Starship Asterisk*

[img3="Credit: ALMA (ESO/NAOJ/NRAO)/D. Fedele et al"]https://cdn.eso.org/images/screen/potw1714a.jpg[/img3][hr][/hr]

This image depicts the dusty disc encircling the young, isolated star HD 169142. The Atacama Large Millimeter/submillimeter Array (ALMA) imaged this disc in high resolution by picking up faint signals from its constituent millimetre-sized dust grains. The vivid rings are thick bands of dust, separated by deep gaps.

Optimised to study the cold gas and dust of systems like HD 169142, ALMA’s sharp eyes have revealed the structure of many infant solar systems with similar cavities and gaps. A variety of theories have been proposed to explain them — such as turbulence caused by magnetorotational instability, or the fusing of dust grains — but the most plausible explanation is that these pronounced gaps were carved out by giant protoplanets.

When solar systems form gas and dust coalesce into planets. These planets then effectively spring clean their orbits, clearing them of gas and dust and herding the remaining material into well-defined bands. The deep gaps seen in this image are consistent with the presence of multiple protoplanets — a finding that agrees with other optical and infrared studies of the same system.

Observing such dusty protoplanetary discs with ALMA allows scientists to investigate the first steps of planet formation in a bid to unveil the evolutionary paths of these infant systems.

A team of researchers led by Davide Fedele, Florence National Institute of Astrophysics, has detected the presence of two planets in formation around the young star HD 169142, that is about 470 light years from us. The two planets would masses comparable to that of Jupiter ...

• Astronomy & Astrophysics 600:A72 (Apr 2017) DOI: 10.1051/0004-6361/201629860
  arXiv.org > astro-ph > arXiv:1702.02844 > 09 Feb 2017

[img3="Credit: ESA/Hubble & NASA, L. Lamy"]https://cdn.spacetelescope.org/archives ... w1714a.jpg[/img3][hr][/hr]

Ever since Voyager 2 beamed home spectacular images of the planets in the 1980s, planet-lovers have been hooked on extra-terrestrial aurorae. Aurorae are caused by streams of charged particles like electrons, that come from various origins such as solar winds, the planetary ionosphere, and moon volcanism. They become caught in powerful magnetic fields and are channelled into the upper atmosphere, where their interactions with gas particles, such as oxygen or nitrogen, set off spectacular bursts of light.

The alien aurorae on Jupiter and Saturn are well-studied, but not much is known about the aurorae of the giant ice planet Uranus. In 2011, the NASA/ESA Hubble Space Telescope became the first Earth-based telescope to snap an image of the aurorae on Uranus. In 2012 and 2014 astronomers took a second look at the aurorae using the ultraviolet capabilities of the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.

They tracked the interplanetary shocks caused by two powerful bursts of solar wind travelling from the Sun to Uranus, then used Hubble to capture their effect on Uranus’ aurorae — and found themselves observing the most intense aurorae ever seen on the planet. By watching the aurorae over time, they collected the first direct evidence that these powerful shimmering regions rotate with the planet. They also re-discovered Uranus’ long-lost magnetic poles, which were lost shortly after their discovery by Voyager 2 in 1986 due to uncertainties in measurements and the featureless planet surface.

This is a composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the aurorae.

[img3="Credit: NASA, ESA, and A. Simon (GSFC)"]https://cdn.spacetelescope.org/archives ... c1708a.jpg[/img3][hr][/hr]

On April 3, 2017, as Jupiter made its nearest approach to Earth in a year, NASA’s Hubble Space Telescope viewed the solar system’s largest planet in all of its up-close glory. At a distance of 415 million miles (668 million kilometers) from Earth, Jupiter offered spectacular views of its colorful, roiling atmosphere, the legendary Great Red Spot, and it smaller companion at farther southern latitudes dubbed “Red Spot Jr.”

The giant planet is now at “opposition,” positioned directly opposite the Sun from the Earth. This means that the Sun, Earth, and Jupiter line up, with Earth sitting between the Sun and the gas giant. Opposition also marks Jupiter’s closest point to us, and the planet appears brighter in the night sky than at any other time in the year.

This positioning allowed a team led by Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, to observe Jupiter using Hubble’s Wide Field Camera 3. Hubble photographed exquisite details in Jupiter’s atmosphere, as small as about 80 miles (129 kilometers) across.

With its immense and powerful storms and hundreds of smaller vortices, the atmosphere of Jupiter is divided into several distinct, colorful bands, parallel to the equator. These bands, with alternating wind motions, are created by differences in the thickness and height of the ammonia ice clouds; the lighter bands rise higher and have thicker clouds than the darker bands. The bands are separated by winds that can reach speeds of up to 400 miles (644 kilometers) per hour.

Jupiter is best known for the Great Red Spot, an anticyclone that has raged for at least 150 years. This famous storm is larger than Earth. However, the Great Red Spot is slowly shrinking -- a trend seen since the late 1800s. The reason for this phenomenon is still unknown. Hubble will continue to observe Jupiter in hopes of solving this riddle.

The images are part of the Outer Planets Atmospheres Legacy program or OPAL. This program provides yearly Hubble global views of the outer planets to look for changes in their storms, winds, and clouds. It began in 2014 with Uranus, and has been studying Jupiter and Neptune since 2015. In 2018, it will begin viewing Saturn.

The team timed the Hubble observation to coincide with when NASA’s space probe Juno would be near its closest point to Jupiter, so that scientists could get concurrent observations.

[img3="Credit: ESO/P. Horálek"]https://cdn.eso.org/images/screen/potw1715a.jpg[/img3][hr][/hr]

The skies above ESO’s Paranal Observatory resemble oil on water in this ESO Picture of the Week, as greens, yellows, and blues blend to create an iridescent skyscape.

The rocky, barren landscape below evokes an image of an alien world, perfectly complementing the shimmering cosmic display occurring above. The main feature is our beautiful home galaxy, the Milky Way, arching across the Chilean night sky and framing the awestruck observer on the left. The light from billions of stars combines to create the Milky Way’s glow, with huge clouds of dark dust blocking the light here and there and creating the dark and mottled pattern we observe. A natural effect known as airglow is responsible for the swathes of green and orange light that appear to be emanating from the horizon.

ESO’s Very Large Telescope (VLT) can be seen as a speck in the distant background to the right atop Cerro Paranal. Its neighbour, slightly lower down, is the Visible and Infrared Survey Telescope for Astronomy (VISTA).

[img3="Credit: ESA/Hubble & NASA"]https://cdn.spacetelescope.org/archives ... w1715a.jpg[/img3][hr][/hr]

Despite all efforts galaxy formation and evolution are still far from being fully understood. Fortunately, the conditions we see within certain galaxies — such as so-called starburst galaxies — can tell us a lot about how they have evolved over time. Starburst galaxies contain a region (or many regions) where stars are forming at such a breakneck rate that the galaxy is eating up its gas supply faster than it can be replenished!

NGC 4536 is such a galaxy, captured here in beautiful detail by the Hubble’s Wide Field Camera 3 (WFC3). Located roughly 50 million light-years away in the constellation of Virgo (The Virgin), it is a hub of extreme star formation. There are several different factors that can lead to such an ideal environment in which stars can form at such a rapid rate. Crucially, there has to be a sufficiently massive supply of gas. This might be acquired in a number of ways — for example by passing very close to another galaxy, in a full-blown galactic collision, or as a result of some event that forces lots of gas into a relatively small space.

Star formation leaves a few tell-tale fingerprints, so astronomers can tell where stars have been born. We know that starburst regions are rich in gas. Young stars in these extreme environments often live fast and die young, burning extremely hot and exhausting their gas supplies fairly quickly. These stars also emit huge amounts of intense ultraviolet light, which blasts the electrons off any atoms of hydrogen lurking nearby (a process called ionisation), leaving behind clouds of ionised hydrogen (known in astronomer-speak as HII regions).

[img3="Credit: ESO/T. Stallard"]https://cdn.eso.org/images/screen/potw1716a.jpg[/img3][hr][/hr]

So big it could engulf several Earths, Jupiter’s Great Red Spot is a gigantic storm that has been raging for centuries with winds blasting at over 600 kilometres per hour. But it has a rival: astronomers have discovered that Jupiter has a second Great Spot, this time a cold one.

In the polar regions of the planet, astronomers using the CRIRES instrument on ESO's Very Large Telescope, along with other facilities, have found a dark spot in the upper atmosphere (below the aurora to the left) about 200 °C cooler than its surroundings. Aptly nicknamed the “Great Cold Spot”, this intriguing feature is comparable in size to the Great Red Spot — 24 000 km across and 12 000 km tall. But data taken over 15 years show that the Great Cold Spot is much more volatile than its slowly-changing cousin. It changes dramatically in shape and size over days and weeks — but never disappears, and always stays roughly in the same location.

The Great Cold Spot is thought to be caused by the planet’s powerful aurorae, which drive energy into the atmosphere in the form of heat that flows around the planet. This creates a cooler region in the upper atmosphere, making the Great Cold Spot the first weather system ever observed to be generated by aurorae.

[img3="Credit: ESA/Hubble & NASA"]https://cdn.spacetelescope.org/archives ... w1716a.jpg[/img3][hr][/hr]

This entrancing image shows a few of the tenuous threads that comprise Sh2-308, a faint and wispy shell of gas located 5200 light-years away in the constellation of Canis Major (The Great Dog).

Sh2-308 is a large bubble-like structure wrapped around an extremely large, bright type of star known as a Wolf-Rayet Star — this particular star is called EZ Canis Majoris. These type of stars are among the brightest and most massive stars in the Universe, tens of times more massive than our own Sun, and they represent the extremes of stellar evolution. Thick winds continually poured off the progenitors of such stars, flooding their surroundings and draining the outer layers of the Wolf-Rayet stars. The fast wind of a Wolf-Rayet star therefore sweeps up the surrounding material to form bubbles of gas.

EZ Canis Majoris is responsible for creating the bubble of Sh2-308 — the star threw off its outer layers to create the strands visible here. The intense and ongoing radiation from the star pushes the bubble out further and further, blowing it bigger and bigger. Currently the edges of Sh2-308 are some 60 light-years apart!

Beautiful as these cosmic bubbles are, they are fleeting. The same stars that form them will also cause their death, eclipsing and subsuming them in violent supernova explosions.

[img3="Credit: NASA/JPL-Caltech/SwRI/MSSS/Bjorn Jonsson"]https://photojournal.jpl.nasa.gov/jpeg/PIA21389.jpg[/img3][hr][/hr]

This enhanced color Jupiter image, taken by the JunoCam imager on NASA's Juno spacecraft, showcases several interesting features on the apparent edge (limb) of the planet.

Prior to Juno's fifth flyby over Jupiter's mysterious cloud tops, members of the public voted on which targets JunoCam should image. This picture captures not only a fascinating variety of textures in Jupiter's atmosphere, it also features three specific points of interest: "String of Pearls," "Between the Pearls," and "An Interesting Band Point." Also visible is what's known as the STB Spectre, a feature in Jupiter's South Temperate Belt where multiple atmospheric conditions appear to collide.

JunoCam images of Jupiter sometimes appear to have an odd shape. This is because the Juno spacecraft is so close to Jupiter that it cannot capture the entire illuminated area in one image -- the sides get cut off.

Juno acquired this image on March 27, 2017, at 2:12 a.m. PDT (5:12 a.m. EDT), as the spacecraft performed a close flyby of Jupiter. When the image was taken, the spacecraft was about 12,400 miles (20,000 kilometers) from the planet.

This enhanced color image was created by citizen scientist Bjorn Jonsson.