Super-Earths at HD 7924

The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924
http://arxiv.org/abs/0901.4394

Abstract wrote:

We report the discovery of the first low-mass planet to emerge from the NASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarf HD 7924. Keplerian modeling of precise Doppler radial velocities reveals a planet with minimum mass M_P sin i = 9.26 M_Earth in a P = 5.398 d orbit. Based on Keck-HIRES measurements from 2001 to 2008, the planet is robustly detected with an estimated false alarm probability of less than 0.001. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 7924 is photometrically constant over the radial velocity period to 0.19 mmag, supporting the existence of the planetary companion. No transits were detected down to a photometric limit of ~0.5 mmag, eliminating transiting planets with a variety of compositions. HD 7924b is one of only eight planets known with M_P sin i < 10 M_Earth and as such is a member of an emerging family of low-mass planets that together constrain theories of planet formation.

Awesome. A nice aperitif before February 3rd.

Moreover a second planet in outer and highly eccentric orbit is to be confirmed.

Edasich wrote:

Moreover a second planet in outer and highly eccentric orbit is to be confirmed.

The best two-planet fits don't really affect the orbital parameters of the first planet. The high eccentricities of the second planet in a two-planet fit, as well as a false alarm probability > 20%, suggest that a second planet may not be statistically significant. According to the paper, they'll continue to monitor the star to gather more RV data.

I've made a vels file with radial velocities sets available.
The 12.3, 16.7 and 38.3 days peaks seem convincing (both the 12.3 and the 16.7 may even represent a close-in Trojan pair), whereas the 145.7 days periodicity does not. Moreover I don't get such high eccentricities.

But as the paper says, more observation and more precise data are needed to perform further analysis, even to confirm or discard additional companions.

However it's another nearby K-dwarf hosting super-Earth(s). We're discovering new neighbors in the Solar neighborhood

Problem with 1-planet systems is that you can't use dynamics to put an upper bound on the mass. Is this a super-Earth or a hot Neptune? Who knows...

Indeed. And even in multiple planet systems, it may be that dynamics simply aren't as observable as they might be at Gliese 876 for example.

Well, it's not a single-planet system anymore!
More super-Earths!

Three Super-Earths Orbiting HD 7924
http://arxiv.org/abs/1504.06629

Quote :

We report the discovery of two super-Earth mass planets orbiting the nearby K0.5 dwarf HD 7924 which was previously known to host one small planet. The new companions have masses of 7.9 and 6.4 M⊕, and orbital periods of 15.3 and 24.5 days. We perform a joint analysis of high-precision radial velocity data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to robustly detect three total planets in the system. We refine the ephemeris of the previously known planet using five years of new Keck data and high-cadence observations over the last 1.3 years with the APF. With this new ephemeris, we show that a previous transit search for the inner-most planet would have covered 70% of the predicted ingress or egress times. Photometric data collected over the last eight years using the Automated Photometric Telescope shows no evidence for transits of any of the planets, which would be detectable if the planets transit and their compositions are hydrogen-dominated. We detect a long-period signal that we interpret as the stellar magnetic activity cycle since it is strongly correlated with the Ca II H and K activity index. We also detect two additional short-period signals that we attribute to rotationally-modulated starspots and a one month alias. The high-cadence APF data help to distinguish between the true orbital periods and aliases caused by the window function of the Keck data. The planets orbiting HD 7924 are a local example of the compact, multi-planet systems that the Kepler Mission found in great abundance.

Ratio between planet c with 15.3 day period and potential signal 17.1 d gives 1.117 too close in my opinion 17.1 to be exist as planet

Quote :

Ratio between planet c with 15.3 day period and potential signal 17.1 d gives 1.117 too close in my opinion 17.1 to be exist as planet

That's basically what's said in the paper? It is clearly stated that the authors prefer activity as the cause of the signals…

Anyway… I think, nowadays, the detection of additional low-mass planets in this system is not particularly surprising, because short-period low-mass planets are very rarely alone. That's not to detract from the value of this discovery, of course; for that, I must congratulate the Eta-Earth team.

APF is definitely showing strength, both as a precision spectrograph in general and as a complement to HIRES. The latter, due to the enormous pressure on observing time with Keck, tends to suffer from poor observational cadence, which obviously hampers the detection of small signals. The two spectrographs' complementary observing styles coupled with HIRES' already extensive observational history mean I expect quite a lot of good will come from their co-operation.

I believe the 1.65 m/s signal of d is the lowest amplitude signal detected without any prior information on the orbital parameters by HIRES. This is certainly a good thing, though I must point out that a 600-point dataset is… quantitatively enormous. I stress that improvements to the observational cadence and, optimistically, stellar noise removal of HIRES-related datasets would make detection of <2 m/s signals far more possible. Nevertheless, the significance of all three signals is testament to HIRES' capability as a high-precision spectrograph, and the involvement of APF will only improve on the issues mentioned previously.

We are beginning to approach a proper census of the solar neighbourhood's planetary systems. Unsurprisingly, they are looking a lot like the Kepler systems; but the near-future prospects of much improved characterization of these nearby systems shows the merit of the enormous observational times required to detect these nearby worlds. Despite the successes of Kepler for photometry, we cannot downplay radial velocities as a planet detection tool, which I mildly suspect has been occurring in recent years… but, with the next generation of spectrographs soon coming, there can only be successes to come!

There has to be an error in exoplanet catalog radius for planet HD7924 b  1.05 j it is very unlikely so low mass planet with so extremely small density I did not find any reference about radius(transit this planet)