Friday, February 12, 2016

A Stranger at Home

An interesting consequence of being asked the same question repeatedly, is that you stop thinking about the answer. Instead, you find yourself reciting a variant of the same prefabricated response that you gave the previous twelve times. Naturally, in this mode of operation, your brain is susceptible to being stumped by otherwise trivial inquiries, simply because you haven’t heard them before and they don’t automatically register in the existing database. Recently, I found myself in exactly this situation. 

A recurrent question about which Mike and I have thought extensively is “what if you’re wrong?” For an extended discussion about this possibility, scroll down to the previous post. But the perplexing question, posed to me by a reporter some time ago, was a different one: “what if you are right?” In all honesty, my first reaction was “huh? What does this even really mean?” Of course, we hope that we are right! We hope that the dynamical mechanism connecting the alignment of the distant Kuiper belt orbits, the detachment of Sedna-type ellipses from Neptune and the mysteriously inclined trajectories of large semi-major axis Centaurs, is a chaotic web of mean-motion resonances facilitated by Planet Nine. Moreover, we hope that Planet Nine will be observationally detected, like, as soon as possible, ya know what I’m sayin’?… 

But on second thought, it is evident that I was being dopey. This question has considerable depth. If we are right, the clockwork of our solar system is about to acquire a very aberrant new gear, and this has profound implications for how our strange cosmic home fits into its extrasolar context. More specifically, the detection of Planet Nine would render our solar system a slightly less abnormal member of the Galactic planetary census.

In order to understand just how unusual the architecture of the known solar system is, it is useful to dial the clock back to late November of 1995 - that is, to the discovery of the first planet around another sun-like star. With a mass slightly larger than that of Saturn, this object (dubbed 51 Peg b) is bonafide giant planet. However, unlike Jupiter and Saturn, that require more than a decade to finish a single revolution around the sun, 51 Peg b completes its orbital trek in a little over four days. Indeed, the first proof that planets around other main-sequence stars are extant also provided the first hint that orbital architectures of extrasolar planets can be very different from that of solar system’s planets.

Observational characterization of more expansive giant planet orbits during the subsequent decade and a half continued to yield surprises. Evidently, long-period giant planets tend to occupy eccentric, rather than circular, orbits. The figure below shows the semi-major axis - eccentricity distribution of well-characterized extrasolar planets. While the solar system giants would be found scraping the bottom of this figure, exoplanets clearly occupy the the entire eccentricity range, with a nearly parabolic orbit of HD 20782 b at the helm of the population. 

Figure 1: semimajor axis - eccentricity distribution of well-characterized extrasolar planets. The predicted orbit of Planet Nine is shown as well.

More recently, the triumphant success of the Kepler mission showed that the default mode of planet formation in the galaxy generates objects that are somewhat smaller than Uranus and Neptune, but are substantially more massive than the Earth. In other words, planetary masses of order ~10 Earth masses are not only prevalent in the exoplanet catalog, they are dominant. Although the transit technique limits the observational window of Kepler to orbits inside ~1AU, there is little reason to suggest that more distant orbits should be devoid of such planets.

Figure 2: the catalog of planetary candidates detected by Kepler. The sizes of the depicted points are representative of the corresponding planetary radii. The semi-major axes are shown on a logarithmic scale. Figure from Batygin & Laughlin 2015.


Cumulatively, a distinct picture of the Galactic planetary census is beginning emerge, wherein the ordered orbits of the known planets of the solar system are starting to appear increasingly abnormal. On the other hand, with a characteristic mass approximately 10 times greater than the Earth and an eccentricity of ~0.6, Planet Nine fits into this extrasolar planetary album seamlessly. Intriguingly, this yet-unseen world may provide the closest link between our solar system, and the extrasolar realm. Indeed, Planet Nine may constitute the closest thing to the solar system’s very own extrasolar planet.

41 comments:

  1. 51 Peg b has a name (Dimidium) which it is probably best to use (in addition) when writing for the general public.

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  2. Ugh. I would prefer to use that name never. But maybe with time it will grow on me.

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    1. Not my favourite either. But then, some people didn't like Eris very much! You have to take the rough with the smooth. :)

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    2. Just to poke back, one could make the counterargument that Earthlings may have to know their own planets, but placing *all* foreign planets is not supposed to be common knowledge. E.g. ask Europeans to place all the states of US, or USians to place all the nations of Europe.

      I'm glad that I can place some asteroids (like Ceres), some moons (like Moon!), et cetera. And now I have some Planet Nine minions (like Sedna) memorized...

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  3. I've heard of 51 Peg b, but "dimidium" sounds like a new addition to the periodic table.

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    1. I've just looked it up - what a dog's breakfast!
      https://www.newscientist.com/article/dn28676-public-vote-renames-exoplanets-for-gods-monsters-and-scientists/

      OH, and dimidium IS a chemical. And (for Pete's sake!) a rock band...

      THIS member of the public is sticking to Peg 51 b (and I can't see me referring to a planet as Poltergeist either. What the blue blazes was anyone thinking!!!)

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    2. I like Poltergeist - and Draugr and Phobetor - for pulsar planets, together with Lich for the pulsar. Seem appropriate for a dead stellar system. I voted for them myself! :)

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    3. Oh wow - Dimidium (the band) is not half bad! :-)

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    4. Oh my, Konstantin - love you as an astronomer/astrophysicist/planet-locator... But, errr, very different taste in music! I listened to this (well, one minute of this) with incredulity!

      http://youtu.be/Frb42gq81mY

      The band otherwise known as 51 Peg b !!

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    5. Konstantin has to speak for himself of course, but his very interesting webinterview (vinterview? um, no) had him claim eclectic music taste, tailored to the context.

      That said, I spent the first 30 s wondering when the band would start to produce music... I'm too imprinted on music of common dance floors.

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    6. My friend had/has a band 51 Peg, let's not get them confused! https://en.m.wikipedia.org/wiki/51_Peg_(band) interestingly enough, then now stay reunion on January 20, 2016. Coincidence, synchronicity, Or causality ?

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  4. Now, about the actual blog post!
    Great article, Konstantin, thanks very much. I hadn't realised how very different our own solar system (as known to date) is to the rest of the known systems.

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    1. Thanks - the realization that our solar system is an outlier is admittedly, inherently uncomfortable. At the same time, with exoplanet data being as plentiful as it now is, there seems to be no way around it.

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    2. Terry Pratchett wrote something about every probability curve having to have a far end (about his Discworld creation), but, yes, it is a rather strange sensation to consider the real possibility that the Sol solar system is a space oddity.

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    3. Can then ask the question as to whether the Solar system being an outlier has any bearing on there being no signs of intelligent life elsewhere.

      As for our outlier status: is this affected by long period exoplanets being hard to spot using the transit or radial velocity methods?

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    4. One of my interests is astrobiology, so I feel compelled to respond here.

      First, Konstantin is the one that has researched system architectures! But as an interested layman I can't say that I have to agree with a claim of our system being abnormal or even an outlier in a distribution as a definitive claim. As far as I know we know too little about more distant planets and system formation both.

      To show that this may be an open question, I think the opposite claim can be made. There is a paper that claimed that the K1 database (in 2014) could be synthesized to a bimodal planet distribution of either 0-1 gas giants or 4-8 mixed planets, the so called "Kepler dichotomy". [ http://arxiv.org/abs/1410.4192 ] A dynamic process that predicts that could be that systems tend to start out as ours, but in some cases have ejections of planets (which sounds familiar, doesn't it).

      Also, the recent pebble formation models that bridges the meter-sized gap in planet formation reproduces our own system well, with Mars sizes and all, at 3 sigma outcomes. (I.e. our system is not a too rare result.) But I assume having an ejected planet and a miniNeptune would tend to bring our system into the more common distribution of outcomes. Microlensing studies gives an average of one nomad (ejected planet) for every system.

      Second, Fermi's response to his own question - "where are they" - included the difficulties of interstellar travel. Here is what Robert Gray have to say on the history and area of the Fermi question and the still so popular Hart-Tipler argument Proxmire used to torpedo SETI for so long:

      "As for the paradox, there is none, even in Hart’s and Tipler’s arguments. There is no logical contradiction between the statement “E.T. might exist elsewhere” and the statement “E.T. is not here” because nobody knows that travel between the stars is possible in the first place."

      [ http://blogs.scientificamerican.com/guest-blog/the-fermi-paradox-is-not-fermi-s-and-it-is-not-a-paradox/ ]

      In other words, Fermi's response is still valid.

      [I would add that since we can't estimate the frequency of false negatives for this question - economy of interstellar distance seem to allow very little in the way of large scale civilization, say - it isn't even a robustly constrained question to answer in the first place. But that is my take.]

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    5. Exoplanets are indeed harder to spot further out. But eccentricity only changes the shape of the RV signal, not its strength. I think the notion that distant giant planets are quite often eccentric is a pretty well grounded one.

      An interesting related point is that according to the Nice model, at the epoch of the late heavy bombardment, giant planets had rather eccentric orbits, and if not for damping from the massive planetesimal disk (the remnants of which became the Kuiper belt), our solar system would have fit into the exoplanet catalog substantially better.

      As with close-in Kepler planets, their relationship to the solar system is still unclear. But here is one attempt to reconcile the two: http://arxiv.org/pdf/1503.06945v2.pdf

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  5. Mike and Konstantin,

    I have a slightly longer question for you guys. What if you are more than right? What if there is more than just a single planet in the far reaches of our solar system?

    When I was younger and with less kids I was told that there was "missing mass" within our solar system. Does P9 account for most of that mass, or is it already explained away, or more profoundly, does it not account for all of it and there may be more out there?

    Bests,

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  6. Konstantin -- I love Planet 9. Congrats to you and Mike! I just wanted to mention that, dynamically speaking, there is a pathway for the capture of Planet 9 from another star while the Sun was still in its birth cluster: http://planetplanet.net/2016/02/03/planet-nine-an-intruder-among-us/

    cheers, Sean (Raymond)

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    1. Fascinating set of blog posts, Sean! I only recently watched your talk "Terrestrial planet formation at home and abroad" on YouTube http://youtu.be/7dRLvSzDHo8 So it's fun finding your blog.

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    2. Wow, thanks for the note Margarita!

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    3. Hey Sean - thanks for the note and fascinating blog post. I agree that extrasolar capture is a viable formation process for P9. Thanks for writing about it.

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    4. Sean raised another point, that if P9 was born in our solar system then its mass is likely to closer to 5-earth as it wouldn't have time to accrete mass from the gas-disks.

      so what can we say with confidence now about the mass-range for planet-9 and where does it place its orbit?

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  7. Hi Mike,..it could be right choice,...that area around right cosmic rays hot spot - around Aldebaran from Milagro cosmic rays,...(for XI).Maybe could be good to get more detailed infos from Milagro, HAWC cosmic rays surways,.. from Gus Sinnis, Brenda Dingus,...Right Milagros cosmic rays hot spot is not broader than cca 15min. and moved per 7 years of operation cca max. 40minutes, so cca 6mi. per y.,,more precis. only 20min/7years,..cca 3min per year if,... Neptune, Pluto makes 20min/year, so more than 7x5billions km,..Left hot spot shifted cca 100min/7y.,..it looks so, ...X wasn't farther (in 2009) than Pluto. XI is more than 35 milliards km from us,... Prolongation of Left hot spot should be cca where constel. Lion is,...and X should be closer than Pluto now!!! Pavel Smutny

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  8. http://www.nibiruupdate.com/forums/topic/the-1987-new-science-and-invention-encyclopedia/ From R. Harrington researche. X-cca where left hot spot is, XI where right hot spot is. (aprox for 1987). Coincidence with Milagro's results,..

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  9. Could a belt of asteroids instead of a ninth planet also explain everything?

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    1. Unfortunately, no. We thought about self-gravity as a means of generating the observed structure early in the project (see a related paper by Madigan & McCourt). You need far more material than we know is out there.

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  10. http://arxiv.org/abs/1602.06116

    some science on saturn adn eliminating points in the orbit of p9 from the possiblity of being found at.

    I have wondered, due to such resonance that established these unusual orbits for TNOs via P9 would it be easier to infer the part of the orbit P9 is in as well by where these TNOs are in their orbits as well?

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  11. Interesting new preprint on Planet 9's intrinsic luminosity...

    http://arxiv.org/abs/1602.07465

    Evolution and Magnitudes of Candidate Planet Nine

    ...with this punchline:

    Conclusions. If candidate Planet 9 has a significant H/He layer and an efficient energy transport in the interior, then its luminosity is dominated by the intrinsic contribution, making it a self-luminous planet. At a likely position on its orbit near the aphelion, we estimate for a mass of 5, 10, 20, and 50 Mearth a V magnitude from the reflected light of 24.2, 23.7, 23.2, and 22.5 and a Q magnitude from the intrinsic radiation of 15.6, 12.4, 9.8, 6.2. The latter would probably have been detected by past surveys.

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  12. Interesting...

    HTTP://arxiv.org/and/1603.03196

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  13. This comment has been removed by the author.

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  14. Astronomers await (P9,P10) Jovian or Earth like planets. Maybe this is one from main reasons why (P9,P19)wern't discovered till now. If these bodiees are other, superdensty, how it is depicted on ancient sources,..so there must be other methods used for detection. Maybe we must use strong lasers, or even proton cannons for to make P9,P10 glow. Such way it was also very probably done in pre deluvian times,..it is also so depicted,... Pavel Smutny

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  15. Why there is also track (hot spots very suitable for orbital motions of P9, and more distant P10)on Milagro's cosmic rays survay result sky map?! Very heavy but probably also very small P9,P10 accelerate mainly protons (cosmic rays),.. and those were detectedy by in Milagro's water pool placed detectors,...also in pyramidal system,..detectors,.... Pavel Smutny

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  17. Great post, apologies for the late comment, but I wanted to note this result might be another facet of the weak anthropic principle -- we probably live in a solar system with little eccentricity because eccentric orbits have too much temperature/radiation/gravitational flux to support life. When we look outside, we're surprised to see more eccentricity than at home, but we probably shouldn't be.

    I suspect we're going to learn this century that the rise of intelligent, technology-using life capable of trying to answer the question "are we alone?" is so rare in time and space that the vast majority of observable universes never experience it.

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