A solar system that's backwards!

[Loren Pechtel]

https://www.syfy.com/syfywire/astro...rbiting-a-white-dwarf-and-its-far-bigger-than

The planet is larger than the star!

 

[Jimmy Higgins]

To address the obvious first question of how can a large planet (any planet) be that close to the star and survive it going all poofy, before transitioning to a White Dwarf...

It likely formed farther out from the star all those eons ago. When the star died, the planetary system became unstable, with planets moving around. An encounter with another massive planet could've dropped WD 1856b closer in to the star, putting it on the tight orbit it's on now.

i.e., they don't know, but it'd seem almost certain that the planet wasn't this close during the puff phase.

 

[Loren Pechtel]

To address the obvious first question of how can a large planet (any planet) be that close to the star and survive it going all poofy, before transitioning to a White Dwarf...

It likely formed farther out from the star all those eons ago. When the star died, the planetary system became unstable, with planets moving around. An encounter with another massive planet could've dropped WD 1856b closer in to the star, putting it on the tight orbit it's on now.

i.e., they don't know, but it'd seem almost certain that the planet wasn't this close during the puff phase.

Yeah, it would have been inside the star. This is a drag-in or capture event.

 

[Jimmy Higgins]

To address the obvious first question of how can a large planet (any planet) be that close to the star and survive it going all poofy, before transitioning to a White Dwarf...

It likely formed farther out from the star all those eons ago. When the star died, the planetary system became unstable, with planets moving around. An encounter with another massive planet could've dropped WD 1856b closer in to the star, putting it on the tight orbit it's on now.

i.e., they don't know, but it'd seem almost certain that the planet wasn't this close during the puff phase.

Yeah, it would have been inside the star. This is a drag-in or capture event.

Or Jesus... don't forget Jesus.

 

[lpetrich]

Planet discovered transiting a dead star - "Evidence has been found of a planet circling the smouldering remains of a dead star in a tight orbit. The discovery raises the question of how the planet survived the star’s death throes — and whether other planets also orbit the remains."

Nontechnical and non-paywalled.

Enter Vanderburg et al., who used data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission to detect the periodic dimming of the white dwarf WD 1856+534. This dimming is caused by a planet passing between the white dwarf and Earth. Because white dwarfs are so small, the planetary transit is very ‘deep’: 56% of the white dwarf’s light is blocked, compared with the typical 1–2% that is blocked by gas-giant planets around normal stars. In the case of WD 1856+534, the transiting planet is similar in size to Jupiter, and therefore has a diameter about ten times that of the white dwarf (Fig. 1).

But the planet's transit is only 8 minutes long, meaning that one needs monitoring by satellites like Kepler and TESS.

A big problem is that the white dwarf is too cold to produce spectral lines convenient for measuring radial velocities, so it's hard to get a mass for the planet.

One of the biggest questions to emerge from Vanderburg and colleagues’ study is how the planet ended up so close to the white dwarf. The planet is located just 4 solar radii from the white dwarf (or roughly 20 times closer to the white dwarf than Mercury is to the Sun). Assuming that the inner planetary system was swallowed by the expanding star, it seems extremely unlikely that the planet has always been this close to its star.

Vanderburg et al. suggest two possible explanations. The first is that the planet avoided destruction by tearing off the outer layers of the expanding star when it was engulfed. The second is that several distant planets survived the death of the star, but their altered orbits caused them to interact with each other — whereupon the observed planet was thrown towards the white dwarf by another planet. This latter explanation seems the most likely, and offers the tantalizing prospect of detecting additional planets in this system in the future. Given that WD 1856+534 is only 25 parsecs (82 light years) from Earth, the gravitational effects of any further planets on the white dwarf could be detectable by missions such as the European Space Agency’s Gaia space observatory. This system therefore opens up an entirely new field of exoplanetary research.

A giant planet candidate transiting a white dwarf | Nature
Abstract:

Astronomers have discovered thousands of planets outside the Solar System1, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star2, but more distant planets can survive this phase and remain in orbit around the white dwarf3,4. Some white dwarfs show evidence for rocky material floating in their atmospheres5, in warm debris disks6,7,8,9 or orbiting very closely10,11,12, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted13. Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our findings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs.

 

[lpetrich]

The star WD 1856+534 is inferred as having mass 0.52 solar masses and radius 1.4 Earth radii or 0.13 Jupiter radii or 0.013 solar radii. Its effective surface temperature is 4700 K, implying a luminosity 7.6*10^(-5) times the Sun's.

Its cooling age is about 6 billion years, meaning that it has been a white dwarf for longer than the Solar System has existed.

The planet orbits at 0.02 AU, about 330 radii of its star. It receives about 0.18 of the light flux that the Earth receives from the Sun, and its equilibrium temperature is 160 K.

By comparison, the Earth orbits at about 215 solar radii. So the star would look a little smaller than the Sun from its planet.