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A Solar System Analog?

We always have to watch our preconceptions, an early one in the exoplanet game being that solar systems around other stars would look pretty much like our own. Then we started the whole exoplanet discovery binge by finding planets around a pulsar, of all things, and went on to the terrifically odd world of ‘hot Jupiters,’ whose existence had not been predicted by most theorists. Now we’ve gotten used to the idea that solar systems come in huge variety, but finding one that looks more or less like ours would still be comforting, and would make it seem more likely that there are other ‘Earths’ out there, perhaps teeming with life.

Today and tomorrow we look at two such finds, noting the resemblance to what we have around Sol and pondering the implications for the broader planet search. First up is not a single but a double planetary find, two worlds that inhabit a place much like that of Jupiter and Saturn in our Solar System. Not only is this an intriguing discovery in itself, but note the distance involved: We’re dealing with a system fully 5000 light years from Earth. The only method that will snare that kind of catch is gravitational microlensing, a method triumphantly vindicated by these recent findings.

Microlensing happens when a star crosses in front of a more distant star, the gravity of the nearer object magnifying the light being received from the more distant. Few examples of microlensing thus far can equal this one, which resulted in a magnification of 500 times, revealing the presence of the two planets. The planets actually orbit the nearer star, but cause their own changes to the light intensity of the more distant object. We’ve found Jupiter-class worlds with microlensing before, but the additional planet is the result of the unusual opportunity afforded by this event. Says Scott Gaudi (Ohio State University):

“This is the first time we had a high-enough magnification event where we had significant sensitivity to a second planet — and we found one. You could call it luck, but I think it might just mean that these systems are common throughout our galaxy.”

Microlensing explained

If common, such systems give credence to the idea that systems like ours may eventually start filling up our exoplanet catalogs, but do note the differences: The two worlds, assumed to be gas giants, are roughly 80 percent as big as Jupiter and Saturn, and the star they orbit is roughly half the size of Sol, releasing a mere five percent as much light as the Sun. But the similarities are also noteworthy. The smaller of the planets is twice the distance from its star as the larger (this is the case of Jupiter and Saturn in our Solar System), and the closer planetary orbits mean that the temperatures of these worlds are similar to those of Jupiter and Saturn as well.

Image: Microlensing works by using the gravity of the near star (shown at the bottom left), and its orbiting planets as a lens, magnifying light from a background star. By studying how the complex patterns of magnified light change as the near star and the two planets of the planetary system move in front of the distant star (top right), we can determine the properties of the planets and their star. Credit: KASI/CBNU/ARCSEC.

Intriguing news for planet formation theorists? Believe it. Gaudi again:

“The temperatures are important because these dictate the amount of material that is available for planet formation. Most theorists think that the biggest planet in our solar system formed at Jupiter’s location because that is the closest to the sun that ice can form. Saturn is the next biggest because it is in the next location further away, where there is less primordial material available to form planets.

“Theorists have wondered whether gas giants in other solar systems would form in the same way as ours did. This system seems to answer in the affirmative.”

But get this: Gaudi believes that next generation microlensing experiments will be able to find Solar System analogs down to the Mercury level. That means all known planets but one in our system, and would include any terrestrial worlds in the exosystem under investigation. The work grows out of observations made by the Optical Gravitational Lensing Experiment, with follow-up by the Microlensing Follow Up Network (MicroFUN), which helped to coordinate worldwide observations.

You can call this a solar system analog or a ‘scaled solar system’ (Gaudi uses both terms), though I would prefer to wait until we know more about this system before using that term. Nor do we have any way of knowing whether the optimistic thought that such systems are common will really hold. But since this is the first time microlensing has been able to operate at this level of magnification, that does mean that the first time we were able to see a potential Solar System analog with microlensing, we did. Exciting stuff, which coupled with the discovery of a close Jupiter analog (and here the word ‘analog’ really fits) that we’ll describe tomorrow points to how fast we’re moving toward detecting planetary environments not dissimilar from our own.

The paper is Gaudi et al., “Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing,” Science Vol. 319, No. 5865 (February 15, 2008), pp. 927-930 (abstract).

Comments on this entry are closed.

  • andy February 14, 2008, 14:30

    Another significant result from this detection is the measurement of the orbital motion of the outer companion, which suggests the system is not a wildly eccentric one. Microlensing is a very interesting technique and has made important discoveries, such as the first “cold super-Earth” OGLE-2005-BLG-390Lb.

    In a similar vein, but from the traditional radial velocity technique, the exoplanet HD 154345b is apparently also a Jupiter analogue – a planet with minimum mass 0.95 times that of Jupiter, located in a 4.2 AU orbit around a G8 star.

    arXiv: The Jupiter Twin HD 154345b

  • Administrator February 14, 2008, 16:16

    Reading my mind, Andy. HD 154345b is the topic of tomorrow’s post.

  • Hans Bausewein February 16, 2008, 5:13

    Are microlensing events already being predicted?

    That would help a lot in getting more observations.

  • andy February 17, 2008, 5:37

    In fact, there’s an analogue of the OGLE-2006-BLG-109Lb planet much closer to home: Gliese 849b, located at a mere 29 light years from Earth. The radial velocity measurements of Gliese 849 also exhibit a linear trend, which indicates that the system may have an additional, more distant companion, which hasn’t completed an orbit over the timescale of the measurements.

  • ljk February 19, 2008, 10:19

    BLG-109: A Distant Version of our own Solar System

    Illustration Credit: KASI, CBNU, ARCSEC, NSF

    Explanation: How common are planetary systems like our own? Perhaps quite common, as the first system of planets like our own Solar System has been discovered using a newly adapted technique that, so far, has probed only six planetary systems. The technique, called gravitational microlensing, looks for telling brightness changes in measured starlight when a foreground star with planets chances almost directly in front of a more distant star. The distant star’s light is slightly deflected in predictable ways by the gravity of the closer system. Recently a detailed analysis of microlensing system OGLE-2006-BLG-109 has related brightness variations to two planets that are similar to Jupiter and Saturn of our own Solar System. This discovery carries the tantalizing implication that interior planets, possibly including Earth-like planets, might also be common. Pictured above is an artistic conception of how the BLG-109 planetary system might look.

    http://antwrp.gsfc.nasa.gov/apod/ap080218.html

  • ljk February 20, 2008, 11:22

    Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Del, xi Aql, and HD 81688

    Authors: Bun’ei Sato, Hideyuki Izumiura, Eri Toyota, Eiji Kambe, Masahiro Ikoma, Masashi Omiya, Seiji Masuda, Yoichi Takeda, Daisuke Murata, Yoichi Itoh, Hiroyasu Ando, Michitoshi Yoshida, Eiichiro Kokubo, Shigeru Ida

    (Submitted on 19 Feb 2008)

    Abstract: We report the detection of 3 new extrasolar planets from the precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, namely, 18 Del (G6 III), xi Aql (K0 III) and HD 81688 (K0 III-IV), are located at the clump region on the HR diagram with estimated masses of 2.1-2.3 M_solar.

    18 Del b has a minimum mass of 10.3 M_Jup and resides in a nearly circular orbit with period of 993 days, which is the longest one ever discovered around evolved stars. xi Aql b and HD 81688 b have minimum masses of 2.8 and 2.7 M_Jup, and reside in nearly circular orbits with periods of 137 and 184 days, respectively, which are the shortest ones among planets around evolved stars. All of the substellar companions ever discovered around possible intermediate-mass (1.7-3.9 M_solar) clump giants have semimajor axes larger than 0.68 AU, suggesting the lack of short-period planets.

    Our numerical calculations suggest that Jupiter-mass planets within about 0.5 AU (even up to 1 AU depending on the metallicity and adopted models) around 2-3 M_solar stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars.

    Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial or due to engulfment by central stars. Deriving reliable mass and evolutionary status for evolved stars is highly required for further investigation of formation and evolution of planetary systems around intermediate-mass stars.

    Comments: 28 pages, 9 figures, accepted for publication in PASJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2590v1 [astro-ph]

    Submission history

    From: Bunei Sato [view email]

    [v1] Tue, 19 Feb 2008 05:19:59 GMT (136kb)

    http://arxiv.org/abs/0802.2590

  • ljk March 10, 2008, 23:52

    q1 Eri: a solar-type star with a planet and a dust belt

    Authors: R. Liseau, C. Risacher, A. Brandeker, C. Eiroa, M. Fridlund, R. Nilsson, G. Olofsson, G. Pilbratt, P. Thebault

    (Submitted on 9 Mar 2008)

    Abstract: Only very few solar-type stars exhibiting an infrared excess and harbouring planets are known to date. Indeed, merely a single case of a star-planet-disk system has previously been detected at submillimeter (submm) wavelengths. Consequently, one of our aims is to understand the reasons for these poor statistics, i.e., whether these results reflected the composition and/or the physics of the planetary disks or were simply due to observational bias and selection effects. Finding more examples would be very significant.

    The selected target, q1 Eri, is a solar-type star, which was known to possess a planet, q1 Eri b, and to exhibit excess emission at IRAS wavelengths, but had remained undetected in the millimeter regime. Therefore, submm flux densities would be needed to better constrain the physical characteristics of the planetary disk. Consequently, we performed submm imaging observations of q1 Eri. The detected dust toward q1 Eri at 870 micron exhibits the remarkable fact that the entire SED, from the IR to mm-wavelengths, is fit by a single temperature blackbody function (60 K). This would imply that the emitting regions are confined to a narrow region (ring) at radial distances much larger than the orbital distance of q1 Eri b, and that the emitting particles are considerably larger than some hundred micron. However, the 870 micron source is extended, with a full-width-half-maximum of roughly 600 AU. Therefore, a physically more compelling model also invokes a belt of cold dust (17 K), located at 300 AU from the star and about 60 AU wide. The minimum mass of 0.04 Mearth (3 Mmoon) of 1 mm-size icy ring-particles is considerable, given the stellar age of about 1 Gyr. These big grains form an inner edge at about 25 AU, which may suggest the presence of an unseen outer planet (q1 Eri c).

    Comments: 4 pages, 2 colour figures, Letter for Astronomy and Astrophysics, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1294v1 [astro-ph]

    Submission history

    From: Ren\’e Liseau [view email]

    [v1] Sun, 9 Mar 2008 13:35:25 GMT (162kb)

    http://arxiv.org/abs/0803.1294

  • ljk July 1, 2008, 12:32

    Prospects for the habitability of OGLE-2006-BLG-109L

    Authors: Renu Malhotra, David A. Minton

    (Submitted on 27 Jun 2008)

    Abstract: The extrasolar system OGLE-2006-BLG-109L is the first multiple-planet system to be discovered by gravitational microlensing (Gaudi et al., 2008); the two large planets that have been detected have mass ratios, semimajor axis ratios, and equilibrium temperatures that are similar to those of Jupiter and Saturn; the mass of the host star is only 0.5 M_sun, and the system is more compact than our own Solar system.

    We find that in the habitable zone of the host star, the two detected planets resonantly excite large orbital eccentricities on a putative earth-mass planet, driving such a planet out of the habitable zone.

    We show that an additional inner planet of ~>0.3M_earth at <~0.1 AU would suppress the eccentricity perturbation and greatly improve the prospects for habitability of the system.

    Thus, the planetary architecture of a potentially habitable OGLE-2006-BLG-109L planetary system — with two “terrestrial” planets and two jovian planets — could bear very close resemblance to our own Solar system.

    Comments: 11 pages including 4 figures; accepted for publication in ApJ-Letters

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.4409v1 [astro-ph]

    Submission history

    From: Renu Malhotra [view email]

    [v1] Fri, 27 Jun 2008 00:14:21 GMT (32kb)

    http://arxiv.org/abs/0806.4409

  • ljk August 7, 2009, 14:43

    Formation of the Terrestrial Planets from a Narrow Annulus

    Authors: Brad Hansen

    (Submitted on 5 Aug 2009)

    Abstract: We show that the assembly of the Solar System terrestrial planets can be successfully modelled with all of the mass initially confined to a narrow annulus between 0.7 and 1.0 AU.

    With this configuration, analogues of Mercury and Mars often form from the collisional evolution of material diffusing out of the annulus under the scattering of the forming Earth and Venus analogues.

    The final systems also possess eccentricities and inclinations that match the observations, without recourse to dynamical friction from remnant small body populations.

    Finally, the characteristic assembly timescale for Earth analogues is rapid in this model, and consistent with cosmochemical models based on the $^{182}$Hf–$^{182}$W isotopes.

    The agreement between this model and the observations suggests that terrestrial planet systems may also be formed in `planet traps’, as has been proposed recently for the cores of giant planets in our solar system and others.

    Comments: 37 pages, 16 figures. to appear in ApJ

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

    Cite as: arXiv:0908.0743v1 [astro-ph.EP]

    Submission history

    From: Brad Hansen [view email]

    [v1] Wed, 5 Aug 2009 20:11:46 GMT (327kb)

    http://arxiv.org/abs/0908.0743