ESO (European Southern Observatory) releases another report this week:

Stellar Outburst Brings Water Snow Line Into View 

The Atacama Large Millimeter/submillimeter Array (ALMA) has made the first ever resolved observation of a water snow line within a protoplanetary disc. This line marks where the temperature in the disc surrounding a young star drops sufficiently low for snow to form. A dramatic increase in the brightness of the young star V883 Orionis flash heated the inner portion of the disc, pushing the water snow line out to a far greater distance than is normal for a protostar, and making it possible to observe it for the first time. The results are published in the journal Nature on 14 July 2016.

Young stars are often surrounded by dense, rotating discs of gas and dust, known as protoplanetary discs, from which planets are born. The heat from a typical young solar-type star means that the water within a protoplanetary disc is gaseous up to distances of around 3 au from the star [1] — less than 3 times the average distance between the Earth and the Sun — or around 450 million kilometres [2]. Further out, due to the extremely low pressure, the water molecules transition directly from a gaseous state to form a patina of ice on dust grains and other particles. The region in the protoplanetary disc where water transitions between the gas and solid phases is known as the water snow line [3].

But the star V883 Orionis is unusual. A dramatic increase in its brightness has pushed the water snow line out to a distance of around 40 au (about 6 billion kilometres or roughly the size of the orbit of the dwarf planet Pluto in our Solar System). This huge increase, combined with the resolution of ALMA at long baselines [4], has allowed a team led by Lucas Cieza (Millennium ALMA Disk Nucleus and Universidad Diego Portales, Santiago, Chile) to make the first ever resolved observations of a water snow line in a protoplanetary disc.

Zooming on the young star V883 Orionis. This star is currently in outburst, which has pushed the water snow line further from the star and allowed it to be detected for the first time with ALMA. Credit: ESO/Digitized Sky Survey 2/N. Risinger (skysurvey.org)/M. Kornmesser. Music: Johan B. Monell

The sudden brightening that V883 Orionis experienced is an example of what occurs when large amounts of material from the disc surrounding a young star fall onto its surface. V883 Orionis is only 30% more massive than the Sun, but thanks to the outburst it is experiencing, it is currently a staggering 400 times more luminous — and much hotter [5].

Lead author Lucas Cieza explains:

“The ALMA observations came as a surprise to us. Our observations were designed to look for disc fragmentation leading to planet formation. We saw none of that; instead, we found what looks like a ring at 40 au. This illustrates well the transformational power of ALMA, which delivers exciting results even if they are not the ones we were looking for.”

The bizarre idea of snow orbiting in space is fundamental to planet formation. The presence of water ice regulates the efficiency of the coagulation of dust grains — the first step in planet formation. Within the snow line, where water is vapourised, smaller, rocky planets like our own are believed to form. Outside the water snow line, the presence of water ice allows the rapid formation of cosmic snowballs, which eventually go on to form massive gaseous planets such as Jupiter.

The discovery that these outbursts may blast the water snow line to about 10 times its typical radius is very significant for the development of good planetary formation models. Such outbursts are believed to be a stage in the evolution of most planetary systems, so this may be the first observation of a common occurrence. In that case, this observation from ALMA could contribute significantly to a better understanding of how planets throughout the Universe formed and evolved.

Notes

[1] ] 1 au, or one astronomical unit, is the mean distance between the Earth and the Sun, around 149.6 million kilometres.This unit is typically used to describe distances measured within the Solar System and planetary systems around other stars.

[2] This line was between the orbits of Mars and Jupiter during the formation of the Solar System, hence the rocky planets Mercury, Venus, Earth and Mars formed within the line, and the gaseous planets Jupiter, Saturn, Uranus and Neptune formed outside.

[3] The snow lines for other molecules, such as carbon monoxide and methane, have been observed previously with ALMA, at distances of greater than 30 au from the protostar within other protoplanetary discs. Water freezes at a relatively high temperature and this means that the water snow line is usually much too close to the protostar to observe directly.

[4] Resolution is the ability to discern that objects are separate. To the human eye, several bright torches at a distance would seem like a single glowing spot, and only at closer quarters would each torch be distinguishable. The same principle applies to telescopes, and these new observations have exploited the exquisite resolution of ALMA in its long baseline modes. The resolution of ALMA at the distance of V883 Orionis is about 12 au — enough to resolve the water snow line at 40 au in this outbursting system, but not for a typical young star.

[5] Stars like V883 Orionis are classed as FU Orionis stars, after the original star that was found to have this behaviour. The outbursts may last for hundreds of years.

A new report from ESO (European Southern Observatory):

Deepest Ever Look into Orion

ESO’s HAWK-I infrared instrument on the Very Large Telescope (VLT) in Chile has been used to peer deeper into the heart of Orion Nebula than ever before. The spectacular picture reveals about ten times as many brown dwarfs and isolated planetary-mass objects than were previously known. This discovery poses challenges for the widely accepted scenario for Orion’s star formation history.

An international team has made use of the power of the HAWK-I infrared instrument on ESO’s Very Large Telescope (VLT) to produce the deepest and most comprehensive view of the Orion Nebula [1] to date. Not only has this led to an image of spectacular beauty, but it has revealed a great abundance of faint brown dwarfs and isolated planetary-mass objects. The very presence of these low-mass bodies provides an exciting insight into the history of star formation within the nebula itself.

The famous Orion Nebula spans about 24 light-years within the constellation of Orion, and is visible from Earth with the naked eye, as a fuzzy patch in Orion’s sword. Some nebulae, like Orion, are strongly illuminated by ultraviolet radiation from the many hot stars born within them, such that the gas is ionised and glows brightly.

The relative proximity of the Orion Nebula [2] makes it an ideal testbed to better understand the process and history of star formation, and to determine how many stars of different masses form.

This video gives a close-up view of a spectacular new image of the Orion Nebula star-formation region that was obtained from multiple exposures using the HAWK-I infrared camera on ESO’s Very Large Telescope in Chile. This is the deepest view ever of this region and reveals many more very faint planetary-mass objects than expected. Credit: ESO/H. Drass et al. Music: Johan B. Monell (www.johanmonell.com)

Amelia Bayo (Universidad de Valparaíso, Valparaíso, Chile; Max-Planck Institut für Astronomie, Königstuhl, Germany), a co-author of the new paper and member of the research team, explains why this is important:

“Understanding how many low-mass objects are found in the Orion Nebula is very important to constrain current theories of star formation. We now realise that the way these very low-mass objects form depends on their environment.“

This new image has caused excitement because it reveals a unexpected wealth of very-low-mass objects, which in turn suggests that the Orion Nebula may be forming proportionally far more low-mass objects than closer and less active star formation regions.

This video starts from a wide view of the night sky and zooms in on the famous constellation of Orion. The final part gives a close-up view of a spectacular new image of the Orion Nebula star-formation region that was obtained from multiple exposures using the HAWK-I infrared camera on ESO’s Very Large Telescope in Chile. This is the deepest view ever of this region and reveals more very faint planetary-mass objects than expected. Credit: ESO/H. Drass et al./N. Risinger (skysurvey.org)/M. Kornmesser. Music: Johan Monell (www.johanmonell.com)

Astronomers count up how many objects of different masses form in regions like the Orion Nebula to try to understand the star-formation process [3]. Before this research the greatest number of objects were found with masses of about one quarter that of our Sun. The discovery of a plethora of new objects with masses far lower than this in the Orion Nebula has now created a second maximum at a much lower mass in the distribution of star counts.

This sequence compares an infrared image of the Orion Nebula star-formation region that was obtained from multiple exposures using the HAWK-I infrared camera on ESO’s Very Large Telescope with a picture of the same part of the sky imaged in visible light with the WFI camera on the MPG/ESO 2.2-metre telescope. The longer wavelength light detected by HAWK-I can penetrate the dusty regions of the nebula and expose many young stars that are normally invisible and also reveal many curious features created by very young stars and the jets that they expel. Credit: ESO/H. Drass/Igor Chekalin. Music: Johan B. Monell (www.johanmonell.com)

These observations also hint tantalisingly that the number of planet-sized objects might be far greater than previously thought. Whilst the technology to readily observe these objects does not exist yet, ESO’s future European Extremely Large Telescope (E-ELT), scheduled to begin operations in 2024, is designed to pursue this as one of its goals.

Lead scientist Holger Drass (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany; Pontificia Universidad Católica de Chile, Santiago, Chile) enthuses:

“Our result feels to me like a glimpse into a new era of planet and star formation science. The huge number of free-floating planets at our current observational limit is giving me hope that we will discover a wealth of smaller Earth-sized planets with the E-ELT.”

Notes

[1] Nebulae such as the famous one in Orion are also known as H II regions to indicate that they contain ionised hydrogen. These immense clouds of interstellar gas are sites of star formation throughout the Universe.

[2] The Orion Nebula is estimated to lie about 1350 light-years from Earth.

[3] This information is used to create something called the Initial Mass Function (IMF) — a way of describing how many stars of different masses make up a stellar population at its birth. This provides an insight into the stellar population’s origins. In other words, determining an accurate IMF, and having a solid theory to explain the origin of the IMF is of fundamental importance in the study of star formation.

A new report from ESO (European Southern Observatory):

A Surprising Planet with Three Suns

A team of astronomers have used the SPHERE instrument on ESO’s Very Large Telescope to image the first planet ever found in a wide orbit inside a triple-star system. The orbit of such a planet had been expected to be unstable, probably resulting in the planet being quickly ejected from the system. But somehow this one survives. This unexpected observation suggests that such systems may actually be more common than previously thought. The results will be published online in the journal Science on 7 July 2016.

This artists impression shows the orbit of the planet in the triple-star system HD 131399. Two of the stars are close together and the third, brighter component is orbited by a gas giant planet named HD 131399Ab.

Luke Skywalker‘s home planet, Tatooine, in the Star Wars saga, was a strange world with two suns in the sky, but astronomers have now found a planet in an even more exotic system, where an observer would either experience constant daylight or enjoy triple sunrises and sunsets each day, depending on the seasons, which last longer than human lifetimes.

This world has been discovered by a team of astronomers led by the University of Arizona, USA, using direct imaging at ESO’s Very Large Telescope (VLT) in Chile. The planet, HD 131399Ab [1], is unlike any other known world — its orbit around the brightest of the three stars is by far the widest known within a multi-star system. Such orbits are often unstable, because of the complex and changing gravitational attraction from the other two stars in the system, and planets in stable orbits were thought to be very unlikely.

Located about 320 light-years from Earth in the constellation of Centaurus (The Centaur), HD 131399Ab is about 16 million years old, making it also one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets.

“HD 131399Ab is one of the few exoplanets that have been directly imaged, and it’s the first one in such an interesting dynamical configuration,”

said Daniel Apai, from the University of Arizona, USA, and one of the co-authors of the new paper.

“For about half of the planet’s orbit, which lasts 550 Earth-years, three stars are visible in the sky; the fainter two are always much closer together, and change in apparent separation from the brightest star throughout the year,”

adds Kevin Wagner, the paper’s first author and discoverer of HD 131399Ab [2].

Kevin Wagner, who is a PhD student at the University of Arizona, identified the planet among hundreds of candidate planets and led the follow-up observations to verify its nature.

The planet also marks the first discovery of an exoplanet made with the SPHERE instrument on the VLT. SPHERE is sensitive to infrared light, allowing it to detect the heat signatures of young planets, along with sophisticated features correcting for atmospheric disturbances and blocking out the otherwise blinding light of their host stars.

Although repeated and long-term observations will be needed to precisely determine the planet’s trajectory among its host stars, observations and simulations seem to suggest the following scenario: the brightest star is estimated to be eighty percent more massive than the Sun and dubbed HD 131399A, which itself is orbited by the less massive stars, B and C, at about 300 au (one au, or astronomical unit, equals the average distance between the Earth and the Sun). All the while, B and C twirl around each other like a spinning dumbbell, separated by a distance roughly equal to that between the Sun and Saturn (10 au).

In this scenario, planet HD 131399Ab travels around the star A in an orbit with a radius of about 80 au, about twice as large as Pluto’s in the Solar System, and brings the planet to about one third of the separation between star A and the B/C star pair. The authors point out that a range of orbital scenarios is possible, and the verdict on the long-term stability of the system will have to wait for planned follow-up observations that will better constrain the planet’s orbit.

“If the planet was further away from the most massive star in the system, it would be kicked out of the system,” Apai explained. “Our computer simulations have shown that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly.”

Planets in multi-star systems are of special interest to astronomers and planetary scientists because they provide an example of how the mechanism of planetary formation functions in these more extreme scenarios. While multi-star systems seem exotic to us in our orbit around our solitary star, multi-star systems are in fact just as common as single stars.

“It is not clear how this planet ended up on its wide orbit in this extreme system, and we can’t say yet what this means for our broader understanding of the types of planetary systems, but it shows that there is more variety out there than many would have deemed possible,” concludes Kevin Wagner. “What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems.”

Notes

[1] The three components of the triple star are named HD 131399A, HD 131399B and HD 131399C respectively, in decreasing order of brightness. The planet orbits the brightest star and hence is named HD 131399Ab.

[2] For much of the planet’s year the stars would appear close together in the sky, giving it a familiar night-side and day-side with a unique triple sunset and sunrise each day. As the planet moves along its orbit the stars grow further apart each day, until they reach a point where the setting of one coincides with the rising of the other — at which point the planet is in near-constant daytime for about one-quarter of its orbit, or roughly 140 Earth-years.

This sequence takes the viewer deep into the constellation of Centaurus in the southern skies. We settle on the unremarkable-looking star called HD 131399, which is a triple star whose brightest component is orbited by a unique exoplanet called HD 131399Ab.