Three ‘Super-Earths,’ One Star

by Paul Gilster on June 16, 2008

The much anticipated Nantes conference on Extrasolar Super-Earths is already paying off big in the form of a triple system of such planets. Found around the star HD 40307, the planets are among the 45 candidate worlds recently identified by European scientists using the HARPS instrument, a spectrograph mounted on the European Southern Observatory’s 3.6-meter telescope at La Silla. The survey focused on F, G and K-class stars, finding 45 potential planets, all of which are below 30 Earth masses and show an orbital period shorter than fifty days.

What’s happening here testifies to the growing sophistication of our tools. While most of the 300+ positively identified exoplanets have been found around Sun-like stars, they have so far tended to be gas giants. Teasing smaller planets out of the data requires long observing runs — HD 40307, for example, has been under active study for five years — and it also requires the greater precision of instruments like HARPS. “With the advent of much more precise instruments such as the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla,” says planet-hunter Stéphane Udry (Geneva Observatory), “we can now discover smaller planets, with masses between 2 and 10 times the Earth’s mass.”

Orbits around HD 40307

HD 40307 is slightly less massive than the Sun, and located 42 light years away in the southern sky, in the direction of the constellations Doradus and Pictor. The three planets turn out to have masses of 4.2, 6.7 and 9.4 times that of the Earth, and they’re in tight orbits of 4.3, 9.6, and 20.4 days, respectively. Consider how sensitive our exoplanetary studies have become: The smallest of the planets has a mass one hundred thousand times less than that of the star it circles, inducing a ‘wobble’ of only a few meters per second. But to get the big picture, bear in mind that planets in close orbits are at this point easier for our instrumentation to detect than those in wide, long-period orbits. How much are we still unable to detect?

Image: Schematic view from above of the orbits of the three super-Earths around their host star. The scale is in astronomical units (AU), the mean distance between the Earth and the Sun. All planets are thus well within the orbit of Mercury around the Sun (which has an orbital period of 88 days). Credit: European Southern Observatory.

Thus we get into some exciting quotes, this one from Michel Mayor (Geneva Observatory), a familiar name as one of the discoverers of a planet around the star 51 Pegasi in 1995. That event kicked off the exoplanet hunt, one that seems to grow in intensity with each passing day. Looking not only at the super-Earths HARPS has disclosed but at the entire HARPS sample and its 45 candidate planet result, Mayor comments:

“Clearly these planets are only the tip of the iceberg. The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days.”

Good news indeed, especially when you follow it up with this from colleague Udry, who factors in the limitations of our instruments and currently available data:

“It is most probable that there are many other planets present: not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet. Add to it the Jupiter-like planets already known, and you may well arrive at the conclusion that planets are ubiquitous.”

The same team also discussed two other planetary systems at the Nantes conference. One involves a 7.5 Earth-mass world around HD 181433 in a 9.5 day orbit, accompanied by a Jupiter-like planet in a three-year orbit. The other is a 22 Earth-mass planet in a system that also includes a Saturn-class world, also with a three-year period. The conference runs until the 18th and will doubtless occupy us in coming days with a series of further results. The above work from the Geneva team has been submitted in the form of two papers to Astronomy & Astrophysics. Full references here when they become available.


dad2059 June 16, 2008 at 10:06

“…you may well arrive at the conclusion that planets are ubiquitous.”

Excellent news indeed. We may find eventually that Earth-like worlds are ‘ubiquitous’ also.

stargazerdude22 June 16, 2008 at 11:10

I wonder if these “Hot SuperEarths” followed their “Hot Jupiters”, with the result that the “Hot Jupiters” ended up being swallowed by their star. The “Hot Super Earths” would have been water worlds at one time, formed past the snow-line from mostly volitiles (ices) with a sprinkling of metals.

kurt9 June 16, 2008 at 13:12

Its not a star. Its not a gas giant. Its Super-Earth!

Sorry, I couldn’t resist.

TheoA June 16, 2008 at 14:18

Is the habitable zone still open. If so this appears to be a system not crushed by its Jupiter Migration. There should be more rocky planets further out.

Actually one third of stars having planets @ 50 day orbits can also be bad news as this is outside the habitable zone except for red dwarf types, who have their own challenges.

There is no hint of these planets being tidally locked is there?

One thing I would point out is the complete lack of data from the COROT team.

Considering the exciting stuff from HARPS and soon Kepler maybe they might be redundant fairly quickly.

Eric Goldstein June 16, 2008 at 15:32

TheoA, there are papers that suggest that Hot Jupiter migration won’t prevent the formation of habitable zone worlds. For example, see Centauri-Dreams’ coverage here:

Dave Moore June 16, 2008 at 16:03

One third of the sample has Neptune sized or super-Earth sized planets in under 50 day orbits. Obviously, the planetary formation process took a fundametally different path to our solar system’s. I’m beginning to think we need to ask why we don’t have 3-4 planets orbiting inside Mercury.

Adam June 16, 2008 at 16:29

Hi Dave

Good question! It’s quite the puzzle, though according to current theories it’s to be expected around lower mass stars because their luminosity is much lower and the snow-line is further in. In the Sun’s case the snow-line marks the formation of Jupiter, but because the disk is much smaller around lower mass stars the snow-line leads to Super-Earths – albeit icy ones.

Adam June 16, 2008 at 16:38


HD 40307 (Gliese 2046) is quite a bit dimmer than the Sun – it’s about 1/10 the Sun’s visual luminosity and maybe double that with bolometric correction. So it’s about ~ 0.7 Solar masses. Roughly the average stellar mass.

TheoA June 16, 2008 at 18:04


Thanx for pointing that out.

I thought the jury was still out on that situation, esp as such worlds have not been discovered in association with Hot Jupiters, though technology may have more to do with that.

In any case a Hot Jupiter through the inner system is a huge disruption, esp. as we are pretty sure it did not happen to our system.

Administrator June 16, 2008 at 18:24

TheoA, you might find ‘A Hot Jupiter in Our Solar System?’ interesting in this context:

Nothing definitive, but there is the possibility that we did indeed have such at one time. And you’re right, the level of our technology has a lot to say about what we can detect in systems with known ‘hot Jupiters.’ We’re getting better and better at smaller planetary detections, but it’s still tough when you’re dealing with orbits further out from the primary star.

John Hunt June 16, 2008 at 20:26

Could there (is there) a formula which combines AU distance and planet size to create a “habitability index”. Many people speak about the “habitable zone”. This may (or may not) be useful for considering the origin of life but it is not that useful in identifying planets which may be habitable using human technology. For example, Mars is outside of the habitable zone but will be inhabited by people one day.

This Habitability Index could arbitrarily set 100 as the habitability score for Earth. Then we could speak about a range in which life would be difficult if the planet were too small or too cold or conversely too large or too hot. Just like the lower limit for neonatals have been dropping, likewise the habitability range would be widened as our technology improves. From an interstellar mission standpoint this is important. One day we’ll find the closest exoplanet. But it might be outside of the habitability range.

Why does Mars get so much more attention than Venus? Probably because we all know that it’s much more habitable to humans than Venus. Our future will first be on Mars and maybe later Venus. Likewise, the first real interstellar mission might be to a human-habitable exoplanet rather than to an exoplanet closer but outside of the human-habitable range.

But what is the criteria for human habitability. Shouldn’t we develop this criteria. Has anyone done this? If we can assign a human-habitability score to each new exoplanetary finding then we will be constantly introducing the idea of the feasibility of an interstellar mission.


Adam June 17, 2008 at 2:52

Hi John

Your idea has a lot of merit and perhaps you should try doing it yourself. Stephen Dole’s classic, “Habitable Planets For Man”, is available for free from the RAND corporation and can be downloaded as a PDF file. Dole discusses the criteria for habitability in some depth – albeit from the perspective of 1964. It’s a good place to start.

Danlo June 17, 2008 at 5:33

Wow, this is sooo interesting! Why don´t we just send a solarsail-probe there? Why dont we even send a probe to Europa? Why does not anything happen?! Because latest fashion is more interesting to most people.. :(

Ronald June 17, 2008 at 8:09

@Adam and Dave Moore: luminosity ánd metallicity, most probably.

High metallicity leads to more planetary material, leads to more and bigger planets and possibly more/bigger planets in close orbit.

I have had the idea, that there may be a kind of threshold metallicity beyond which you get giant planets in close orbit, either ‘original’ or by means of inward migration. But that may have to be reconsidered in combination with luminosity.

So many planets in close orbit around solar type stars seems to be mixed (good and bad) news, even though indeed inward migration does not seem to rule out terrestrial planets in stable orbits anymore.

@John Hunt: I like the idea of a Habitability Index, thoug it would definitely have to incorporate stellar characteristics such as, in particular: luminosity, variability, multiplicity and component separation (i.e. binary, so many AU apart at min. separation), …

And I suppose, before you start considering stellar missions, you would have a lot more direct data on a planet to determine habitability, such as temperature, atmosphere, water, etc., than just these indirect parameters that we are using here.

Adam June 17, 2008 at 15:54

Hi Danlo

For “most people” on planet Earth, at the moment, getting enough food and running water are more important than sending probes to anywhere. Before we invest in serious interstellar infrastructure an energy breakthru is sorely needed here on Earth – cheap solar or nuclear fusion. Once we have that, we save the world AND can launch interstellar probes cheaply. A two-for-one deal in which we all win.

Dave Moore June 17, 2008 at 18:06

Talking about habitability, this star is a prime candidate for a habitable planet.
Given the regularity of the planets aready discovered, I would wager a small amount of money that there are planets with approximately 45, 98, and 217 day orbits. From the sensitivity of the measurements, we know there are no Jupiter or even Neptunian sized planets in those orbits. Any planets would be, at most, super Earth in size, so gravitational pertibations wouldn’t be enough to cause jumbling of the orbits or planetary ejection.

The 98 day and 217 day orbits may well fall within the star’s habitable zone.

Ronald June 18, 2008 at 3:24

@Dave: what you say about “planets with approximately 45, 98, and 217 day orbits” is quite intriguing, can you elaborate on that a bit more?

I mean why those orbits, is it a variation of Bode’s Law?

Ronald June 18, 2008 at 5:04

Further to HD 40307: based on its luminosity and in comparison with our own HZ, its HZ would be from about 0.40 – 0.52 AU.

How would that relate to orbital periods?

OK, Kepler’s 3rd: P^2 ~ a^3

P = orbital period of planet
a = semimajor axis of orbit
~ = proportional

So the 45-day-planet would be (almost) 1.7 times as far away as the 20.4-day-planet (which is at 0.12 AU), i.e. 0.20 AU; the 98-day-planet would be 2.84 times as far away i.e. 0.34 AU; the 217-day-planet would be 4.83 times as far away i.e. 0.58 AU (?)

The two outer planets would in that case both be just outside the HZ (resp., on the inside and on the outside)

Isn’t it so that those orbits depend on the planets’ masses as well, with a larger planet moving relatively faster and/or closer in?

andy June 18, 2008 at 8:21

Doing a linear regression on planet number and log(period), the next three orbits would be expected to be 45, 98 and 213 days. For a stellar mass of 0.77 times that of the Sun, and requiring the velocity semi-amplitude to be less than 1 m/s (working backwards from the stellar and planetary masses and the orbital periods, the smallest semi-amplitude among the known planets orbiting the star is 1.2 m/s for circular orbits), the mass limits on these three orbits are 4.7, 6.1 and 7.9 Earth masses respectively (again for circular orbits)

Of course, without actually having the data and the actual semi-amplitudes this analysis is on fairly shaky grounds – there might be additional linear/quadratic trends in the data which may indicate more massive planets. Plus there is no guarantee that the trend in orbital periods can be extrapolated beyond the three known planets – in principle such relationships predict an infinite number of planets at ever-increasing distances, which is not plausible.

Dave Moore June 18, 2008 at 18:54

@Ronald. From what we can tell, looking at the orbits of multiplanet systems with low ellipticities, there appears to be a regularity in the planets spacing. Andy has done a much rigorous analysis than I have.

What I do is look at the ratios of the orbital distances as a rough guide. Then use the ratio to get a rough idea where the next planet out may be or use it to look at gaps where there could be stable orbits to see whether planets would fit in using the natural progression. Each system seems to have its own spacing ratio, which varies from about 1.5 to over 2 in the case of 55 Cancri.

I’d like to thank Andy for working out the upper bounds for the masses for those orbits. I should have said that this system is a good prospect for a habitable Earth-like planet, perhaps the best we’ve had so far in that from what we know of the system there is a good possibility that there will be a super-Earth or something smaller in the habitable zone.

Of the other star systems we have discovered planets around, the habitable zone is either occupied with a Neptunian or larger planet, the orbit is unstable because of a nearby large planet, or, if an Earth-like planet could occupy a stable orbit in the habitable zone, there is nothing from what we know about the system to infer that this the case.

ljk June 25, 2008 at 13:56

Another Earth – it is only a matter of time:

But why another Earth? I mean, we’ve already got one.

We’ve got a whole crazy Universe out there, so why not
go for something more exotic? Otherwise we should just
stay home and see who wins the next American Idol.

ljk July 1, 2008 at 12:41

The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 Super-Earths (4.2, 6.9, & 9.2 Earth masses)

Authors: M. Mayor, S. Udry, C. Lovis, F. Pepe, D. Queloz, W. Benz, J.-L. Bertaux, F. Bouchy, C. Mordasini, D. Segransan

(Submitted on 27 Jun 2008)

Abstract: 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.

Comments: Submitted to A&A (6 pages)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0806.4587v1 [astro-ph]

Submission history

From: Stephane Udry [view email]

[v1] Fri, 27 Jun 2008 18:04:42 GMT (224kb,D)

ljk September 24, 2008 at 8:07

Extrasolar planets and brown dwarfs around A-F type stars V. A planetary system found with HARPS around the F6IV-V star HD 60532

Authors: M. Desort, A.-M. Lagrange, F. Galland, H. Beust, S. Udry, M. Mayor, G. Lo Curto

(Submitted on 23 Sep 2008)

Abstract: Aims: In the frame of the search for extrasolar planets and brown dwarfs around early-type stars, we present the results obtained for the F-type main-sequence star HD 60532 (F6V) with HARPS.

Methods: Using 147 spectra obtained with HARPS at La Silla on a time baseline of two years, we study the radial velocities of this star.
Results: HD 60532 radial velocities are periodically variable, and the variations have a Keplerian origin. This star is surrounded by a planetary system of two planets with minimum masses of 1 and 2.5 Mjup and orbital separations of 0.76 and 1.58 AU respectively. We also detect high-frequency, low-amplitude (10 m/s peak-to-peak) pulsations. Dynamical studies of the system point toward a possible 3:1 mean-motion resonance which should be confirmed within the next decade.

Comments: 7 pages, 11 figures, accepted for publication in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0809.3862v1 [astro-ph]

Submission history

From: Morgan Desort [view email]

[v1] Tue, 23 Sep 2008 08:22:24 GMT (1267kb,D)

ljk October 27, 2008 at 23:45

The HARPS search for southern extra-solar planets XVII. Six long-period giant planets around BD -17 0063, HD 20868, HD 73267, HD 131664, HD 145377, HD 153950

Authors: C. Moutou, M. Mayor, G. Lo Curto, S. Udry, F. Bouchy, W. Benz, C. Lovis, D. Naef, F. Pepe, D. Queloz, N.C. Santos

(Submitted on 26 Oct 2008)

Abstract: We report the discovery of six new substellar companions of main-sequence stars, detected through multiple Doppler measurements with the instrument HARPS installed on the ESO 3.6m telescope, La Silla, Chile.

These extrasolar planets are orbiting the stars BD -17 0063, HD 20868, HD 73267, HD 131664, HD 145377, HD 153950. The orbital characteristics which best fit the observed data are depicted in this paper, as well as the stellar and planetary parameters. Masses of the companions range from 2 to 18 Jupiter masses, and periods range from 100 to 2000 days.

The observational data are carefully analysed for activity-induced effects and we conclude on the reliability of the observed radial-velocity variations as of exoplanetary origin.

Of particular interest is the very massive planet (or brown-dwarf companion) around the metal-rich HD 131664 with M2sini= 18.15 MJup, and a 5.34-year orbital period. These new discoveries reinforces the observed statistical properties of the exoplanet sample as known so far.

Comments: accepted in A&A, 7 Pages + online tables

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0810.4662v1 [astro-ph]

Submission history

From: Claire Moutou [view email]

[v1] Sun, 26 Oct 2008 05:24:09 GMT (217kb)

ljk December 8, 2008 at 2:33

Habitability of Super-Earth Planets around Other Suns: Models including Red Giant Branch Evolution

Authors: W. von Bloh, M. Cuntz, K.-P. Schroeder, C. Bounama, S. Franck

(Submitted on 4 Dec 2008)

Abstract: The unexpected diversity of exoplanets includes a growing number of super- Earth planets, i.e., exoplanets with masses of up to several Earth masses and a similar chemical and mineralogical composition as Earth. We present a thermal evolution model for a 10 Earth mass planet orbiting a star like the Sun.

Our model is based on the integrated system approach, which describes the photosynthetic biomass production taking into account a variety of climatological, biogeochemical, and geodynamical processes. This allows us to identify a so-called photosynthesis-sustaining habitable zone (pHZ) determined by the limits of biological productivity on the planetary surface.

Our model considers the solar evolution during the main-sequence stage and along the Red Giant Branch as described by the most recent solar model. We obtain a large set of solutions consistent with the principal possibility of life.

The highest likelihood of habitability is found for “water worlds”. Only mass-rich water worlds are able to realize pHZ-type habitability beyond the stellar main-sequence on the Red Giant Branch.

Comments: 40 pages, 6 figures; Astrobiology (in press)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0812.1027v1 [astro-ph]

Submission history

From: Manfred Cuntz [view email]

[v1] Thu, 4 Dec 2008 21:28:33 GMT (260kb)

ljk December 10, 2008 at 1:24

arXiv: 0812.1608

Date: Tue, 9 Dec 2008 02:06:22 GMT (240kb,D)

Title: The HARPS search for southern extra-solar planets. XVII. Super-Earth and Neptune-mass planets in multiple planet systems HD47186 and HD181433

Authors: F. Bouchy, M. Mayor, C. Lovis, S. Udry, W. Benz, J-L Bertaux, X.Delfosse, C. Mordasini, F. Pepe, D. Queloz, D. Segransan

Categories: astro-ph

Comments: 5 pages, 5 figures, accepted in A&A

This paper reports on the detection of two new multiple planet systems around solar-like stars HD47186 and HD181433. The first system includes a hot Neptune of 22.78 M_Earth at 4.08-days period and a Saturn of 0.35 M_Jup at 3.7-years period. The second system includes a Super-Earth of 7.5 M_Earth at 9.4-days period, a 0.64 M$_Jup at 2.6-years period as well as a third companion of 0.54M_Jup with a period of about 6 years.

These detections increase to 20 the number of close-in low-mass exoplanets (below 0.1 M_Jup) and strengthen the fact that 80% of these planets are in a multiple planetary systems. , 240kb

andy January 14, 2009 at 9:49

Are these “Super-Earths” or “Mini-Neptunes”? Tidal evolution concerns favour the latter… The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?

ljk January 30, 2009 at 1:14

The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924

Authors: Andrew W. Howard, John A. Johnson, Geoffrey W. Marcy, Debra A. Fischer, Jason T. Wright, Gregory W. Henry, Matthew J. Giguere, Howard Isaacson, Jeff A. Valenti, Jay Anderson, Nikolai E. Piskunov

(Submitted on 28 Jan 2009)

Abstract: 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.

Comments: ApJ accepted, 10 pages, 10 figures, 4 tables

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0901.4394v1 [astro-ph.EP]

Submission history

From: Andrew Howard [view email]

[v1] Wed, 28 Jan 2009 03:36:01 GMT (90kb)

ljk February 4, 2009 at 11:28

HD45364, a pair of planets in a 3:2 mean motion resonance

Authors: A.C.M. Correia, S. Udry, M. Mayor, W. Benz, J.-L. Bertaux, F. Bouchy, J. Laskar, C. Lovis, C. Mordasini, F. Pepe, D. Queloz

(Submitted on 3 Feb 2009)

Abstract: Precise radial-velocity measurements with the HARPS spectrograph reveal the presence of two planets orbiting the solar-type star HD45364. The companion masses are 0.187 Mjup and 0.658 Mjup, with semi-major axes of 0.681 AU and 0.897 AU, and eccentricities of 0.168 and 0.097, respectively.

A dynamical analysis of the system further shows a 3:2 mean motion resonance between the two planets, which prevents close encounters and ensures the stability of the system over 5 Gyr.

This is the first time that such a resonant configuration has been observed for extra-solar planets, although there is an analogue in our Solar System formed by Neptune and Pluto. This singular planetary system may provide important constraints on planetary formation and migration scenarios.

Comments: 6 pages, 6 figures

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

DOI: 10.1051/0004-6361:200810774

Cite as: arXiv:0902.0597v1 [astro-ph.EP]

Submission history

From: Alexandre Correia [view email]

[v1] Tue, 3 Feb 2009 20:31:53 GMT (1497kb)

Comments on this entry are closed.

{ 1 trackback }