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Are Other Solar Systems Like Our Own?

We’ve identified over 200 planets around other stars, but in many ways we know little about other solar systems. The problem is in extrapolating from our knowledge of one or two planets to an entire planetary system, much of which we cannot detect. Can we expect to find gas giants mixed with small terrestrial worlds around most Sun-like stars? And what about the smaller and far fainter red dwarfs? Clearly, the job of characterizing not just planets but entire systems is going to occupy astronomers for many a decade.

A new paper from the California & Carnegie team takes helpful steps in that direction. New planet finds are always fascinating, and the team does have four of them, the highlight being the pair orbiting the Sun-like star HIP 14810. Greg Laughlin (UC-SC) writes about that system on the systemic site, noting this:

The fact that the orbit is clearly non-circular would be strong evidence for the presence of planet c, even if there weren’t enough data to detect c directly. If planet b was the only significant planet in the system, its orbit would have circularized via tidal dissipation on a timescale that is less than the age of the star.

I mention Laughlin’s comment because it bears on where we are in the planet hunting process. The radial-velocity measurements that have found most known exoplanets depend on collecting data about how a star wobbles as it is gravitationally influenced by the planet(s) around it. Clearly, larger planets close in to their primary star are going to be the first seen with this method because they affect the star more clearly in short time frames.

But as we collect data over longer and longer periods, we begin to see more subtle signatures in the data that can point to other planets. Gas giants in outer system orbits eventually get teased out of the noise, and most intriguingly of all, we start to look for smaller worlds that may be terrestrial in nature. The California & Carnegie team points out that the mass distribution of planets is thought to increase steeply as we move into the range of lower masses. And that means that there are probably a large number of planets in systems already identified that are accompanied by other worlds.

So here are some conclusions from the study: The number of known multiple-planet systems is 22, and the number of exoplanet-bearing stars with trends in the radial velocity data that point to additional planets is also 22. The overall number of nearby stars bearing planets is 152. From the paper:

This means that 30% of known exoplanet systems show significant evidence of multiplicity. Considering that the mass distribution of planets increases steeply toward lower masses…, our incompleteness must be considerable between 1.0 and 0.1 Jupiter-masses. Thus, the actual occurrence of multiple planets among stars having one known planet must be considerably greater than 30%.

And backing out to a wider view:

From an anthropocentric perspective, this frequency of multiplicity suggests that in some respects, the Solar System is not such an aberration. Our Sun has 4 giant planets, and it appears that such multiplicity is not uncommon, although circular orbits are.

The news keeps getting more interesting the more time elapses, for as data accumulates, the planet hunt for a given star becomes that much more precise. The California & Carnegie team points out that its search is only now becoming sensitive to planets like Jupiter in 12-year orbits. Finding a true analog to Saturn would demand another 15 years of observation. It’s hard to be patient when the ultimate goal for some of us is a terrestrial-sized world in a star’s habitable zone, but every passing day yields that much more information that can tell us what other solar systems really look like.

The paper, which will run in The Astrophysical Journal in February, is Wright et al., “Four New Exoplanets, and Hints of Additional Substellar Companions to Exoplanet Host Stars,” now available online in preprint form.

Comments on this entry are closed.

  • Edg Duveyoung November 27, 2006, 11:51

    Though not an absolute, most planets and their stars would be “impure” — that is, not merely composed of hydrogen — in that they’d be composed of heavier elements formed and blown-out-into-space from the novae of the first-ever pure stars, and that this would be measurable in their spectra.

    Help me out here you astro-gurus out there, tell me how “far along” we are with this kind of process. Is our sun made of materials from “pure novae” or were our sun’s parents’ novae themselves impure, and so our sun is, ahem, an impure-grandchild?

    Is the universe populated with mostly children, grandkids, or great grandkids, or ? Seems to me, that after 10 billion years, that most of the first stars would be “gone,” so, naively I ask, are most of “today’s” stars children of pure parents or are they like our sun — grandkids from impure parents?

    Do most stars last 10 billion years like our sun is expected to enjoy? Are novae mostly occurring in galactic centers or do spiral arms have them also despite their not being subjected to nova-inducing-and-accelerating radiation from far closer neighbors in a galactic center? How well distributed are the heavier elements “now?”

    Seems to me that a grandchild star will have about as many planets as our sun does — they’d have the rocks to spit-spin outwards around which atmospheres could accumulate — form like snowflakes around dust motes.

    I wish I were as scholarly in this regard as others here seem to be. If I just need to google myself up to a higher level so’s to not bug folks here with “kiddie questions,” please tell me. But I do await answers with an eager heart, so that counts for something, eh?


  • Aesmael November 27, 2006, 20:09

    I could not speak to how many generations along dear Sol is but I think I can safely say it is third generation at the least; even the oldest stars we can find already have impurities from previous generations. The majority of stars are red dwarfs so it is true most stars will last far longer.

    As for your nova question, again I could not say anything for sure of regular novae but in spiral galaxies most supernovae are found in the arms because that is where most star formation takes place.

  • Stephen November 29, 2006, 14:06

    The trouble is that big, bright stars have short life times. There could be 100th generation stars by now, if they were all big. Small stars are dim, but have life times much greater than the age of the Universe. But, to detect them, and get the spectral data you need to determine it’s age, you need to get enough photons. This is only going to happen if they are near by. If there don’t happen to be any very old small stars in our metal rich part of the Milky Way, well, that’s the breaks, but not any really big surprise.

    There is some thought that the very first generation of stars tended to be huge. If all there was available to make them was hydrogen and helium, then they would collapse in a particular way, and this would tend to make them a particular size. So, there may be very few small, old, first generation stars that have not been subsequently contaminated.

  • Matt January 15, 2007, 16:21

    The more I look at astronomical knowledge and speculation the more it seems that life is somehow inherant to the universe.

  • ljk March 16, 2007, 12:53

    Astrophysics, abstract

    From: Wes Lockwood [view email]

    Date: Thu, 15 Mar 2007 18:45:53 GMT (1259kb)

    Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective

    Authors: G. W. Lockwood, B. A. Skiff, Gregory W. Henry, Stephen Henry, R. R. Radick, S. L. Baliunas, R. A. Donahue, W. Soon

    We examine patterns of variation of 32 primarily main sequence stars, extending our previous 7-12 year time series to 13-20 years by combining b, y data from Lowell Observatory with similar data from Fairborn Observatory. Parallel chromospheric Ca II H and K emission data from the Mount Wilson Observatory span the entire interval. The extended data strengthen the relationship between chromospheric and photometric variation derived previously. Twenty-seven stars are deemed variable. On a year-to-year timescale young active stars become fainter when their Ca II emission increases while older less active stars such as the Sun become brighter when their Ca II emission increases. The Sun’s total irradiance variation, scaled to the b and y filter photometry, still appears to be somewhat smaller than stars in our limited sample with similar mean chromospheric activity, but we now regard this discrepancy as probably due mainly to our limited stellar sample.


  • ljk May 10, 2007, 9:25

    Determination of the size, mass, and density of “exomoons” from photometric transit timing variations

    Authors: A. Simon, K. Szatmary, G.M. Szabo

    (Submitted on 8 May 2007)

    Abstract: Precise photometric measurements of the upcoming space missions allow the size, mass, and density of satellites of exoplanets to be determined.

    Here we present such an analysis using the photometric transit timing variation ($TTV_p$). We examined the light curve effects of both the transiting planet and its satellite. We define the photometric central time of the transit that is equivalent to the transit of a fixed photocenter. This point orbits the barycenter, and leads to the photometric transit timing variations. The exact value of $TTV_p$ depends on the ratio of the density, the mass, and the size of the satellite and the planet. Since two of those parameters are independent, a reliable estimation of the density ratio leads to an estimation of the size and the mass of the exomoon. Upper estimations of the parameters are possible in the case when an upper limit of $TTV_p$ is known. In case the density ratio cannot be estimated reliably, we propose an approximation with assuming equal densities. The presented photocenter $TTV_p$ analysis predicts the size of the satellite better than the mass. We simulated transits of the Earth-Moon system in front of the Sun. The estimated size and mass of the Moon are 0.020 Earth-mass and 0.274 Earth-size if equal densities are assumed. This result is comparable to the real values within a factor of 2. If we include the real density ratio (about 0.6), the results are 0.010 Earth-Mass and 0.253 Earth-size, which agree with the real values within 20%.


    6 pages, 5 figures, to appear in Astronomy and Astrophysics


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0705.1046v1 [astro-ph]

    Submission history

    From: Gyula Szabo [view email]

    [v1] Tue, 8 May 2007 08:36:53 GMT (190kb)


  • ljk May 15, 2007, 17:07

    A dynamical analysis of the 14 Her planetary system

    Authors: K. Gozdziewski, C. Migaszewski, M. Konacki

    (Submitted on 14 May 2007)

    Abstract: Precision radial velocity (RV) measurements of the Sun-like dwarf 14 Herculis in Naef et. al (2004), Butler et. al (2006) and Wittenmyer et al (2007) reveal a Jovian planet in a 1700 day orbit and a trend indicating the second distant object. On the grounds of dynamical considerations, we test a hypothesis that the trend can be explained by the presence of an additional giant planet. We derive dynamical limits to th orbital parameters of the putative outer Jovian companion in an orbit within ~12AU. In this case, the mutual interactions between the Jovian planets are important for the long-term stability of the system. Hence the kinematic model is not adequate to model the RV data. The best self-consistent and stable Newtonian fit corresponds to an edge-on configuration of Jovian planets in about 9AU orbit with a moderate eccentricity ~0.2 and confined to a zone spanned by the low-order mean motion resonances 5:1 and 6:1. This solution lies in a shallow minimum of Chi2 and persists over a wide range of the system inclination. Because the data cover roughly a half of the period (~27 yr) of the orbital solution, the semi-major axis of the outer planet cannot be well constrained. Other stable configurations within 1\sigma confidence interval of the best fit are possible and correspond to the semi-major axis of the outer planet in the range of (6,12) AU. The orbital inclination cannot yet be determined but when it decreases, both planetary masses approach ~10m_J and for i ~30 deg the hierarchy of the masses is reversed. Simultaneously, the border of dynamical stability is shifted beyond 8–9~AU.


    9 pages with low resolution figures suitable for arXiv, submitted to MNRAS, the manuscript with full resolution figures may be downloaded from this http URL (warning! large file, 9MB)


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0705.1858v1 [astro-ph]

    Submission history

    From: Krzysztof Gozdziewski [view email]

    [v1] Mon, 14 May 2007 01:39:33 GMT (468kb)


  • ljk June 27, 2007, 10:23

    An Exo-Jupiter Unveiled

    Category: astro

    Posted on: June 26, 2007 4:07 AM, by Steinn Sigurðsson

    The Extreme Solar System conference at Santorini is off with a bang!

    Number of announcements already in the first session, I’ll catch up on the highlights later, and just give the, in my opinion, most interesting.

    The California-Carnegie-AAT group has a genuine extrasolar Jupiter analog!

    Jason Wright announced it in the first session.

    It is one of several systems that have been monitored for ~ decade and been known to show a long term trend in velocity.

    The velocity variation peaked a few years ago, but had not shown a full cycle, so the orbit was poorly constrained. The second turn just came recently and the orbital solution has collapsed to a quite robust fit, this one is real I think.

    System is HD154345 – a G8V main sequence dwarf.

    About 0.9 solar masses, slightly sub-solar metallicity ( [Fe/H] = -0.1), at a distance of 18 pc

    The planet has projected (m sin(i)) mass of 1 Jupiter mass, in a 10 year orbital period, at about 4.4 AU from the star, with an eccentricity of only 0.07 +- little bit.

    This is a Jupiter – a cold gaseous giant planet in the right place, which does not look to have migrated or done anything messy.

    It is of course a fabulous target for low mass rocky planets interior to the current known giant, including in the habitable zone.

    It is also a very promising indicator that the large number of known “trending” systems being monitored will resolve out to be solar system analogs – maybe 20-30% of stars being monitored may be solar system like if this all pans out – but that is speculative at this stage.

    Really wonderful discovery and announcement. Paper is on its way soon I gather.