunar probe launch.

The name Yutu was chosen from 200,000 proposals submitted over two months worldwide.

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.