HD 40307 - Habitable Planet Candidate in Six-Planet System

The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 Super-Earths (4.2, 6.9, & 9.2 Earth masses)
http://arxiv.org/abs/0806.4587

Abstract wrote:

This paper reports on the detection of a planetary system with three Super-Earths orbiting HD40307. HD40307 is a K2V metal-deficient star at a distance of only 13 parsec, part of the HARPS GTO high-precision planet-search programme. The three planets on circular orbits have very low minimum masses of respectively 4.2, 6.9 and 9.2 Earth masses and periods of 4.3, 9.6 and 20.5 days. The planet with the shortest period is the lightest planet detected to-date orbiting a main sequence star. The detection of the correspondingly low amplitudes of the induced radial-velocity variations is completely secured by the 135 very high-quality HARPS observations illustrated by the radial-velocity residuals around the 3-Keplerian solution of only 0.85 m/s. Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports the planetary interpretation. Contrary to most planet-host stars, HD40307 has a marked sub-solar metallicity ([Fe/H]=-0.31), further supporting the already raised possibility that the occurrence of very light planets might show a different dependence on host star's metallicity compared to the population of gas giant planets. In addition to the 3 planets close to the central star, a small drift of the radial-velocity residuals reveals the presence of another companion in the system the nature of which is still unknown.

If there are radial velocity sets available I should perform a Systemic Console simulation...

The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?
Consideration of tides seems to favour the mini-Neptunes scenario rather than super-Earths. Possibilities for the inner planet b also include a volcanic "super-Io" or the core of an eroded Neptune-type planet.

There's also a Spitzer proposal about attempting to find transits of planet b. Observations would have already taken place. Even if HD 40307b doesn't transit, some of the properties of the planet's atmosphere could be determined (I guess this is similar to the way that the atmospheric redistribution on ups And b could be determined despite the lack of transits).

Wow, I hope the proposal was accepted. I'm surprised that they expect to be able to detect b, with it having such a small size, and probably less irradiation from the host star.

http://www.springerlink.com/content/t644r38324236858/

This page talks of a new HARPS triple super-earth system. The mass range presented is different than that given for HD40307. Maybe it's a misprint...

It seems to be a system with the following parameters (from a Harvard-Smithsonian Center for Astrophysics conference)

P = 4.3d Mass = 3.5 M_e
P = 9.6d Mass = 7.0 M_e
P = 20.5d Mass = 8.0 M_e

In my opinion, identical enough to published HD 40307 system parameters.

http://arxiv.org/abs/1002.4707

Quote :

The Spitzer search for the transits of HARPS low-mass planets - I. No transit for the super-Earth HD 40307b

M. Gillon (1,2), D. Deming (3), B.-O. Demory (4,2), C. Lovis (2), S. Seager (4), M. Mayor (2), F. Pepe (2), D. Queloz (2), D. Segransan (2), S. Udry (2), S. Delmelle (1), P. Magain (1) ((1) University of Liege, Belgium, (2) Geneva Observatory, Switzerland, (3) NASA/Goddard SFC, Greenbelt, USA, (4) MIT, Boston, USA)
(Submitted on 25 Feb 2010)

We have used Spitzer and its IRAC camera to search for the transit of the super-Earth HD 40307b. The transiting nature of the planet could not be firmly discarded from our first photometric monitoring of a transit window because of the uncertainty coming from the modeling of the photometric baseline. To obtain a firm result, two more transit windows were observed and a global Bayesian analysis of the three IRAC time series and the HARPS radial velocities was performed. Unfortunately, any transit of the planet during the observed phase window is firmly discarded, while the probability that the planet transits but that the eclipse was missed by our observations is nearly negligible (0.26%).

A shame. But there are several hot Super-Earths to be checked!

Edit:
Note the title of the paper. Looks like it's the first in a series of such papers. This will be very interesting.

Sirius_Alpha wrote:

A shame. But there are several hot Super-Earths to be checked!

Exactly 10 until August 2010, the first two have already been observed in December 2009 and January 2010.
Kind of a Grey area as far as embargo breach / respect for the related scientists are concerned. -- Sirius Alpha
I'm sorry v_v

Borislav wrote:

Exactly 10 until August 2010, the first two have already been observed in December 2009 and January 2010.

Number of observations increased to 5 stars with a total time of up to 47 hours of Spitzer. There are still in reserve 53 hours prior to August 2010.

In general, it is interesting that in the monitoring plan for this week and in early April is just three stars. By law probability for any of these three stars clearly be detected transit. And on repeated observations can be to learn from some of them.

More planets.

Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307
http://arxiv.org/abs/1211.1617

Quote :

The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The system lacks massive planets and is therefore a potential candidate for having additional low-mass planetary companions. We re-derive Doppler measurements from public HARPS spectra of HD 40307 to confirm the significance of the reported signals using independent data analysis methods. We also investigate these measurements for additional low-amplitude signals. We used Bayesian analysis of our radial velocities to estimate the probability densities of different model parameters. We also estimated the relative probabilities of models with differing numbers of Keplerian signals and verified their significance using periodogram analyses. We investigated the relation of the detected signals with the chromospheric emission of the star. As previously reported for other objects, we found that radial velocity signals correlated with the S-index are strongly wavelength dependent. We identify two additional clear signals with periods of 34 and 51 days, both corresponding to planet candidates with minimum masses a few times that of the Earth. An additional sixth candidate is initially found at a period of 320 days. However, this signal correlates strongly with the chromospheric emission from the star and is also strongly wavelength dependent. When analysing the red half of the spectra only, the five putative planetary signals are recovered together with a very significant periodicity at about 200 days. This signal has a similar amplitude as the other new signals reported in the current work and corresponds to a planet candidate with M sin i = 7 Me (HD 40307 g). ...

Another g planet for Vogt and Butler...

Usual caveats about the weirdness of super-Earth geophysics apply.

And the mini-Neptune vs. super-Earth issue.

Also is it just me or do (e,f,g) look suspiciously similar to (b,c,d) just with slightly smaller masses and a few times longer orbital periods...

Wonder how that happened... it isn't quite as odd as Kepler-20 for example but we have enough trouble figuring out why Mars is such a small planet as it is...

Yeah I noticed the system looks a lot like some of the Kepler systems.

Very the same as kepler compact system and I expect more planets even between f and g

HD 40307 g now listed on the Habitable Planetary Catalog as the fourth more promising exoplanet detected so far:




With an ESI=0.79 is however quite unlikely to host any complex life-forms.

EPE lists this planet with an excentricity of 0.29, so the distance from its star ranges from 0.426 au to 0.774 au! Wouldn't this distance variation imply in extreme temperature and climate changes, hindering the development of complex life? Or any life at all?

It's possible. But notice the "best fit" eccentricities in the paper are greater than the maximum dynamically stable eccentricities. Getting eccentricities for such low amplitude RV signals is difficult and I'm sure they'll be revised as more data comes in.

I've never been particularly convinced by the calculations used for the ESI.

Most of the time estimates of the habitable zone put the Earth quite close to the inner edge, the ESI on the other hand uses a more symmetric distribution.

Of the current list of potentially-habitable exoplanets, for me Gliese 667 Cc is still the most promising candidate (though that isn't saying particularly much). The case for Gliese 581g's existence is very dubious, Kepler-22b is probably a mini-Neptune and HD 85512b is only marginally less irradiated than Venus. Gliese 163 c also has rather uncomfortably high levels of insolation, particularly given the effect on the albedo of the host star's spectrum. HD 40307 g and Gliese 581 d are rather massive and could plausibly be mini-Neptunes, though perhaps in the case of Gliese 581 d a dense hydrogen-rich atmosphere could be beneficial to maintaining liquid water.

And HD 40307g also but both together with kepler 22b they are likely 2.4 -2.5 earth radius and eccentricity cause they are closer and farther from host star and probably lost some volatiles

Signals embedded in the radial velocity noise
http://arxiv.org/abs/1301.3438

Quote :

Bayesian data analysis techniques, together with suitable statistical models, can be used to obtain much more information from noisy data than the traditional frequentist methods. For instance, when searching for periodic signals in noisy data, the Bayesian techniques can be used to define exact detection criteria for low-amplitude signals - the most interesting signals that might correspond to habitable planets. We present an overview of Bayesian techniques and present detailed analyses of the HARPS-TERRA velocities of HD 40307, a nearby star observed to host a candidate habitable planet, to demonstrate in practice the applicability of Bayes' rule to astronomical data.

Tuomi needs to stop re-using the same paper titles haha.