We’d all like to think our Solar System is a run-of-the-mill place, filled with the kind of planets, including our own, likely to be found around other stars. But maybe it’s not so ordinary after all. For recent work suggests that stars like the Sun aren’t all that likely to form planets the size of Jupiter or larger. So while small, rocky worlds may or may not be common — we’re still finding the answer to that one — the combination of rocky worlds and gas giants we take for granted may actually be distinctive.
Once again I’m reminded how many conjectures go into our projections of habitable worlds. Here’s one possibility: Without a large gas giant in the outer solar system to act as a gravitational shield for the inner system, planets in the habitable zone of a star might be so pelted by space debris that life would be unlikely to form on them. So it’s conceivable that any findings about the scarcity of gas giants are a blow to our astrobiological hopes around other stars. At least, around stars like our own.
The work in question looks at the Orion Nebula, that fertile breeding ground for new stars. Only a million years old, an infant in cosmic terms, this is quite a dense place, from which the view must be striking: Pack a thousand stars into a cube several light years to the side and you’ve got the idea. The Sun’s origin some four billion years ago is commonly thought to have occurred in a dense, open cluster like Orion. These clusters come apart with age, their stars gradually separating until no trace of the original cluster remains.
Using the Combined Array for Research in Millimeter Astronomy (CARMA), researchers from the University of California at Berkeley, Caltech, and the Harvard-Smithsonian Center for Astrophysics have found that only eight percent of the stars in Orion’s central region have the surrounding dust needed for a gas giant. That would be a disk with a mass greater than one-hundredth the mass of the Sun. Indeed, the average mass of a protoplanetary disk in this region was only one-thousandth of a solar mass.
Only one in ten of the stars showed radiation characteristic of any dust disk whatsoever. The paper on this work explains the ramifications: “Evidently, giant planet formation is either advanced (having thus depleted the small dust grains in the disk) or impossible around most stars in the ONC [Orion Nebula Cluster].”
Image: While a Hubble Space Telescope image of visible light emitted by a protoplanetary disk in the Orion Nebula called proplyd 170-337 shows hot, ionized gas (red) surrounding and streaming off of a disk (yellow), 1.3 mm radio observations by CARMA and SMA reveal the dust disk hiding within the hot gas (contours). This protoplanetary disk has a mass more than one hundredth that of the sun, the minimum needed to form a Jupiter-sized planet. Credit: Bally et al 2000/Hubble Space Telescope & Eisner et al 2008/CARMA, SMA.
The findings may well relate to how tightly packed the Orion stars are. John M. Carpenter (Caltech), relating the work to previous studies of the Taurus cluster (where twenty percent of the stars showed enough dust to form planets), had this to say about Orion:
“Somehow, the Orion cluster environment is not conducive to forming high mass disks or having them survive long, presumably due to the ionization field from the hot, massive OB stars , which you might expect would photoevaporate dust and lead to small disk masses.”
This (from the paper) is also interesting, while highlighting the obvious need for further work:
… our observations show no clear correlation between stellar mass and disk mass, but suggest that massive disks may be more likely to be found around lower mass stars. The percentage of detected disks is lower for stars more massive than 1 M? , and the most massive disks detected are associated with the relatively low stellar mass stars in the sample. However, larger numbers of stellar and disk mass measurements in the ONC are needed to build up better statistics and further constrain the relationship between stellar and disk properties.
Let’s relate all this this to broader exoplanet findings. Six percent of the stars thus far surveyed have planets the size of Jupiter or larger. This does not mean that smaller disks of the sort that could give rise to rocky planets are not out there, and perhaps in abundance. Improved instrumentation, like the unfinished Atacama Large Millimeter Array (ALMA) now being built in Chile, may tell us how numerous they are. But it does imply that what we assume to be a common way of producing stars like the Sun is less likely to deal up Jupiter-class worlds, with ramifications not yet fully understood. (See the comments re a slight change to the text above).
An abridged version of the paper is available online. It’s Eisner et al., “Proplyds and Massive Disks in the Orion Nebula Cluster Imaged with CARMA and SMA,” accepted for publication this August in the Astrophysical Journal.
Paul you say: “So we’ve got dust disks, but they’re smaller than we might have expected *even* in such a tightly packed young cluster.”
But what I understand from especially the first quote is that is may be *because* of the (too) densely packed cluster.
Ronald, good point, and I don’t like that sentence much either. I think I’ll go back and pull it out, as it’s confusing and in any case, the rest of that paragraph doesn’t really need it.
I think it is much to early to be trying to call the odds just yet.
The idea of Jupiter as a shield is no longer as accepted as it once was.
See Astronomy & Geophysics, Volume 49 Issue 1, Pages 1.22 – 1.27
http://www3.interscience.wiley.com/journal/119417929/abstract
” The idea that Jupiter has shielded the Earth from potentially catastrophic impacts has long permeated the public and scientific mind. But has it shielded us? We are carrying out the first detailed examination of the degree of shielding provided by Jupiter and have obtained some surprising results. Rather than Jupiter acting as a defensive presence, we found that it actually makes little difference – but if Jupiter were significantly smaller, the impact rate experienced by the Earth would be considerably enhanced. Indeed, it seems that a giant planet in the outer reaches of a planetary system can actually pose a threat to the habitability of terrestrial worlds closer to the system’s parent star.
“
Regarding your first conjecture regarding gas giants acting like a gravity shield – and reading what Alastair has linked to, I can’t agree more – there needs to be some sort of quantitative mathematical analysis to back this up.
At the simplest, I can envision that while Jupiter’s gravitational field would be sufficient to alter the course of all inbound debris, that this is still different from gravitationally capturing said debris. And an altered course doesn’t really mean it will now avoid impact.
Good points from both Alastair and Memet, nor do I disagree.
Yet again, the Kepler mission currently scheduled for launch next year may help with this survey although it probably won’t run long enough even in extended mission (TBD) mode to confirm Jupiter 5 AU analogs. But we should see what ‘scaled down’ red dwarf solar systems look like.
Gas giants are not all that scarce. A recent paper by Marcy et al conclusively demonstrates that between 10 and 20% of stars have planets down to Saturn mass with semimajor axes between 0.1 and 20 A.U.
The TEXES Survey For H2 Emission From Protoplanetary Disks
Authors: M.A. Bitner, M.J. Richter, J.H. Lacy, G.J. Herczeg, T.K. Greathouse, D.T. Jaffe, C. Salyk, G.A. Blake, D.J. Hollenbach, G.W. Doppmann, J.R. Najita, T. Currie
(Submitted on 7 Aug 2008)
Abstract: We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory.
We searched for mid-infrared H2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (~10 km/s), centered at the stellar velocity, and spatially unresolved at scales of 0.4 arcsec, which is consistent with origin from a disk at radii 10-50 AU from the star.
In cases where we detect multiple emission lines, we derive temperatures > 500 K from ~1 M_earth of gas. Our upper limits for the non-detections place upper limits on the amount of H2 gas with T > 500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions we are studying and that processes such as UV, X-ray, and accretion heating may be important.
Comments: 24 pages, 16 figures, 5 tables, ApJ accepted
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0808.1099v1 [astro-ph]
Submission history
From: Martin Bitner [view email]
[v1] Thu, 7 Aug 2008 20:00:03 GMT (107kb)
http://arxiv.org/abs/0808.1099
Models of Jupiter’s Growth Incorporating Thermal and Hydrodynamic Constraints
Authors: Jack J. Lissauer, Olenka Hubickyj, Gennaro D’Angelo, Peter Bodenheimer
(Submitted on 29 Oct 2008)
Abstract: [Abridged] We model the growth of Jupiter via core nucleated accretion, applying constraints from hydrodynamical processes that result from the disk-planet interaction. We compute the planet’s internal structure using a Henyey-type stellar evolution code. The planet’s interactions with the protoplanetary disk are calculated using 3-D hydrodynamic simulations.
Previous models of Jupiter’s growth have taken the radius of the planet to be approximately one Hill sphere radius, Rhill. However, 3-D hydrodynamic simulations show that only gas within 0.25Rhill remains bound to the planet, with the more distant gas eventually participating in the shear flow of the protoplanetary disk. Therefore in our new simulations, the planet’s outer boundary is placed at the location where gas has the thermal energy to reach the portion of the flow not bound to the planet.
We find that the smaller radius increases the time required for planetary growth by ~5%. Thermal pressure limits the rate at which a planet less than a few dozen times as massive as Earth can accumulate gas from the protoplanetary disk, whereas hydrodynamics regulates the growth rate for more massive planets. Within a moderately viscous disk, the accretion rate peaks when the planet’s mass is about equal to the mass of Saturn. In a less viscous disk hydrodynamical limits to accretion are smaller, and the accretion rate peaks at lower mass.
To account for disk dissipation, we perform some of our simulations of Jupiter’s growth within a disk whose surface gas density decreases on a timescale of 3Myr. According to our simulations, proto-Jupiter’s distended and thermally-supported envelope was too small to capture the planet’s current retinue of irregular satellites as advocated by Pollack et al. (1979).
Comments: 17 pages 12 figures, 4 tables. To appear in the journal Icarus
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0810.5186v1 [astro-ph]
Submission history
From: Gennaro D’Angelo Dr. [view email]
[v1] Wed, 29 Oct 2008 03:20:44 GMT (1413kb,D)
http://arxiv.org/abs/0810.5186
Long Period Exoplanets
Authors: Caleb Scharf, Kristen Menou
(Submitted on 12 Nov 2008)
Abstract: Recent imaging campaigns indicate the possible existence of massive planets on longer than 1000 year orbits about a few percent of normal stars. Such objects are not easily explained in most current planet formation models.
In this Letter we use a large ensemble of N-body simulations to evaluate the potential for planet scattering during relaxation of dynamically active systems to explain very long period, massive planets.
We find that such a mechanism could indeed be at play, and that statistical samples of long period planets could place interesting constraints on early stage planet formation scenarios. Results from direct imaging and microlensing surveys are complementary probes of this dynamical relaxation process.
Comments: Submitted ApJ Letters
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0811.1981v1 [astro-ph]
Submission history
From: Caleb A. Scharf [view email]
[v1] Wed, 12 Nov 2008 19:39:28 GMT (137kb)
http://arxiv.org/abs/0811.1981