Kepler-91 b - Hot Jupiter soon to be engulfed

Kepler-91b: a planet at the end of its life. Planet and giant host star properties via light-curve variations
http://arxiv.org/abs/1312.3943

Quote :

The evolution of planetary systems is intimately linked to the evolution of their host star. Our understanding of the whole planetary evolution process is based on the large planet diversity observed so far. To date, only few tens of planets have been discovered orbiting stars ascending the Red Giant Branch. Although several theories have been proposed, the question of how planets die remains open due to the small number statistics. In this work we study the giant star Kepler-91 (KOI-2133) in order to determine the nature of a transiting companion. This system was detected by the Kepler Space Telescope. However, its planetary confirmation is needed. We confirm the planetary nature of the object transiting the star Kepler-91 by deriving a mass of $ M_p=0.88^{+0.17}_{-0.33} ~M_{\rm Jup}$ and a planetary radius of $R_p=1.384^{+0.011}_{-0.054} ~R_{\rm Jup}$. Asteroseismic analysis produces a stellar radius of $R_{\star}=6.30\pm 0.16 ~R_{\odot}$ and a mass of $M_{\star}=1.31\pm 0.10 ~ M_{\odot} $. We find that its eccentric orbit ($e=0.066^{+0.013}_{-0.017}$) is just $1.32^{+0.07}_{-0.22} ~ R_{\star}$ away from the stellar atmosphere at the pericenter. Kepler-91b could be the previous stage of the planet engulfment, recently detected for BD+48 740. Our estimations show that Kepler-91b will be swallowed by its host star in less than 55 Myr. Among the confirmed planets around giant stars, this is the planetary-mass body closest to its host star. At pericenter passage, the star subtends an angle of $48^{\circ}$, covering around 10% of the sky as seen from the planet. The planetary atmosphere seems to be inflated probably due to the high stellar irradiation.

There has been some dispute in literature whether or not Kepler-91b is a true giant planet (See Esteves, et al. 2013). Previous reports of its albedo have implied the object is self-luminous and therefore stellar. Now, radial velocity measurements have determined the mass of the planet, and found it to be securely in the planetary regime.

Radial velocity confirmation of Kepler-91 b. Additional evidence of its planetary nature using the Calar Alto/CAFE instrument
http://arxiv.org/abs/1408.1528

Hm.

I was pretty on the fence about this discovery (what with the four-odd papers with differing results), but that is… convincing.

So Hot Jupiters can - and do - exist around giants?! Why haven't we observed one around the several hundred giants in the solar vicinity (HD 102956 doesn't count, not evolved enough)? How rare must they be? So many questions!

Another confirmation of the planet with radial velocity.

Radial Velocity Observations and Light Curve Noise Modeling Confirm That Kepler-91b is a Giant Planet Orbiting a Giant Star
http://arxiv.org/abs/1408.3149

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Kepler-91b is a rare example of a transiting hot Jupiter around a red giant star, providing the possibility to study the formation and composition of hot Jupiters under different conditions compared to main-sequence stars. However, the planetary nature of Kepler-91b, which was confirmed using phase-curve variations by Lillo-Box et al., was recently called into question based on a re-analysis of Kepler data. We have obtained ground-based radial velocity observations from the Hobby-Eberly Telescope and unambiguously confirm the planetary nature of Kepler-91b by simultaneously modeling the Kepler and radial velocity data. The star exhibits temporally correlated noise due to stellar granulation which we model as a Gaussian Process. We hypothesize that it is this noise component that led previous studies to suspect Kepler-91b to be a false positive. Our work confirms the conclusions presented by Lillo-Box et al. that Kepler-91b is a 0.73+/-0.13 Mjup planet orbiting a red giant star.

I am not really surprised that hot Jupiters exist around giant stars. There has to be some intermediate phase between "cold Jupiter" and "swimming in the convective zone." Though of course they're probably rare as they get consumed on astronomically short lifetimes, explaining their apparent rarity.

Hey, that's even more convincing. There's certainly no arguments on the companion's nature now.

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I am not really surprised that hot Jupiters exist around giant stars. There has to be some intermediate phase between "cold Jupiter" and "swimming in the convective zone." Though of course they're probably rare as they get consumed on astronomically short lifetimes, explaining their apparent rarity.

I do agree, but that still means they must be extraordinarily rare for this to be the first one to be discovered.

I suppose, due to their large radii, short-period companions to Giants will be transiting at a far larger range of inclinations than for a less evolved star. As the associated radial velocity variations are very large, too, there shouldn't be many cases where any such companions are missed. So perhaps there just aren't any in the nearest 50 pc?

The absence of Hot Jupiters around giants does seem to agree with the small population of such planets known around A-early F stars, though incompleteness is much more of a problem there.

Could the apparent rarity be partially explained by red giants not being included in most RV planet searches?

No, because most giants brighter than V = 7-8 that are amenable to RV searches (read: G through mid-K) are being/have been observed under dedicated giant star RV searches. That's several hundred giants, at the least.

Just think of all the purported AU jovians around giants that have been announced. Quite a lot. A Hot Jupiter would be easier to detect around the same stars, but there aren't any!

Note that the mass of Kepler-91 suggests it was probably a late-F on the main sequence rather than an early-type star.

Remember that Trojan candidate for the system (which for some reason I posted in a different thread)? The one which was rejected due to unphysically high temperatures?

Interesting follow-up on EXONEST here: http://www.mdpi.com/1099-4300/19/10/559/htm

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We believe that improperly modeling stellar illumination was a problem in our earlier study involving a proposed Trojan partner to the planet Kepler-91b. In that analysis, the Bayesian evidence weighed heavily in favor of a Trojan partner, but the modeled planetary temperatures were unphysically high, which led us to discount the possibility of a Trojan partner. The star Kepler-91 is an asymptotic giant branch star with a radius of approximately 6.30±0.16R_⊙ (where R_⊙ is the solar radius), and the semi-major axis of the hot Jupiter Kepler-91b is a=0.072^+0.007_−0.002 AU. The planet Kepler-91b has a radius of R_p=1.322^+0.094_−0.086R_J, where R_J represents Jupiter’s radius. By Equations (15)–(17), this implies that 29.5%±1.0% of Kepler-91b is in full daylight, 40.2%±2.1% is in partial penumbral light and 30.4%±1.0% is in full darkness, so that approximately 69.6% of the planet is illuminated at any point in time (as is illustrated in Figure 4B). By not properly accounting for this illumination, the modeled thermal emissions would be forced to compensate with increased day-side and night-side temperatures possibly explaining the unphysically high temperatures obtained in the analysis, which led to the Trojan hypothesis being discounted. This is supported by the fact that other studies of this system have led to unphysically high temperatures. For example, a study of KOI-2133b (Kepler-91b) by Esteves et al. found that the nightside temperature was 3100±200 K, which was greater than the expected equilibrium temperature of 1570 K, leading them to hypothesize that Kepler-91b, which was later re-confirmed as a planet, was self-luminous.

So maybe this one will end up getting resurrected?

Above link is now available on arXiv: https://arxiv.org/abs/1712.08894