HD 40307 - Habitable Planet Candidate in Six-Planet System

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.

Conference proceedings vs. paper...

The Tau Ceti section is quite interesting. 14, 35, 300, 600 days? The first two correspond to "b" and "c", the 300 and 600 days do not correspond to any of the claimed planets.

A dynamical study on the habitability of terrestrial exoplanets II: The super Earth HD 40307 g
http://arxiv.org/abs/1403.5868

Quote :

HARPS and it Kepler results indicate that half of solar-type stars host planets with periods P<100 d and masses M < 30 M_E. These super Earth systems are compact and dynamically cold. Here we investigate the stability of the super Earth system around the K-dwarf HD40307. It could host up to six planets, with one in the habitable zone. We analyse the system's stability using numerical simulations from initial conditions within the observational uncertainties. The most stable solution deviates 3.1 sigma from the published value, with planets e and f not in resonance and planets b and c apsidally aligned. We study the habitability of the outer planet through the yearly-averaged insolation and black-body temperature at the pole. Both undergo large variations because of its high eccentricity and are much more intense than on Earth. The insolation variations are precession dominated with periods of 40 kyr and 102 kyr for precession and obliquity if the rotation period is 3 d. A rotation period of about 1.5 d could cause extreme obliquity variations because of capture in a Cassini state. For faster rotation rates the periods converge to 10 kyr and 20 kyr. The large uncertainty in the precession period does not change the overall outcome.

Some of the presentations from the Rencontres du Vietnam conference are up. I'm going through them now. The first result I see is that with 110 new RV measurements, the presence of the 35, 52 and 192 day planets are not confirmed. They do find a fourth planet and a fifth promising signal at 28.7 d, which of course doesn't correspond to any of the three new planets in Tuomi's paper.

If I'm reading it (this) right, the 52-day planet is confirmed. Looking at the page titled "HD40307: Different analysis & results", the 200 d and 28.7 d peaks are present in both periodograms, but their strengths are switched; they might aliases, which is plausible because the 28.7 day peak is at expectations for a monthly alias. So I think it's a bit quick to call on g… I can't say anything on e without a proper look at the residuals after fitting g/whatever because e has the smallest semi-amplitude of the system.

Is the system from the page titled "A few stars are extremely stable" new? I don't recognise it, and for all of my work with the archive I can't think of what star it could be - very few stars have been observed with such a high cadence.

I don't recognise it either.

Inner one looks like it could be GJ 3634b. Would explain why the 35.6-day one is highlighted, as it would be in the HZ.

As regards HD 40307g, that one was always fairly uncertain.

After a little archive-fu, I have identified the system:


Though I admit it is probably not desired by the HARPS team, this is publicly available data, so I'll name the star: HD 32564 (G6V).

The inner system is very compact (orbital spacings of ~0.05 AU), so their eccentricities cannot be very large without crossing orbits. P_e/P_d and P_f/P_e ≈ 1.5, indicating that they may be resonant. The outer planet converges on a very high eccentricity, but I'm not convinced it is real; e = 0 implies a minimum mass of about 14 M_⊕.

Anyway, that's enough of that derail. This thread's for HD 40307!

Maybe it's my bad english but I'm lost... Why are we talking about HD 32564?

I sent you a pm.

How many planets is there?   

Played with this one in Systemic using the public data from the HARPS archive. It does rather look like the "e" and "g" planets aren't there: after fitting the first three planets and the long term trend (for a sinusoid, the period comes out as ~4000 days, probably related to the stellar magnetic cycle), the next planet is the 51-day one ("f") and then the 28.7-day one, which when using the Keplerian model results in a high eccentricity that crosses the other planetary orbits.

But would probably need to do some proper modelling of the activity effects to go further.