Kepler-87: A gas giant and a very low density super-Earth

An Independent Planet Search In The Kepler Dataset. II. An extremely low-density super-Earth mass planet around Kepler-87
http://arxiv.org/abs/1310.2064

Quote :

[ABRIDGED]: Aims: two candidates in the KOI 1574 system are relatively long-period (about 114d and 191d) and in 5:3 resonance. We therefore search for TTVs in this particularly promising system. Methods: The full Kepler data was used, allowing to search for TTVs as well as for additional transit-like signals. Results: We detect strong anti-correlated TTVs of the 114d and 191d signals, dynamically confirming them as members of the same system. Dynamical simulations reproducing the observed TTVs allow us to also determine the masses of the planets. KOI 1574.01 (hereafter Kepler-87 b) was found to have a radius of 13.49 +/- 0.55 R_earth and a mass of 324.2 +/- 8.8M_earth, and KOI 1574.02 (Kepler-87 c) was found to have a radius of 6.14 +/- 0.29R_earth and a mass of 6.4 +/- 0.8M_earth. Both planets have low densities of 0.729 and 0.152 g cm^-3, respectively, which is non-trivial for such cold and old (7-8 Gyr) planets. Specifically, Kepler-87 c is the lowest- density planet in the super-Earth mass range. Both planets are thus particularly amenable to modeling and planetary structure studies, and also present an interesting case were ground-based photometric follow-up of Kepler planets is very desirable. Finally, we also detect two more short period super-Earth sized planetary (< 2R_earth) candidates in the system, making the relatively high multiplicity of this system notable against the general paucity of multiple systems in the presence of giant planets like Kepler-87 b.

any idea of the star magnitude, Kp? Is it a bright star for future characterization?

Well kepler 87 c low density is rather big surprise ,but density is low mainly by large radius.Due to small amount of transits mass of the planet can be not well determined like in case kepler 11 at the beginning

pochimax wrote:

any idea of the star magnitude, Kp? Is it a bright star for future characterization?

Kp mag of 14.6.
Using the KOI data off this site, I see they show three candidates of periods 114.73653 d, 573.96 d, and 5.83343 d. The paper mentions the 5.8-day candidate as well as another, but it doesn't match the 574-day candidate. I wonder what happened to the 574-day candidate?

It's likely to be confirmed. However this star has many similarities with nearby G subgiant Beta Hydri (mass, radius, T_eff, luminosity).

Original KOI data.

Parameter	KOI-1574.01	KOI-1574.02	KOI-1574.03
Period	114.7365 d	573.96 d	5.83343 d
Radius	6.2 Re	3.2 Re	0.78 Re

In the original data, the star is given a radius of 0.83 solar radii and an age of 0.5 Gyr. Ofir et al derived a much higher value for the stellar radius and age though. I wonder if Kepler-87c being an outlier in M/R parameter space supports the original, young, low-radius model for the star?

As for the difference in the reported period of KOI-1752.02, I realised that the old value is almost an integer multiple of the new value, leading me to wonder if some transits had been missed. Digging through the paper, it looks like that's the case.

Quote :

We use the full Kepler data – quarters 1 through 16 – in our analysis. The additional data includes a few more transits of KOI1574.01 relative to OD13, and importantly - the third and forth transit events of KOI 1574.02 (unfortunately the star fall on the inactive Module 3 during Quarters 7, 11 and 15 and transits that did occur were not observed).

So the 573 day signal is a sort of alias of the true, 192 day signal, and so there's no reason to believe that there is another planet at 573 days.

a very good job, Sirius.

it' s a pity the star it' s not bright enough for future atmospheric characterizations.

So, a low-density Jupiter and a sub-Neptune. Star is less metal-rich than the Sun but not to a particularly extreme degree. Where did all the rocks go?

Well  planets are large so probably to the smaller bodies in the system .I do not think calling planet with radius over 6 times earth as super earth is good idea.

If planet with period 191 days was prevously 573 days so radius is wrong.There is nothing interesting in characterisation of hydrogen envelopes of such gas giants