Space Sunday: tiny stars & giant planets, and an interstellar visitor

An artist’s impression of the Neptune-sized world orbiting white dwarf WDJ0914+1914,. While the star is “dead” it is still hot enough for its solar wind to be slowly ripping away the planet’s outer atmosphere, most of which is lost to space, while some of it swirls into a disc, itself accreting onto the white dwarf, further heating it. Credit: ESO / M. Kornmesser

When a star like our own reaches the end of its life, two things happen: first, in a desperate attempt to keep itself burning after using its hydrogen and helium, it expands outwards into a red giant as it burns heavier elements in turn (our Sun will expand to a size sufficient to consume Mercury, Venus and Earth) before it collapses into a hot, white dwarf, a fraction of its former size (perhaps no bigger than the Earth).

But what of any gas giants orbiting the star well beyond the limits of its red giant expansion? What happens to them following the star’s collapse to a white dwarf? Do they simply continue until such time as their own internal heating fails? Might they have some additional interaction with their former parent?

A team from Warwick University, England, appear to have the answer. They’ve discovered a Neptune-sized planet some 4 times larger than its white dwarf host star, and the two have entered into what is – at this point in our understanding of such situations – a unique relationship.

The star is called WDJ0914+1914 and is some 2,000 light years away. Whilst reviewing data on it gathered by the Sloan Digital Sky Survey (SDSS), the astronomers came across something odd: the star was apparently giving off oxygen, sulphur and hydrogen emissions. While the oxygen was to be expected – by this time in a star’s life most of what is left is actually oxygen and carbon – the hydrogen and sulphur simply shouldn’t have been there.

Turning to the Very Large Telescope (VLT), the Warwick team found the emissions corresponded to to a ring of gas surrounding the star. At first they thought they had discovered a binary system in which the mass of one star was being drawn off by the other, forming a dust ring around both. However, further analysis revealed the composition of the disc matches the deeper layers of planets in our own Solar System like Neptune and Uranus, suggesting a planetary body still exists orbiting the star and material from that planet is feeding the disc, allowing it to survive.

While fusion has long since ended at WDJ0914+1914, the star is still radiating at some 28,000ºC – enough energy to tear material from the upper layers of a planet’s atmosphere. Much of this atmosphere would trail outwards from the planet as a hot plume – which the Warwick team detected – while some would collapse to feed the disc of material surrounding the star.

Putting their calculations together, the Warwick team worked out that the planet – which cannot be directly sighted – is likely to be around the size of Neptune, and it is losing its atmosphere at a rate of around 2,700 tonnes per second to both to the disc of material around the star, and eventually onto the star itself – “feeding” it, if you will. Although this sounds a lot, it actually adds up to a relative small amount given the size of the planet, and so the loss is unlikely to alter its overall structure as the star continues to cool.

This discovery at WDJ0914+1914 is unique at the moment – but it makes the case that other white dwarf stars may also be survived by planets, some of which we may be able to detect using the transiting method of observation (WDJ0914+1914 is simply too dim for this to work). Certainly, the Warwick team’s research has opened the door on this form of research, one that could help with our understanding of exoplanet atmospheres.  It also offers a cold look at the far future (roughly 4.5 to 5 billion years from now) of our own solar system.

New Dates For Commercial Crew Test Flights

NASA has issued new dates for the final test flights for the SpaceX Crew Dragon and Boeing CST-100 Starliner that, if passed, should allow both vehicles to move on towards actually transporting astronauts to and from the International Space Station.

On December 20th, 2019, a United Launch Alliance Atlas V will launch the first CST-100 Starliner into orbit on an uncrewed orbital test flight (OTF) to the International Space Station. As well as testing the Starliner’s avionics and flight systems, the flight will also test a new docking system that is intended to become the “”standard docking system for sending humans to Gateway and to Mars” as a part of the Artemis programme, and used to deliver additional supplies and some Christmas / New Year’s extras to the ISS crew.

The first Boeing CST-100 vehicle being transported from Boeing’s fabrication centre at Kennedy Space Centre on its way to the Space Launch Complex 41 Vertical Integration Facility at Cape Canaveral Air Force Station in readiness for its flight, November 21st, 2019. Credit: Boeing

Also flying on the vehicle will be a flight test dummy christened “Rosie the Rocketeer”, named for “Rosie the Riveter”, the iconic role model for U.S. women working in factories and on production lines in WW II. The dummy is fitted with an array of sensors to measure critical data including G-forces endured during the flight to inform the team about the stresses a human crew will experience during an ascent to orbit on the vehicle. Results from this data, and all telemetry gathered during the flight will help inform NASA and Boeing on the Starliner’s readiness to commence crewed flights.

The vehicle will not spend long at the ISS – it will be undocked on December 28th and make a return to Earth in a full dress-rehearsal for a crewed landing for the CST-100 capsule. Should weather interfere with the planned launch, both December 21st and 23rd offer suitable windows for the launch to take place.

The first Boeing CST-100 mounted atop its United Launch Alliance Atlas V booster at Space Launch Complex 41, Cape Canaveral Air Force Station, December 4th, 2019. Credit: Boeing

Then, on January 4th, 2020, SpaceX is expected to complete an in-flight abort test. For that test, a Falcon 9 will lift off from Launch Complex 39A at Kennedy Space Centre carrying a test Crew Dragon vehicle – which has previously performed  a successful static fire test of its SuperDraco escape motors in November. Around 90 seconds into the flight, and the time of maximum dynamic pressure on the vehicle, the escape system will be triggered, the capsule hopefully escaping the rocket to make a safe splash-down under parachute.

SpaceX had hoped to complete this test before the end of the year, but assorted delays – including that of the CRS-19 resupply mission, which launched earlier in December (see: On the ISS – mighty mice and robots) – meant that target could not be met. If the abort flight test is successful, it should allow NASA and SpaceX to determine when crewed flights to the ISS can commence – an uncrewed test flight of the vehicle to the ISS having been completed in March 2019.

Overall. NASA would like both Boeing and SpaceX to complete their first crewed flights to the ISS – also regarded as test flights – by mid-2020.

Borisov Passes Perihelion

C/2019 Q4 (2I/Borisov, the “i” being for “interstellar” – or just “Borisov for short), the second confirmed extra-solar object to pass through our solar system (see Exoplanets, exocomets and Titan’s craters for more), made its closest approach to the Sun in early December and is now officially making its way back out towards interstellar space.

The object was first observed by Russian amateur astronomer Gennady Borisov – from whom it takes its name – on August 30th, 2019. Data gathered on the object revealed it to be travelling with a hyperbolic excess velocity of about 34 km per second – or London to New York in under 3 minutes – marking it as extra-solar in origin (see: exoplanets, exocomets and Titan’s craters for more about the early sightings).

Mapping the progress of C/2019 Q4 (2I/Borisov) through the solar system. Credit: NASA/JPL

What is particularly interesting about Borisov is that, unlike the first confirmed extra-solar object we’ve spotted zipping through the solar system (`Oumuamua – see here (October 2017), here (November 2017) and here and here (both November 2018) for more), it has been out-gassing and forming a tail, just like a solar system comet.

In the months since its discovery and confirmation, Borisov has been the subject of intense study. It’s now believed to be around 1 km across, putting it at the upper end of the scale for the estimated length of `Oumuamua – although it could be as large as 6 km across. This makes it somewhat smaller than original estimates, which suggested the comet could be between 2 and 16 km across. By comparison, the the asteroid believed responsible for the Cretaceous–Paleogene Extinction Event 65 million years ago is thought to have been between 10 and 15 km across; however, thanks to acceleration under gravity during its approach to the Sun, 2I/Borisov has accelerated to 45 km/s, roughly twice the velocity of the Cretaceous–Paleogene asteroid.

2I/Borisov captured by the Hubble Space Telescope on November 16th 2019 with the spiral galaxy 2MASX J10500165-0152029 in the background. The galaxy appears blurred as Hubble was focused on 2I/Borisov. This image was taken at a distance of 325 million km, and is coloured blue to better reveal the comet’s coma and tail to help with analysis of their form and composition. Credit: NASA / ESA

Precisely determining the size of the comet has been made difficult both due to the fact that is it surrounded by a coma, a cloud of dust and material ejected by the sunward side of the object as it is heated in its approach to the Sun, and because the comet never comes close enough to Earth for radar to be reliably used to directly penetrate the coma and provide data on its size and shape.

However, spectrographic analysis of the coma and tail indicate that 2I/Borisov has a similar composition to that of solar system comets originating the the Oort Cloud, and is very similar to that of carbon-chain depleted comets that are common to the Jupiter family of short period comets. The gases being ejected appear to contain water vapour, plus traces of gases such as diatomic carbon and cyanide. The latter caused some headlines when announced, although cyanide is typically one of the first gases detected being ejected by solar system comets.

2i/Borisov imaged by Hubble on December 9th, 2019 at a distance of 296 million km. This image appears to show the comet survived perihelion intact, again revealing a bright central coma, with fainter out-gassing around it and a dim tail. The image has again been tinted blue to help with analysis of the coma and tail. Credit: NASA / ESA

Some estimates of the comet’s size had suggested that it might disintegrate as it reached perihelion at a distance of around 2 AU (astronomical units – the average distance between the Earth and the Sun), as a result of tidal forces. These suggestions were based on the behaviour of solar comets thought to be of a similar size and composition which have disintegrated on reaching / passing 2 AU from the Sun. However, images captured by the Hubble Space Telescope taken post-perihelion appear to show 2I/Borisov to be intact.

2I/Borisov is now a southern hemisphere night-time object, although its extremely low magnitude makes it hard to spot with anything but larger telescopes. While it has make its closest approach to the Sun, it will not do the same with Earth until close to the end of December, when it will be about 1.9 AU from us. It is liable to remain a subject of study through until around September 2020 as it continues its outward trek through the solar system, eventually becoming too dim to be usefully observed.

Space Video for the Week

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.

– Douglas Adams, The Hitch-Hiker’s Guide to the Galaxy

Most of us are probably familiar with those words – but just how big is space really? In 2018, to mark the opening of the European Southern Observatory’s Supernova Planetarium in Germany, astronomers and animators put together a film based on stellar and galactic observation’s by the ESO’s telescopes. It takes the observer on a journey from the building housing the planetarium in Germany to “the edge of the universe”.

In it we see Earth slowly recede from planet to point of light before some of the other planets pass by, also little more than point of light. Then the disc of the Sun appears, surrounded by the bright dots of the gas giants before it also fades into the distance as we accelerate out through the Sagittarius arm of our own galaxy. Then the galaxy recedes, merging with its nearest neighbours as we race through the local galactic cluster before the film executes a final burst of speed and the myriad of galaxies in the universe race by, each no bigger than a dot.

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