Although I can’t make the journey just then, I wish I could attend an upcoming conference at Arecibo (Puerto Rico) called Planets around Stellar Remnants. The meeting takes place twenty years after the discovery of the first exoplanets, the worlds orbiting the millisecond pulsar PSR B1257+12. I’ve been interested in the fate of planets around older stars ever since reading H.G. Wells’ The Time Machine as a boy and encountering his image of a swollen, dying Sun. We also have interesting questions to ask about the kind of planets that exist around white dwarfs, and whether new planets (and chances for life) may eventually occur in their systems.
It’s appropriate that the conference be organized by Penn State, for it was that university’s Alexander Wolszczan, working with Dale Frail of the National Radio Astronomy Observatory, who made the discovery of those two and possibly three planets that launched the modern exoplanet era. Nor has Wolszczan slowed in his efforts. The most recent work out of Penn State is the discovery of planets around three dying stars — HD 240237, BD +48 738, and HD 96127 — one of which has an interesting and still unidentified object, perhaps a brown dwarf, in orbit around it.
All three of the stars are much further along in their lifelines than our Sun, says Wolszczan:
“Each of the three stars is swelling and has already become a red giant — a dying star that soon will gobble up any planet that happens to be orbiting too close to it. While we certainly can expect a similar fate for our own Sun, which eventually will become a red giant and possibly will consume our Earth, we won’t have to worry about it happening for another 5 billion years.”
What we get by studying highly evolved stars like these is an updated window into planet formation. 30 known planets and brown dwarf-mass companions are now known to exist around giant stars (all three stars studied here are K-class giants). The object around BD +48 738 is tricky because a long-term radial velocity trend here indicates a distant companion but the data are not yet sufficient to decide between a planet and a low-mass star as the culprit. The star is also orbited by a planet of about 90 percent Jupiter’s mass at roughly 1 AU in a 400-day orbit.
I’ll pause on this because we’re finding a number of companions to giant stars that have minimum masses of about 10 Jupiter masses, making them either brown dwarf candidates or massive planets. It will take further work to identify the object in the outer system of BD +48 738, but if it does turn out to be a brown dwarf, then we have another case of a system with a Jupiter-class inner planet and a distant brown dwarf orbiting the same star. The paper on this work discusses the implications in terms of our primary theories of planet formation:
In principle, such a system could form from a sufficiently massive protoplanetary disk by means of the standard core accretion mechanism (Ida & Lin 2004), with the outer companion having more time than the inner one to accumulate a brown dwarf like mass. A more exotic scenario could be envisioned, in which the inner planet forms in the standard manner, while the outer companion arises from a gravitational instability in the circumstellar disk at the time of the star formation (e.g. Kraus et al. 2011). In any case, it is quite clear that this detection, together with the other ones mentioned above, further emphasizes the possibility that a clear distinction between giant planets and brown dwarfs may be difficult to make…
The three stars appear jittery under observation because they oscillate more than younger stars like the Sun. That made the planet hunt a challenge, but also allowed for the discovery of a negative correlation between the star’s metallicity and the degree to which it oscillates. Wolszczan says that the less metal content the team found in each star, the more noisy and jittery it turned out to be. The paper relates this to p-mode (pressure-driven) oscillations at the surface of the star:
The origin of this trend is most likely related to the fact that higher metallicity (opacity) of the star lowers its temperature, which decreases the amplitude of p-mode oscillations, while lower metallicity has the opposite effect.
This Penn state news release quotes Wolszczan on the future of our own Solar System as the Sun swells to become a red giant and swallows the inner planets. Somewhere in the remote future, says the astronomer, perhaps one to three billion years from now, we may consider moving to Europa, an icy wasteland that under the gaze of a swollen Sun will become a world of ‘vast, beautiful oceans.’ It’s an enchanting thought, and one we’ll doubtless think more about as we continue our investigations of giant stars and the fate of planetary systems around them.
The paper is Gettel et al., “Substellar-Mass Companions to the K-Giants HD 240237, BD +48 738 and HD 96127,” accepted by the Astrophysical Journal (preprint).