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First Reconnaissance Of An Exoplanetary System

The views expressed are those of the author and are not necessarily those of Scientific American.


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HR 8799

The planets of HR 8799 (labeled). The starlight has been suppressed and mostly removed (Credit: Project 1640, Oppenheimer et al.)

Using cutting edge techniques, a team of astronomers has directly imaged a distant system of four planets, and made history by obtaining simultaneous spectra of these worlds.

This first comparative look reveals that the objects each have distinct atmospheric compositions, none of which directly match any previously known class of astrophysical body.

 

 

Only a tiny handful of exoplanetary systems exist where we have been able to spot planets directly. Detecting their emitted, or reflected, light is awfully tough when it’s millions to billions of times fainter than the radiation pouring off their parent suns.

One such system surrounds HR 8799 – a very young, variable, and moderately large star, weighing in at about 1.5 times the Sun’s mass, and located about 128 light years away. Back in late 2008 it was announced that 3 planets had been imaged using advanced adaptive optics at the great Keck and Gemini telescopes in Hawaii. A couple of years later and a fourth planet was spotted.

These are not the kinds of planets we’re used to. For one thing they are all hot – glowing still from their recent formation within the last 30 million years or so, with temperatures between about 800 and 1000 Kelvin (980F to 1300F). They are gas giants, but they weigh in at anywhere between about 5 or 7 times the mass of Jupiter to as much as 20-30 times. In fact, there has been some debate about whether they really are planets or more massive objects known as brown dwarfs.

Using older data from the Hubble Telescope the orbital periods have been estimated (NASA/Space Telescope)

This system also seems pretty alien if we look at where these planets orbit. Labeled b, c, d, and e in order of discovery, their present distances from HR 8799 are about 68 AU, 38 AU, 24 AU, and 15 AU – where an AU is the distance of the Earth from the Sun.

This means that the closest planet to the star, ‘e’, orbits about midway between where Saturn and Uranus orbit in our solar system, planet ‘b’ would be out where our Kuiper belt ends. It’s a radically different architecture than what we’re used to.

Getting a handle on the composition and atmospheric structure of these mysterious gassy worlds would be a tremendous advance. A spectrum of the light from such hot worlds would provide just such a fingerprint. But efforts to measure this have had limited success, until now.

Enter Project 1640, a multi-year effort using the 200-inch wide Palomar telescope in California. It applies state-of-the-art adaptive optics to minimize the blurring effects of Earth’s atmosphere, a sophisticated coronagraph to block HR 8799′s light in order to reveal the planets, and a spectrograph that turns the pixels of each image into 37, 146 spectra.

The Project 1640 instrument (left) about to be installed at the 200-inch Palomar (right) (Credit: Project 1640/AMNH)

If that sounds technical, well it is. The upshot is that not only can the planets be seen in a system like HR 8799, but, with some skill, a spectrum can be obtained for each simultaneously – a direct probe of their actual composition and nature.

The results of this exploration are reported In a new paper by Ben Oppenheimer and colleagues, to appear in The Astrophysical Journal. It’s pretty jaw-dropping. Although the four planets are glowing similarly bright, they are each quite different from their siblings.

There are signatures of compounds like methane and ammonia, but also of things that might be acetylene and hydrogen cyanide – it’s a real mix. To quote Oppenheimer et al. – their analyses suggest that the planets are like this:

• b: contains ammonia and/or acetylene as well as CO2 but little methane.

• c: contains ammonia, perhaps some acetylene but neither CO2 nor substantial methane.

• d: contains acetylene, methane and CO2 but ammonia is not definitively detected.

• e: contains methane and acetylene but no ammonia or CO2.

You might be glazing over with this, so what does it mean? First, it means that these objects look more like planets than they do brown dwarfs. They’re also clearly, and remarkably, distinct from each other – despite (presumably) all being big, hot, gas giants. The only one that looks vaguely familiar is ‘e’ – whose spectrum is a bit like that of the night-side of Saturn.

Exactly how and why these worlds are so varied is a juicy puzzle. The researchers suggest that it might in part be a result of ultra-violet light flooding the system in bursts from the youthful star HR 8799. A thousand times brighter than the equivalent from our Sun, this radiation can drive all sorts of chemical and physical changes in planetary atmospheres.

In fact there is tentative evidence for the planetary spectra changing over a period of just a couple of months. This could be the effect of the changing stellar radiation. It could also perhaps be that most familiar of planetary properties, the phenomenon we call weather.

This is a gorgeous piece of astronomy, and it represents a new era of discovery, one where the diversity of other worlds is going to keep us very, very busy.

Not bad for a spot of early reconnaissance.

[Note: Full disclosure - it's only fair to say that I have been familiar with Project 1640 since its inception, and count several of the authors as good colleagues, so I am naturally biased in my excitement]

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. SteveO 3:09 pm 03/11/2013

    Holy cow! Caleb, I don’t know the authors and I can tell you that I am gobsmacked as well! I thought we would have to wait a few more years to get results like this!

    Now the wait for the first exoplanet shown to have a non-steady state atmosphere really heats up. (I am looking at YOU oxygen…)

    Link to this
  2. 2. lump1 12:27 am 03/12/2013

    This is absolutely amazing! Like SteveO, I thought that it would be many years before we ever get data like this. Now I want more! If we built a special space telescope just for this, I don’t think it would be a waste – even if it’s expensive.

    Link to this
  3. 3. jtdwyer 6:14 am 03/12/2013

    Very nicely done, and very interesting! Thanks for the links and imagery!

    The first image seems to indicate by the flux within the orbital range that these young objects may still be contained within a protoplanetary disk, which might also be indicated by their apparent nearly circular orbits seen in the second image, or perhaps if more properly illustrated the circular rotation of the accretion disk.

    In this case, I’d have to wonder whether these objects formation is complete. Much of the disk material may yet be accreted by these and/or other objects. As the potential disk material is further localized, the detected objects’ local gravitational bindings would also change, potentially radically changing their orbits, or even expelling some of them. Certainly, if the detected objects continue to accrete disk material their composition may change.

    We may be observing the early formation and development of a planetary system here – perhaps we shouldn’t jump to conclusions about its final characteristics yet…
    Thanks!

    Link to this
  4. 4. David Marjanović 9:33 am 03/12/2013

    800 to 1000 K? Wow. They’re literally glowing.

    Link to this
  5. 5. bgrnathan 10:22 am 03/12/2013

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    Author of popular Internet article, TRADITIONAL DOCTRINE OF HELL EVOLVED FROM GREEK ROOTS

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    Link to this
  6. 6. Caleb A. Scharf in reply to Caleb A. Scharf 10:28 am 03/12/2013

    The HR 8799 system is probably about 30 million years old, there is definitely still a lot of ‘debris’ in terms of dust and disk matter – but it’s well along the formation pathway so the vast majority of the disk (gas) is already gone, if there is further accretion onto planets it is relatively minor. However the star is still variable, and the orbital parameters are such that dynamically the system may not have yet ‘cooled’ to a more long-term stable state….watch this space.

    Link to this
  7. 7. Caleb A. Scharf in reply to Caleb A. Scharf 10:31 am 03/12/2013

    Re 800-1000K Yup – we think that most/all gas giant planets start hot due to the accretion and compression of their gaseous envelopes (as well as major collisions). Jupiter will have been this way and is indeed still leaking out the latent heat of formation from 4.5 Gya. It makes young systems excellent targets for infrared imaging – such as this- because the planets are far less faint compared to their parent stars at this age.

    Link to this
  8. 8. jtdwyer 1:24 pm 03/12/2013

    Still, isn’t the simplest explanation for the observation that all the objects are comoving (violating Keplerian dynamics, especially at such distant radii) is that they are all contained within a gravitationally binding material disk?

    Link to this
  9. 9. Caleb A. Scharf in reply to Caleb A. Scharf 1:40 pm 03/12/2013

    erm, I don’t think they’re co-moving. The observed motion is consistent with Keplerian orbits.

    Link to this
  10. 10. jtdwyer 1:42 pm 03/12/2013

    I just found in the research report, page 6:
    “Finally, the probability that a star has a massive debris disk like HR 8799 declines with age” and page 9:
    “The cooling of hydrogen-helium brown dwarfs and giant planets is generally well understood; however, the initial conditions associated with the formation of objects from collapsing molecular clouds or core accretion inside a disk are uncertain. Consequently, theoretical cooling tracks of objects at young ages may not be reliable.”

    On page 10:
    “The exceptionally dusty debris disk around HR 8799 may indicate that the proto-planetary disk was massive and had a high surface density, factors conducive to planet formation…”

    Fig 2. Indicates the existence of a peripheral dust disk at a separation distance of 87 AU.

    Link to this
  11. 11. Caleb A. Scharf in reply to Caleb A. Scharf 1:55 pm 03/12/2013

    Right, I can see how this might confuse. The ‘cooling tracks’ refers to a graphical representation of the temperature of objects plotted versus time – not spatial trajectories. Although the HR 8799 disk is still very dusty it is unlikely to be more than a few percent of the total mass of the original disk (most of which was gas). Orbits may still be evolving, but not on the timescales we’ve been studying the system for (a few years). So, while it’s true that we need to consider the long-term dynamical effects of a remaining dust/rocky fragment disk, this doesn’t mean the planets aren’t on Keplerian orbits – just that those orbits may evolve.

    Link to this
  12. 12. jtdwyer 2:03 pm 03/12/2013

    I may have misunderstood – Fig 2 states:
    “All three companions are confirmed as co-moving with HR 8799…”

    However, the discussion on page 7 seems to indicate that the discussion of co-motion relates to the proper motion of all HR8799 objects in relation to background objects, thus indicating that they are gravitational bound together. Also on page 7 & 8 there seems to be a specific discussion of orbital speeds that I’m not able to comprehend…

    Also on page 7, it is stated:
    “Additionally, the data show that it is orbiting counter-clockwise. It moved 25 ± 2 mas/year (0.98 AU/year) southeast during the 4 year period. Its detected orbital motion is near perpendicular to the line connecting the planet and primary, suggesting that the system is viewed nearly pole-on and that the orbit is not very eccentric. The near face-on perspective is further supported by the slow projected rotational velocity of HR 8799″

    However, Fig 2 does seem to indicate that all three companions’ orbits appear to be circular – I think suggesting that they may also be gravitationally interacting with the peripheral disk…

    Link to this
  13. 13. jtdwyer 2:06 pm 03/12/2013

    Sorry I’d missed your response. You might want to consider the discussion on pages 7 & 8 for possible confirmation… Thanks!

    Link to this
  14. 14. jtdwyer 2:14 pm 03/12/2013

    Sorry to continue on, but I read the ‘cooling tracks’ discussion to suggest that established models of gas giant cooling considered only independent, closed systems, that the cooling characteristics of such objects contained within a “massive disk” (as being suggested for HR 8799) would likely deviate from those models’ results.

    “The cooling of hydrogen-helium brown dwarfs and giant planets is generally well understood; however, the initial conditions associated with the formation of objects from collapsing molecular clouds or core accretion inside a disk are uncertain. Consequently, theoretical cooling tracks of objects at young ages may not be reliable.”

    Link to this
  15. 15. jtdwyer 2:17 pm 03/12/2013

    Again, page 10:
    “The exceptionally dusty debris disk around HR 8799 may indicate that the proto-planetary disk was massive and had a high surface density, factors conducive to planet formation…”

    Link to this
  16. 16. sunspot 3:33 pm 03/12/2013

    “…Project 1640 … a spectrograph that turns the pixels of each image into 37, 146 spectra.”

    Dr. Scharf,
    Excellent report, as always. The resolution of the pixels appears to be much better than 1 AU at a distance of 129 light years. Please confirm. If so, can a pixel resolve emissions from planetary rings or natural satellites? For reference, Saturn is about 9.5 AU from the Sun, and resolving Titan would require a resolution better than .01 AU.

    Link to this
  17. 17. Caleb A. Scharf in reply to Caleb A. Scharf 8:22 am 03/14/2013

    Re the pixel scale: the pixels correspond to 20 milliarcsec on the sky, which is about 0.8 AU at the distance of HR 8799 – so not good enough to resolve out anything like rings or moons, unfortunately!

    Link to this
  18. 18. Caleb A. Scharf in reply to Caleb A. Scharf 8:25 am 03/14/2013

    Re the dusty ‘disk’: This is from Ben Oppenheimer himself ‘The disk is cleared out at the planet orbits and exists interior and
    exterior to those orbits (like an asteroid belt and a kuiper belt). The intro to the paper discusses some of this.

    Co-moving means that they have common proper motion and are thus
    physically associated with the star. Oribtal motion has been measured
    from the mid/late 90s through now (using archival data). We only have
    estimates of the 7 orbital parameters for each planet, but we are trying
    to constrain them. We can probably do a common parallax
    measurement as well, with 1640, but essentially no need at this point.’

    It is also true that some of the 4 planets may be close to mean-motion orbital resonance with each other – which can help with stability.

    Link to this

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