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Planets Around Sun-like Stars

What jumps out at the reader when examining yesterday’s exoplanet news is not so much that we’ve found as many as six low-mass planets, but that the two stars involved are both near twins of the Sun. Steven Vogt (UC-Santa Cruz) and Paul Butler (Carnegie Institution of Washington) led this work, and Vogt is quick to point out that two of the planets are ‘super-Earths,’ the first we’ve ever found around stars so similar to our own. Vogt notes this has implications for the broader hunt for planets that could sustain life:

“These detections indicate that low-mass planets are quite common around nearby stars. The discovery of potentially habitable nearby worlds may be just a few years away.”

A Bonanza Around 61 Virginis

And Sun-like they are, these stars. 61 Virginis, 28 light years from Earth, has long fascinated astrobiologists because it is more similar to the Sun than any of our nearest neighbors in terms of age, mass and other properties. Moreover, a separate team working with the Spitzer Space Telescope has discovered a thick ring of dust around 61 Vir at about 80 AU, twice the distance of Pluto from the Sun. No doubt we’re looking at the result of cometary collisions in the outer reaches of this interesting system.

61 Vir is now known, from this ground-based radial velocity work using data from the Keck Observatory as well as the Anglo-Australian Telescope, to host at least three planets, ranging in mass from five to 25 times that of Earth. And UC-Santa Cruz postdoc Eugenio Rivera notes that “Spitzer’s detection of cold dust orbiting 61 Vir indicates that there’s a real kinship between the Sun and 61 Vir.” Indeed, Rivera’s simulations on this system show that a habitable Earth-like world could readily exist in the region between the newly discovered planets and the outer dust disk.

61Virbcd_orbits

Image: A comparison of the orbits of the planets of 61 Vir with the inner planets in our Solar System. All three planets discovered to date in this system would lie inside the orbit of Venus. Credit: University of New South Wales/Carnegie Institution.

The Case of HD 1461

The second star in question is HD 1461, like 61 Vir a near twin of the Sun, some 76 light years away. We’ve found one definite planet there, a world of 7.5 Earth masses, although we lack the data to know whether this is a ‘super-Earth’ or something closer to Uranus or Neptune. Indications of two other planets are also present. Whatever the case, note the precision involved in this work. The inner 61 Vir planet, says Butler, produced a planetary signal that is among the two or three lowest-amplitude signals that have ever been identified. Butler goes on to make this remarkable observation:

We’ve found there is a tremendous advantage to be gained from combining data from the AAT and Keck telescopes, two world-class observatories, and it’s clear that we’ll have an excellent shot at identifying potentially habitable planets around the very nearest stars within just a few years.”

This is good news indeed, especially coupled with the growing belief that as many as half of nearby stars have detectable planets with masses less than or equal to Neptune’s. Ponder the fact that this is all being done with ground-based telescopes using the radial-velocity method. In this case, the team then matched their work with measurements from robotic telescopes in Arizona to ensure there was no brightness variability in either of the stars in question. The fact that there was not rules out patterns of dark spots on these stars that could have thrown the planetary findings into question.

Tuning Up Ground-Based Detections

You’ll want to read Greg Laughlin’s brief comment on the finds on his systemic site (Laughlin was a member of the UC-Santa Cruz team and a co-author of the two papers the work has produced). He notes that our ground-based observing techniques have now been refined to the point where Earth-mass objects around nearby stars are within reach:

“It’s come down to a neck-and-neck race as to whether the first potentially habitable planets will be detected from the ground or from space,” Laughlin said. “A few years ago, I’d have put my money on space-based detection methods, but now it really appears to be a toss-up. What is truly exciting about the current ground-based radial velocity detection method is that it is capable of locating the very closest potentially habitable planets.”

For newcomers here, I’ll also note that the Lick-Carnegie Exoplanet Survey Team has created an excellent tool available to the public that allows anyone so inclined to search for extrasolar planet signals by exploring real data sets. The Systemic Console is available through the systemic site.

The papers, both slated for publication in the Astrophysical Journal, are Vogt et al., “A Super-Earth and two Neptunes Orbiting the Nearby Sun-like star 61 Virginis,” (preprint), and Rivera et al., “A Super-Earth Orbiting the Nearby Sun-like Star HD 1461,” (preprint).

A Jupiter in a Circular Orbit

But the good news doesn’t stop here. A Jupiter-mass planet has been discovered around the G5V star 23 Librae in a fourteen-year orbit similar to Jupiter’s ten-year period. This one is especially intriguing because it shows we’re beginning to be able to detect the signatures of solar systems that are much like our own. Hugh Jones (University of Hertfordshire) comments:

“Since Jupiter dominates the signal from our Solar System, we are now in a position to quantify how common planets like Jupiter are around stars like our Sun. Compared to the Solar System, most extrasolar systems look odd, with planets in very small or very elliptical orbits. In contrast, this new planet has an orbit that is both large, and nearly circular — for the first time we are beginning to see systems that resemble our own.”

This work is also a collaboration between the Anglo-Australian Telescope and the Keck Telescope, and the system they’re reporting on is an unusual one in other ways. A previously known giant planet has been discovered orbiting 23 Lib in an eccentric eight-month orbit. “This makes this yet another unexpected oddball – a system with a circular Jupiter-like planet, but with an interior planet in an eccentric orbit,” notes Rob Wittenmyer of the University of New South Wales.

The paper on the 23 Lib discovery is Jones et al., “A long-period planet orbiting a nearby Sun-like star,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).

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Comments on this entry are closed.

  • terry December 15, 2009, 12:06

    well, I’m going to have to rewrite a short story I wrote set at 61 Vir.

  • Carl December 15, 2009, 12:08

    61 Virginis’ comet cloud is interesting for possible refueling schemes. The Enzmann design https://www.centauri-dreams.org/?p=9961 is one that may utilize the remote volatile bodies for deuterium content.

    Even though this system is seven times the distance of Alpha Centauri, notions employing this advanced rocketry come to mind.

  • Zen Blade December 15, 2009, 13:28

    Regarding the planets around 61Vir, did the authors speculate on the off-centered nature of the planets (or atleast of two of them).

    I don’t know nearly enough to speculate about what influence that orbit would have on nearby planets, but would a stable habitable orbit still be possible given either the influence of 61 VirD or the influence of whatever is responsible for the VirD orbit?

  • Administrator December 15, 2009, 14:59

    There’s quite a bit of speculation about the orbits in the 61 Vir paper. Note this:

    Given the three-planet model, we can look either for solutions in which the planetary orbits are circular or solutions where the eccentricities are allowed to float. Inclusion of eccentricities provides only a modest improvement to the orbital fit, and we conclude that a significant amount of additional Doppler velocity monitoring will be required to improve measurement of the eccentricities.

    More data needed!

  • fbj December 15, 2009, 16:56

    “for the first time we are beginning to see systems that resemble our own.”

    Not true!

    http://exoplanet.eu/planet.php?p1=HD+154345&p2=b

  • Ronald December 15, 2009, 20:11

    Fascinating times!

    But, to be honest, I am not only fascinated, but also a bit disappointed.

    What I mean is this: 61 Virginis is not coincidentally one of my favourite stars, in my top-10 of ‘most promising solar type star candidates for earth-like planets’. Stable G5 V star, 80% of solar luminosity, about 90% of solar mass, metallicity 93 – 95% of solar, estimated age about 6 – 6.5 gy, not too old for such a G5 star. And only a cosmic hop away at 27.8 ly ;-)

    No reason, one would guess from metallicity, to have a super-earth and two subgiants in such close orbits. I thought that was more typical of high metallicity stars.

    Questions that come up in my mind are now:

    – What really causes (sub)giants in close orbit, if not just overall metallicity? Is there another predominant mechanism determining this in early planet-formation history? Such as particular elements.

    – More important: can there still exist earth-like planets in stable wider orbits, that is, in the Habitable Zone of 61 Virginis? I estimate the HZ of 61 Virginis extends from about 0.85 to 1.1 AU (or a bit further out). Or do these inner big planets spoil it for such earth-like planets in more earth-like orbits?

  • spaceman December 15, 2009, 21:12

    Dear Administrator,

    In this paper the authors refer to the population of Neptune and super-Earth planets found by Mayor et al as “postulated” and “putative.” Have you or anyone else who frequents this forum heard that the Swiss results may be in conflict with, say, the NASA-UC Eta-Earth survey? In other words, are there any doubts in the radial velocity planet search community about the veracity of the Swiss results which imply that close in low-medium mass planets are quite common?

  • Ronald December 16, 2009, 5:16

    What is further interesting about 23 Librae (HD 134987), is that the earlier discovered planet (b) is a giant (at least 1.5 times Mj) at 0.8 AU, which is roughly the HZ of that G5 star (ok, not taking into account eccentricity).

    This could also imply some interesting exomoons.

  • Ronald December 16, 2009, 10:20

    I would like to know whether and to what extent the location (i.e. small orbit) of the discovered planets around 61 Virginis is the result of inward migration, as is probably the case with true giant planets in close orbit (hot Jupiters).
    I suppose that this would make an essential difference with regard to the chances of earthlike planets in the HZ.

    Further, I am curious to know what is known, in this regard, about Alpha Mensae (Gl 231, HD 43834).
    This star (also relatively nearby) is quite similar to 61 Virginis, also G5/G6, similar mass, luminosity, metallicity and (I think) age.

  • andy December 16, 2009, 14:18

    The discovery of another transiting “super-Earth” has been reported in Nature. GJ 1214b is rather a different kind of planet to CoRoT-7b: it has 6.55 Earth masses and seems to possess a hydrogen atmosphere. Some “super-Earths” are “mini-Neptunes” it would seem.

  • Mike Lorrey December 18, 2009, 1:10

    I suspect that close in planet around 23 Lib in the eccentric orbit is likely two planets in resonance, which some papers have suggested is likely for many eccentric orbit discoveries, as two resonant planets can show the same signal as one larger planet in an eccentric orbit.

  • andy December 18, 2009, 18:16

    Mike Lorrey: this is definitely something to consider. One example of something like this happening is Mu Arae (HD 160691): the discovery of an extra planet close to or perhaps in a 2:1 resonance with one of the known planets resulted in a configuration which revised the eccentricities downwards quite significantly.

    Seems to have been a bit of a bumper week for exoplanets: another circumbinary planet, this time orbiting the AM Herculis-type cataclysmic variable DP Leonis, bringing the total number of known circumbinary planets up to 5 in a total of 4 systems (the others being PSR B1620-26, QS Virginis and the double planet system HW Virginis). Also a paper which announces three planets around metal-poor stars which contains the interesting conclusion that long-period Jupiters are not uncommon around moderately metal-poor stars.

  • ljk January 3, 2010, 0:12

    The Calan-Hertfordshire Extrasolar Planet Search

    Authors: J.S. Jenkins, H.R.A. Jones, K. Gozdziewski, C. Migaszewski, J.R. Barnes, M.I. Jones, P. Rojo, D.J. Pinfield, A.C. Day-Jones, S. Hoyer

    (Submitted on 29 Dec 2009)

    Abstract: The detailed study of the exoplanetary systems HD189733 and HD209458 has given rise to a wealth of exciting information on the physics of exoplanetary atmospheres. To further our understanding of the make-up and processes within these atmospheres we require a larger sample of bright transiting planets.

    We have began a project to detect more bright transiting planets in the southern hemisphere by utilising precision radial-velocity measurements.

    We have observed a constrained sample of bright, inactive and metal-rich stars using the HARPS instrument and here we present the current status of this project, along with our first discoveries which include a brown dwarf/extreme-Jovian exoplanet found in the brown dwarf desert region around the star HD191760 and improved orbits for three other exoplanetary systems HD48265, HD143361 and HD154672.

    Finally, we briefly discuss the future of this project and the current prospects we have for discovering more bright transiting planets.

    Comments: 4 pages, 2 figures, to appear in the conference proceedings “New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets” Shanghai 2009

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

    Cite as: arXiv:0912.5325v1 [astro-ph.EP]

    Submission history

    From: James Jenkins Dr [view email]

    [v1] Tue, 29 Dec 2009 16:58:02 GMT (141kb)

    http://arxiv.org/abs/0912.5325