# What's the big deal with WD 1856+534 b?

A few days ago (september 2020) a planet candidate was announced orbiting white dwarf WD 1856+534. Some media outlets announce it as the First Possible ‘Survivor’ Planet around a white dwarf, while others even say it is the first planet around a white dwarf. Some add that if further confirmed, (...) shows that some planets could survive the destruction of their sun-like stars. I can't find the source, but I even recall reading an article saying that the host being a stellar remnant was what made this finding relevant.

However:

1. the first confirmed exoplanets (1992) orbit a stellar remnant (PSR B1257+12),
2. there are other planet candidates around white dwarfs, and
3. it is not confirmed that the planet survived the death of its host star (as opposed to being formed or captured later).

Another highlight given is that it is still to be explained how such a planet survives being so close to its star, but do we know the planet is beyond its Roche limit?

I am puzzled: what exactly makes this planet's finding so special/a first? Is it just de degree of certainty about one of the previous assertions? (First around a WD, first survivor.) Is it something else? Or was it just a case of recency bias that made the discovery more visible?

I'll address the three sub-questions individually, as a way of fully answering the title question.

1. While there are indeed three planets orbiting PSR B1257+12, note that it's a pulsar, the compact remnant of an energetic event involving the system's progenitor. However, that event would likely have destroyed any planets that originally orbited the star, meaning that the three planets we see today likely formed afterwards (see e.g. Podsiadlowski 1993). By contrast, it seems that WD 1856+534 b may have formed prior to its parent star becoming a white dwarf, after which its orbit was disrupted and it moved into a tighter orbit.

2. While there are other candidate exoplanets orbiting white dwarfs, some consist of planetesimals, asteroids or other debris (e.g. WD 1145+017), some have only been detected indirectly (e.g. WD J0914+1914), and some are being actively disrupted by the star (e.g. WD 1145+017). On the other hand, WD 1856+534 b seems to be somewhat stable and is quite substantial $$(M\sim10\text{-}15M_J$$), likely falling into the giant planet regime.

3. It's certainly true that we don't know that the planet formed before the progenitor became a white dwarf, and it definitely can't be ruled out. The authors (Vanderburg et al. 2020) don't explicitly discuss a capture model, but they do attempt to make a statistical estimate of how close a nearby star is likely to have come within the past ~6 billion years, i.e. since the star became a white dwarf: $$D\sim\left(\pi vt_{\text{cool}}n\right)^{-1/2}$$ with $$v$$ the mean local stellar velocity, $$t_{\text{cool}}\approx5.85\;\text{Gyr}$$ the cooling age of the white dwarf, and $$n$$ the local stellar number density$$^{\dagger}$$. Assuming $$v\approx60\;\text{km s}^{-1}$$, they find that in the last ~5.85 billion years, the closest a nearby star would be likely to come to the system is roughly 600 AU. If we consider the capture model, an encounter at that distance seems unlikely to transfer a planet to a comparatively tight orbit.

That said . . . we obviously can't rule out that sort of interaction. The same holds for a bunch of other proposed scenarios which call for "either finely-tuned or a priori unlikely initial conditions", as Vandenburg et al. put it. So, sure, capture is possible. It just would presumably require a fairly close approach. The authors also don't (I think) explicitly address the idea of formation after the white dwarf evolved to its present state.

All of that said, yes, this isn't an enormous leap into unexplored territory, but it does seem that we have solid evidence of a massive exoplanet in a stable, close orbit around a white dwarf, and models involving it forming prior to the progenitor's evolution to a white dwarf phase appear to hold water.

$$^{\dagger}$$ As the system is only $$\sim25\;\text{pc}$$ away, they make safe assumptions about the values using data from Gaia in the solar neighborhood, i.e. $$n\approx0.1\;\text{pc}^{-3}$$.

• Where does the 600 au/6 billion year number cone from? – ProfRob Sep 21 '20 at 19:00
• @RobJeffries My bad, I forgot to link to the paper - specifically, it's in the "Other theories" subsection of the Methods section. – HDE 226868 Sep 21 '20 at 19:13
• @RobJeffries Ah, I see. I should have been clearer - it's just a statistical estimate based on conditions in the stellar neighborhood. – HDE 226868 Sep 21 '20 at 20:52
• I need to read the paper. You shouldn't make arguments about an extraordinary object that are based on mean behaviour. Just because an event is unlikely doesn't mean it isn't the explanation for a single example of a peculiar object. One would need to show that planets exist around lots (or at least a few more) white dwarfs before the capture idea could be excluded. – ProfRob Sep 21 '20 at 21:43
• @RobJeffries I agree. My understanding is that they can't rule it out but simply find an in situ model more likely. It seems like they throw a lot of things into the "unlikely but maybe with certain parameters it can work" bin. – HDE 226868 Sep 21 '20 at 21:59