A ‘Super Earth’ Around GJ 436?

by Paul Gilster on January 23, 2008

The closest we’ve come so far to identifying Earth-like planets around other stars is in the identification of so-called ‘super Earths.’ Calculations designed to model the composition of such planets say that worlds up to about ten Earth masses are rocky rather than gaseous. Some of these, as we have in the case of Gliese 581, have even excited interest in their possible habitability. We’d like to find ways beyond the now conventional radial velocity and transit studies to identify more such worlds.

Now a new planet may have been found around GJ 436, a red dwarf already known to host a Neptune-mass planet in a tight 2.6 day orbit. This is interesting work because of the methods used. Ignasi Ribas (Institut de Ciències de l’Espai, Spain) and team have taken a close look at the known planet and are arguing it is possible to identify a second world, a super-Earth, through the telltale variations in the transit duration of GJ 436b, the already known ‘hot Neptune.’

Giving the game away is the fact that GJ 436b’s orbit, while scorchingly close to the primary, is not perfectly circular. Why should a planet in such an orbit show an eccentricity as high as 0.15? Possible perturbations from other objects in this system have been investigated by others, but Ribas’ team found room to work in the fact that GJ 436b barely crosses the disk of its star as seen from Earth. Tiny changes in the orbital inclination angle can readily be observed, and if that angle is indeed changing, that would explain why this transit has been so hard to confirm — the 2007 transit detection came as a surprise that contradicted earlier results. Say the authors:

Assuming this hypothesis, it is reasonable to explore the possibility of a perturber that could be responsible for both the relatively large eccentricity and the inclination change, while remaining undetected by the radial velocity measurements.

The pieces of the puzzle begin to come together in a second planet for the GJ 436 system. GJ 436c shows a minimum mass of 4.8 Earth masses, with a 0.6 Earth mass play in the numbers. The authors are the first to point out that they do not consider this an ‘extremely solid detection,’ but argue that the case is strong because the existence of GJ 436c explains the inner planet’s orbit. If the finding is borne out, that would make GJ 436c the least massive planet known to orbit a main sequence star.

The radial velocity data on this system is consistent with a planet that matches up with these properties (but see systemic for a more detailed look, and reservations on this), and may indicate still more planets:

Indeed, the system around GJ 436 shows striking resemblances to that around the M-type star Gl 581 (Udry et al. 2007), and thus its planets may experience changes in the orbital elements, perhaps eventually undergoing transits in spite of a previously null result… Our study provides yet another illustration of the variety of exoplanet systems and highlights the potential for complex dynamical histories that imply sizeable variations of the planets’ orbital elements, like the eccentricity, over timescales of decades.

Thus the value of a ‘near-grazing transit’! Note what’s happening here: We’re examining what we know about one planet and using its characteristics to find the signature of a smaller world. Space-based transit studies should be quite useful in working with such tight transits, and that pushes the limit on what we can detect down to even smaller objects, at least in systems as helpful as the one around GJ 436.

The paper is Ribas et al., “A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits,” submitted to Astrophysical Journal Letters and available online.

{ 16 comments }

David January 24, 2008 at 5:27

HI,

Greg Laughlin has a nice overview of the paper
at http://oklo.org/?p=266#comments.
He questions their methodology, but also states its
well worth investigating further.

david January 24, 2008 at 6:54

I wonder, given in many cases we only know of such worlds due their affect on their parent stars, just how many of those worlds are single planets versus a planet/moon combination. And if such combinations are common just how many of those moons might be in the earth mass range.

It might be the case that many of those super earths are water worlds with moons that are more earth like, with both water and continents.

In such a system life might originate on the more massive world with the large ocean providing more opportunities for life to develop and get spread to the moon.

And might not the gravitational forces keep such a moon tectonically active for longer than normal?

MaDeR January 24, 2008 at 15:34

Well, so close to star these planets have rather small chance of retaining any moons of considerable size…

philw January 24, 2008 at 15:41

I wonder if the nearby Neptunian allows for a resonance that has such planets actually rotate instead of being tidally locked?

Alastair McKinstry January 25, 2008 at 2:21

David,

Its probably unlikely that the moons of super Earths would themselves be habitable – most moons are around 0.01 % the mass of their planets (Luna is believed a fluke caused by a collision; we need to see …). If this is the case, they’re probably too small to hold an atmosphere.

As for super Earths being water worlds: I’d say almost certainly. Earth is actually quite dry: 3.7 km average ocean depth translates to 0.01% water by mass as a whole. Mechanisms we know for water loss are more likely for _smaller_ planets (eg Mercury, Mars, .. Venus is an interesting case); as you head out from the star the chances of a planet being wet increase, but even if a super-Earth had only 0.01% water, it would be covered.

andy January 25, 2008 at 8:54

If a moon forms from a collision with an ocean planet, presumably most of the material blasted into orbit would be from the icy mantle of the ocean world. Since that ice is kept solid because of pressure, things could get interesting when that pressure is released. Depending on whether such a disc could assemble a moon fast enough to counter the evaporation of material, you might be able to form a largely icy moon which could in principle become a low mass ocean world itself. (Not sure what the lower mass limit is for ocean worlds at ~1 AU – if you’re prepared to allow the world to slowly evaporate over the gigayears, it could be pretty low).

However, if we take the moon formation, which is admittedly going to be a rather drier event in the first place (probably dealing with a terrestrial-terrestrial collision), bear in mind that the lunar rocks are very dry, which suggests that water blasted into orbit doesn’t survive to be incorporated into the satellite. If that’s the case, then the prediction would be that ocean worlds tend not to have (large) moons due to the lack of moon-forming material that results from a collision.

Adam January 25, 2008 at 17:17

Hi All

Alastair, the amount of water that terrestrials receive is likely to be highly variable – much of it arrives as a late veneer of material according to current simulations of accretion. Super-Earths will be either dry or wet, purely as a probabilistic thing.

andy, high pressure ices can be quenched to normal pressure, though I’d say a collision would probably shatter the lot. Question is: do large intact chunks get put into orbit? Simulations show the Moon formed from rock vapour mixed with intact pieces of mantle, so perhaps in a cold enough situation a Super-Earth might retain an icy moon. But it is rather hard to imagine a cloud of superheated water snowing out on the equatorial plane to form a moon. Perhaps a little bit further out from the Sun?

daniel January 26, 2008 at 9:04

DEEG H., OCANA B., KOZHEVNIKOV V., CHARBONNEAU D., O’DONOVAN F., & DOYLE L. , 2008 (update : 16 January 2008)

Extrasolar planet detection by binary stellar eclipse timing: evidence for a third body around CM Draconis

Astron. & Astrophys. , – , –

accepted

arXiv:0801.2186v2 [astro-ph] 16 Jan 2008
Astronomy & Astrophysics manuscript no. CMDraOCx
c ESO 2008
January 16, 2008
Extrasolar planet detection by binary stellar eclipse timing:
evidence for a third body around CM Draconis
H. J. Deeg1, B. Oca˜na1,2, V. P. Kozhevnikov3, D. Charbonneau4, F. T. O’Donovan5, and L.R. Doyle6
1 Instituto de Astrof´ısica de Canarias, C. Via Lactea S/N, 38205 La Laguna, Tenerife, Spain
2 Instituto de Radio Astronom´ıa Milim´etrica (IRAM), Av. Divina Pastora 7, N´ucleo Central, 18012 Granada, Spain
3 Astronomical Observatory, Ural State University, Lenin ave. 51, Ekaterinburg, 620083, Russia
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
5 California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
6 SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA
e-mail: hdeeg@iac.es
Received 6 Nov. 2007; accepted 28 Dec. 2007
ABSTRACT
Context. New eclipse minimum timings of the M4.5/M4.5 binary CM Dra were obtained between the years 2000 and
2007. In combination with published timings going back to 1977, a clear non-linearity in observed-minus-calculated
(O-C) times has become apparent. Several models are applied to explain the observed timing behavior.
Aims. Revealing the processes that cause the observed O-C behavior, and testing the evidence for a third body around
the CM Dra system.
Methods. The O-C times of the system were fitted against several functions, representing different physical origins of
the timing variations.
Results. An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting
function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of
observations or from a body on a shorter orbit generating periodicities in O-C times is the most likely source of the
observed O-C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as
the responsible third body. Also, no further evidence of the short-periodic planet candidate described by Deeg et al.
(2000) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate’s mechanism can
be rejected.
Conclusions. A third body, being either a few-Jupiter-mass object with a period of 18.5±4.5 years or an object in the
mass range of 1.5Mjup to 0.1M⊙ with periods of hundreds to thousands of years is the most likely origin of the observed
minimum timing behavior.
Key words. Stars: individual: CM Dra – binaries: eclipsing – Eclipses – planetary systems
1. Introduction
CM Dra (LP 101.15, G225-067, GJ 630.1) is a detached
spectroscopic eclipsing M4.5/M4.5 binary with one of the
lowest known total masses, of 0.44M⊙. With its nearly
edge-on inclination of 89.59◦(see Kozhevnikova et al. 2004
for the most recent orbital and physical elements) it was
chosen as the target of the first photometric search for planetary
transits. Performed by the ’TEP’ project, with an
intense observing campaign during the years 1994 – 1999
(Deeg et al. 1998; Doyle et al. 2000), over 1000 hours of
coverage of that system were obtained with several 1mclass
telescopes. This lightcurve was initially searched for
the presence of transits from planets in circumbinary ’Ptype’
orbits with 5 – 60 day periods, with a negative result
(Doyle et al. 2000). The same lightcurve provided, however,
a further possibility to detect the presence of third bodies,
from their possible light-time effects on the binary’s eclipse
minimum times.
Send offprint requests to: H.J. Deeg

daniel January 26, 2008 at 9:10

Astronomy & Astrophysics manuscript no. CMDraOCx
c ESO 2008
January 16, 2008
Extrasolar planet detection by binary stellar eclipse timing:
evidence for a third body around CM Draconis
H. J. Deeg1, B. Oca˜na1,2, V. P. Kozhevnikov3, D. Charbonneau4, F. T. O’Donovan5, and L.R. Doyle6
1 Instituto de Astrof´ısica de Canarias, C. Via Lactea S/N, 38205 La Laguna, Tenerife, Spain
2 Instituto de Radio Astronom´ıa Milim´etrica (IRAM), Av. Divina Pastora 7, N´ucleo Central, 18012 Granada, Spain
3 Astronomical Observatory, Ural State University, Lenin ave. 51, Ekaterinburg, 620083, Russia
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
5 California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
6 SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA
e-mail: hdeeg@iac.es
Received 6 Nov. 2007; accepted 28 Dec. 2007
ABSTRACT
Context. New eclipse minimum timings of the M4.5/M4.5 binary CM Dra were obtained between the years 2000 and
2007. In combination with published timings going back to 1977, a clear non-linearity in observed-minus-calculated
(O-C) times has become apparent. Several models are applied to explain the observed timing behavior.
Aims. Revealing the processes that cause the observed O-C behavior, and testing the evidence for a third body around
the CM Dra system.
Methods. The O-C times of the system were fitted against several functions, representing different physical origins of
the timing variations.
Results. An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting
function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of
observations or from a body on a shorter orbit generating periodicities in O-C times is the most likely source of the
observed O-C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as
the responsible third body. Also, no further evidence of the short-periodic planet candidate described by Deeg et al.
(2000) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate’s mechanism can
be rejected.
Conclusions. A third body, being either a few-Jupiter-mass object with a period of 18.5±4.5 years or an object in the
mass range of 1.5Mjup to 0.1M⊙ with periods of hundreds to thousands of years is the most likely origin of the observed
minimum timing behavior.
Key words. Stars: individual: CM Dra – binaries: eclipsing – Eclipses – planetary systems

Alastair McKinstry January 26, 2008 at 16:07

Adam,

I suspect that super Earths will be quite wet. I’ve recently started a phd in exoplanets, and volatiles, water. etc is the focus. From what i’ve seen a lot of the planetesimal-forming
sims seem to underestimate the amount of hydrated silicates that current Earth-forming
work assumes is there. Many of the planetesimal simulations have very vague models of
what the planetesimals consist of.

Andy,

I doubt the ice moons will last long in the HZ. I’d expect the water and ices to be
photolysed easily by UV, and lose hydrogen quite rapidly. Interesting idea, though.

James M. Essig January 26, 2008 at 20:47

Hi Alastair and other Folks;

It now appears that our exoplanet detection schemes are now getting sensitive enough to detect planets with masses within roughly a half order of magnitude greater than Earth, a potential super Earth around GJ 436. As we reach out to explore these planets, hopefully one day setting boots down on them, it will be wonderful even from a purely esthetic point of view to see the wide variety of land scapes and sea scapes on them, visit their inhabitants including any ETI, and see any existent varied ecosystems and associated wild life and vegetation. The mind wonders in awe at the beauty we might find including new types of life forms. I once saw a program on the Science Channel or the Discovery Channel or whatever about the possibility of life forms that are a hybrid of plant like and animal like characteristics including some rather amazingly bazaar artistic computer renditions and animations as examples of what we might find on these extrasolar worlds. The potential variety of life forms, ecosystems, and geological features on these worlds is truely awesome.

Thanks;

Jim

ljk February 1, 2008 at 23:24

Dusty Clues: Study suggests no dearth of Earths

A new study suggests that many, or perhaps most,
sunlike stars have planets much like Earth.

http://www.sciencenews.org/articles/20080202/fob2.asp

ljk April 21, 2008 at 15:44

Limits to the planet candidate GJ 436c

Authors: R. Alonso (1), M. Barbieri (1), M. Rabus (2), H.J. Deeg (2), J.A. Belmonte (2), J.M. Almenara (2) ((1) LAM, France, (2) IAC, Spain)

(Submitted on 18 Apr 2008)

Abstract: We report on H-band ground-based observations of a transit of the hot Neptune GJ 436b. Once combined to achieve an equivalent sampling as archived observations taken with Spitzer, our measurements reach comparable precision levels. We analyze both sets of observations in a consistent way, and measure the rate of orbital inclination change to be of 0.02+/-0.04 degrees in the time span between the two observations (253.8 d, corresponding to 0.03+/-0.05 degrees/yr if extrapolated).

By performing simulations of planetary systems including a second planet GJ 436c which has been recently suggested (Ribas et al. 2008), this rate allows to put limits to the relative inclination between the two planets. The allowed inclinations for a 5 M_E super-Earth GJ 436c in a 5.2 d orbit are within ~7 degrees of the one of GJ 436b; for larger differences the observed inclination change can be reproduced only during short sections (<50%) of the orbital evolution of the system. The measured times of three transit centers of the system do not show any departure from linear ephemeris, a result that is only reproduced in <1% of the simulated orbits. Put together, these results argue against the proposed planet candidate GJ 436c.

Comments: 4 pages, 4 figures, submitted to A&A letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.3030v1 [astro-ph]

Submission history

From: Roi Alonso [view email]

[v1] Fri, 18 Apr 2008 14:51:42 GMT (380kb)

http://arxiv.org/abs/0804.3030

ljk May 15, 2008 at 23:00

On the long-term tidal evolution of GJ 436b in the presence of a resonant companion

Authors: Rosemary A. Mardling

(Submitted on 13 May 2008)

Abstract: In order to explain the significant orbital eccentricity of the short-period transiting Neptune-mass planet GJ 436b and at the same time satisfy various observational constraints and anomalies, Ribas, Font-Ribera and Beaulieu have proposed the existence of an eccentric low-mass companion planet at the position of the outer 2:1 resonance.

The authors demonstrate the viability of their proposal using point-mass three-body integrations, arguing that as long as the system appears to be dynamically stable, the short-term secular variations ought to dominate the long-term dissipative evolution. Here we demonstrate that if one includes tidal dissipation, both orbits circularize after a few times the circularization timescale of the inner planet.

We conclude that with or without a nearby companion planet, in or out of the 2:1 resonance, the Q-value of GJ 436b must be near the upper bound estimate for Neptune if the system is as young as 1 Gyr, and an order of magnitude higher if the system is as old as 10 Gyr. We show detail of passage through resonance and conclude that even out of resonance, a companion planet should still be detectable through transit timing variations.

Comments: 5 pages, 5 figures. Submitted to MNRAS Letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.1928v1 [astro-ph]

Submission history

From: Rosemary Mardling [view email]

[v1] Tue, 13 May 2008 20:26:04 GMT (135kb)

http://arxiv.org/abs/0805.1928

andy May 22, 2008 at 19:17

According to the Extrasolar Planets Encyclopaedia, the discovery of GJ 436c has been retracted.

ljk May 28, 2008 at 14:18

Photometric Follow-up Observations of the Transiting Neptune-Mass Planet GJ 436b

Authors: Avi Shporer, Tsevi Mazeh, Joshua N. Winn, Matthew J. Holman, David W. Latham, Frederic Pont, Gilbert A. Esquerdo

(Submitted on 26 May 2008)

Abstract: This paper presents multi-band photometric follow-up observations of the Neptune-mass transiting planet GJ 436b, consisting of 5 new ground-based transit light curves obtained in May 2007. Together with one already published light curve we have at hand a total of 6 light curves, spanning 29 days.

The analysis of the data yields an orbital period P = 2.64386+-0.00003 days, mid-transit time T_c [HJD] =2454235.8355+-0.0001, planet mass M_p = 23.1+-0.9 M_{\earth} = 0.073+-0.003 M_{Jup}, planet radius R_p = 4.2+-0.2 R_{\earth} = 0.37+-0.01 R_{Jup} and stellar radius R_s = 0.45+-0.02 R_{\sun}. Our typical precision for the mid transit timing for each transit is about 30 seconds.

We searched the data for a possible signature of a second planet in the system through transit timing variations (TTV) and variation of the impact parameter. The analysis could not rule out a small, of the order of a minute, TTV and a long-term modulation of the impact parameter, of the order of +0.2 year^{-1}.

Comments: Submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.3915v1 [astro-ph]

Submission history

From: Avi Shporer [view email]

[v1] Mon, 26 May 2008 10:47:55 GMT (63kb)

http://arxiv.org/abs/0805.3915

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