Accretion Of A Giant Planet Onto A White Dwarf Star

The Neptune-sized planet, which orbits an Earth-sized star, is being slowly evaporated by the white dwarf, causing the planet to lose some 260 million tons of material every day.

For the first time, astronomers have discovered evidence for a giant planet orbiting a tiny, dead white dwarf star. And, surprisingly, the Neptune-sized planet is more than four times the diameter of the Earth-sized star it orbits.

"This star has a planet that we can't see directly. But because the star is so hot, it is evaporating the planet, and we detect the atmosphere it is losing. In fact, the searing star is sending a stream of vaporized material away from the planet at a rate of some 260 million tons per day." Boris Gänsicke from the University of Warwick said in a press release.

The new discovery serves as the first evidence of a gargantuan planet surviving a star's transition to a white dwarf. It suggests that evaporating planets around dead stars may be somewhat common throughout the universe. And because our Sun, like most stars, will also eventually evolve into a white dwarf, the finding could even shed light on the fate of our solar system.

The white dwarf in question, dubbed WDJ0914+1914, sits about 1,500 light-years away in the constellation Cancer. Although the white dwarf is no longer undergoing nuclear fusion like a normal star, its lingering heat means it's still a blistering 49,500 degrees Fahrenheit (25,000 Celsius). That’s some five times hotter than the Sun.

Researchers initially flagged the smouldering stellar core for follow-up after sifting through about 7,000 white dwarfs identified by the Sloan Digital Sky Survey. When the team analyzed the unique spectra of WDJ0914+1914, they detected the chemical fingerprints of hydrogen, which is somewhat unusual. But they also picked out signs of oxygen and sulfur — elements they had never seen in a white dwarf before.

In order to get a better grasp of what was happening in the strange system, the team of researchers used the X-shooter instrument on the ESO's Very Large Telescope in Chile to carry out follow-up observations. Based on the more detailed look, the researchers learned that the unusual elements they thought were embedded in the white dwarf were actually coming from a disk of gas churning around the dead star.

"At first, we thought that this was a binary star with an accretion disk formed from mass flowing between the two stars. However, our observations show that it is a single white dwarf with a disk around it roughly 10 times the size of our Sun, made solely of hydrogen, oxygen and sulfur. Such a system has never been seen before, and it was immediately clear to me that this was a unique star." said Gänsicke.

After realizing just how unusual the white dwarf really was, the team shifted their focus to figuring out what the heck could create such a system.

"It took a few weeks of very hard thinking to figure out that the only way to make such a disk is the evaporation of a giant planet," said Matthias Schreiber, an astronomer at the University of Valparaiso in Chile, who was vital to determining the past and future evolution of the bizarre system. Their detailed analysis of the disk's composition matched what astronomers would expect if the guts of an ice giant like Uranus and Neptune were vaporized into space.

Based on Schreiber's calculations, the white dwarf's extreme temperature means it's bombarding the nearby giant planet — which is located 0.07 astronomical unit (AU) from the star, where 1 AU is the Earth-Sun distance — with high-energy photons. This is causing the planet to lose its mass at a rate of more than 3,000 tons per second.

But according to the paper, published Wednesday in Nature, "As the white dwarf continues to cool, the mass-loss rate will gradually decrease, and become undetectable in about 350 million years. And by then, the paper adds, the giant planet only will have lost "an insignificant fraction of its total mass," or about 0.04 Neptune masses.

Because the giant planet is located so close to the white dwarf, the researchers say it should have been destroyed during the stars' red giant phase. That is unless it migrated inward after the star transitioned to a white dwarf. 

"This discovery is major progress because over the past two decades we had growing evidence that planetary systems survive into the white dwarf stage," said Gänsicke. "We've seen a lot of asteroids, comets, and other small planetary objects hitting white dwarfs, and explaining these events requires larger, planet-mass bodies farther out. Having evidence for an actual planet that itself was scattered in is an important step."

The ultimate fate of our solar system. In 5 billion years, when the Sun burns through the last of the hydrogen in its core, it will move on to fusing concentric shells of hydrogen around its now-inert core. This unstable process will cause the Sun to balloon into a red giant, meaning it will swallow Mercury, Venus, and likely Earth.

But as the Sun expands, its gravitational grasp on its outer envelope of material gets more and more tenuous. Eventually, it will shed its outer layers into space. And once it does that, an alien astronomer would see a beautiful planetary nebula surrounding the Sun's burnt-out, incredibly hot core — known as a white dwarf.

In a companion paper also published Wednesday in Astrophysical Journal Letters, Schreiber and Gänsicke explore this scenario, detailing how the future white-dwarf Sun should, like WDJ0914+1914, evaporate our solar system's giant planets.

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Water Vapour on the Habitable-Zone Exoplanet K2-18b

Astronomers have finally uncovered water vapour in the atmosphere of a super-Earth exoplanet orbiting within the habitable zone of its star. The find means that liquid water could also exist on the rocky world's surface, potentially even forming a global ocean.

The discovery, made with NASA's Hubble Space Telescope, serves as the first detection of water vapour in the atmosphere of such a planet. And because the planet, dubbed K2-18 b, likely sports a temperature similar to Earth, the newfound water vapour makes the world one of the most promising candidates for follow-up studies with next-generation space telescopes.

Planet K2-18 b sits some 110 light-years away in the constellation Leo, and it orbits a rather small red dwarf star that's roughly one-third the mass of our own Sun. Red dwarfs are infamous for being active stars that emit powerful flares, but the researchers point out that this particular star appears to be surprisingly docile.

This bodes well for the water-bearing planet, as its 33-day orbit brings it about twice as close to its star as Mercury is to the Sun. Given that the star is much cooler than the Sun, in the end, the planet is receiving similar radiation to the Earth. And based on calculations, the temperature of the planet is also similar to the temperature of the Earth.

Specifically, the paper suggests K2-18 b has a temperature between about –100 °F (–73 °C) and 116 °F (47 °C). For reference, temperatures on Earth can span from below –120 °F (–84 °C) in regions like Antarctica to above 120 °F (49 °C) in regions like Africa, Australia, and the Southwestern United States.

Although K2-18 b flaunts some of the most Earth-like features observed in an exoplanet so far — water, habitable temperatures, and a rocky surface — the researchers point out the world is still far from Earth-like. First off, K2-18 b is roughly twice the diameter of Earth, which makes it about eight times as massive. This puts K2-18 b near the upper limit of what we call a super-Earth — which typically refers to planets between about one and 10 Earth masses.

But the density of K2-18 b is what really cements it as a rocky planet. With a density about twice that of Neptune, K2-18 b has a composition most similar to Mars or the Moon. So, because the planet is believed to have a solid surface, and it's known to have an extended atmosphere with at least some water vapour, researchers say it's feasible that K2-18 b could actually be a water world with a global ocean covering its entire surface.

However, they cannot say for sure. The uncertainty is because Hubble can't probe the atmospheres of distant exoplanets in great detail. For instance, thanks to a sophisticated algorithm, the researchers were able to tease out the undeniable signal of water vapour in the atmosphere of K2-18 b, But they couldn't tell exactly how much water vapour is really there. So, in their paper, they took the conservative approach and gave a broad-range estimate for the abundance of water — somewhere between 0.01% and 50%.

In order to pin down exactly how much water is really on K2-18 b, the researchers say we'll have to wait for the next generation of advanced space telescopes to come online. Specifically, NASA's James Webb Space Telescope, scheduled for launch in 2021, and the European Space Agency's Atmospheric Remote-sensing Infrared Exoplanet Large survey (ARIEL) telescope, planned for launch in the late 2020s, are perfectly suited for the challenge.

The new research was published September 11 in Nature Astronomy

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Top 10 Places for New Human Colonies In Our Solar System

Looking to make a fresh start amongst the stars? Here are a few options and today we are counting down our picks for the Top 10 Places for New Human Colonies In Our Solar System. For this list, we’re looking at planets, moons and dwarf planets orbiting our sun that could potentially be colonized in the future.

Number 10: Mercury As the closest planet to the sun, colonizing Mercury would be complicated but not an insurmountable task. A big problem is that temperatures get up to 800 degrees Fahrenheit during the day and down to -290 degrees at night. 

It’s been suggested that a Mercury colony could be a mobile one that slowly travels the globe, keeping itself in the transitional space of land caught between night and day known as the “twilight belt.” 

Though continuously chasing the sun would be impossible on Earth, Mercury’s rotation takes about 58 days, meaning that the colony could move very slowly and still remain in the safe zone. In the twilight belt, Mercury’s daytime temperature drops to one more comparable to that of Earth.

Number 9: Uranus In the name of humanity’s future, let’s leave the butt jokes out of it. This overlooked ice giant might not rank highly on the list of planets we are anxious to colonize, but if we were to make the effort, a settlement on Uranus could yield substantial rewards.

The planet is rich in Helium-3, a rare substance on Earth that has been proposed as the ideal fuel for interplanetary travel. In Uranus’ atmosphere, the gravity is only 89% that of Earth’s. This has led experts to put forth the idea of floating mining facilities, suspended by hot air balloons or some other mechanism. For more permanent living arrangements, the moons of Uranus would serve well.

Number 8: Triton Speaking of moons, here is one with a lot of potentials. Triton is the largest of Neptune’s moons and the only surface around the planet that offers any sort of solid ground on which to set up a colony.

In terms of why we would want to live there, it shows signs of major geothermal activity and a possible subsurface ocean likely composed of water or ammonia. It’s the coldest body in our solar system, but between the combined resources offered by both it and Neptune, heating wouldn’t be an issue.

There is some concern about the nitrogen geysers that dot the moon’s surface, but hey, no one said that colonization would be easy.

Number 7: Callisto Ice giants like Uranus and Neptune and gas giants like Jupiter are difficult to colonize directly. Whereas Uranus’ gravity allows for colonies suspended in the atmosphere, Jupiter’s unforgiving atmosphere, staggering winds and powerful gravity wells make that sort of approach impossible. Enter Callisto.

The fourth and furthest of the Galilean moons from Jupiter, it’s appealing because it is subject to the least radiation of its peers, like Ganymede. Add to that the presence of water ice and its overall geological stability and you can understand why it has been deemed humanity’s best home base for exploring the outer solar system.

NASA actually has a detailed outline for a theoretical manned mission to Callisto.

Number 6: Ceres/The Asteroid Belt The asteroid belt might sound like an unappealing place to call home, but give it a chance -it might just make you rich! It offers mining possibilities that are honestly hard to quantify in the monetary sense - the word “quintillions” tends to get thrown around though.

Humanity could set up mining stations on any number of larger asteroids, but Ceres, a dwarf planet located within the asteroid belt is the obvious choice. The water ice present is more than adequate to support a sizable colony. Ceres’ small size largely rules out terraforming and artificial gravity would likely be necessary, as would a solution for radiation. Still, it’s very much within the realm of possibility.

Number 5: Venus is a very interesting candidate for colonization in that it is both blessed with appealing features and burdened by some incredibly harsh ones. Venus is similar to earth in size and, by extension, has comparable gravity. 

Unfortunately, the planet’s surface temperature is hot enough to melt lead. It’s also frequently subjected to sulfuric acid rain and home to numerous volcanoes. As such, it’s been suggested that Venus be colonized not on its surface. Instead, humans could establish floating habitats in the Venerian atmosphere, an environment that has been described as “paradise.”

This isn’t the only option though. One strategy involves the construction of an artificial mountain, but arguably the most appealing option is terraforming the planet.

Number 4: Europa Though it might not be as practical as its neighbour Callisto, the smallest of the Galilean moons is without a doubt one of the most exciting bodies in our solar system. Europa has fascinated the scientific community due to the presumed presence of a liquid water ocean.

The surface of the moon is covered in water ice, which is more than enough to support a colony of any size that we could reasonably build. Of all the bodies in our solar system, Europa’s ocean has arguably inspired the greatest hope for finding extra-terrestrial life; and the best way to find it is with a permanent research settlement. Saturn’s moon, Enceladus, is similarly enticing.

Number 3: Titan Second only to Ganymede in terms of size, Titan offers a lot in terms of potential for colonization. Unlike earth’s Moon, Titan boasts a significant atmosphere, making it one of the few entries on our list today that has a built-in solution for the ever-present issue of radiation.

Unfortunately, the outlook is less positive in terms of gravity and proximity to Earth. That being said, its presumed methane lakes offer up a bountiful energy source, which would go a long way in terms of heating and oxygen production.

Fun fact: that low gravity paired with high atmospheric density would enable human flight with a simple set of wings.

Number 2: The Moon We have discussed a number of moons, but as far as Earth is concerned, there’s only one. Here’s why Earth’s natural satellite is such a candidate for our first off-Earth colony. First, it can be travelled to in a matter of days. Proximity to the sun covers energy needs.

Water ice has been found at the poles, so we have got that covered. That just leaves the lack of atmosphere and the accompanying problems of radiation, lack of breathable air and the frequency of small meteorites. But numerous functioning models have actually been presented. Really… we just need someone to foot the bill and to figure out the long-term impacts on human health.

Number 1: Mars With Earth’s Moon literally at our doorstep relative to other bodies in our solar system, why would we choose to fixate on Mars? Well, to put it simply, it presents certain opportunities that the Moon does not. The Martian atmosphere, though far from perfect, is a much better foundation for terraforming and it already provides far better protection from radiation and meteors than the Moon’s.

The planet’s size also gives it a closer gravity to earth’s own, though the two are by no means the same. Arguably the strongest case for colonizing Mars over the Moon, however, is that as a distinct planet, it offers far more opportunity for significant scientific discovery.

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