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Second Smallest Exoplanet Yet Discovered

With the American Astronomical Society meeting now wrapped up in Washington, we’re left to mull over the highlights, particularly the Kepler results. But the Keck Observatory also contributed compelling exoplanet news in the form of HD156668b, a planet some eighty light years from earth in the direction of Hercules. Working with Keck data, a research team led by Andrew Howard (University of California at Berkeley) has brought us a world that is only four times the mass of Earth, making this ‘super Earth’ the second smallest exoplanet yet discovered.

Addendum: See andy’s note below re planets smaller than this one. More on the ‘pulsar planets’ here.

Using the HIRES instrument (High Resolution Echelle Spectrograph) and the 10-meter Keck I telescope at Mauna Kea, the astronomers teased out the presence of the planet through radial velocity methods, which are responsible for the great majority of the planets thus far discovered. The trick is to work down to smaller and smaller worlds using these techniques, something that Howard is delighted to see occurring. Calling the discovery ‘remarkable,’ the astronomer went on to say that it “…shows we can push down and find smaller and smaller planets.”

Image: This graphic shows the data confirming the existence of extrasolar planet HD 156668b as discovered using Keck/HIRES. The planet has a mass of roughly 4.15 Earth masses and is the second smallest exoplanet discovered to date.

HD156668b orbits its star every four days and is thought to have roughly twice the mass of the smallest known extrasolar world, Gliese 581 e. Geoff Marcy (UCB) is behind the Eta-Earth Survey for Low Mass Planets that explicitly targets super-Earths, and has thus far discovered two near Earth-mass worlds. We have Howard’s assurance that more are on the way. More in this Keck Observatory news release.

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Comments on this entry are closed.

  • andy January 8, 2010, 13:30

    This is not the second smallest known exoplanet, nor is Gliese 581 e the smallest known exoplanet.

    Going by the quantity m*sin(i):
    PSR B1257+12A: 0.015 Earth masses (discovered in 1994!)
    Gliese 581e: 1.93 Earth masses
    PSR B1257+12C: 2.9 Earth masses
    PSR B1257+12B: 3.4 Earth masses
    HD 156668b: 4.15 Earth masses

    There is also MOA-2007-BLG-192Lb which has a (rather uncertain) best fit mass of 3.3 Earth masses. The quantity m*sin(i) is unknown because the orbital parameters are unknown.

    In fact it turns out that the inclination of the PSR B1257+12 system is known and the true masses work out at 0.020 Earth masses, 4.3 Earth masses and 3.9 Earth masses for planets A, B and C respectively, but given the unknown inclination, this new planet could easily exceed the mass of planet B. (I estimate the geometric probability that HD 156668b exceeds the mass of PSR B1257+12B is about 74%)

    It is telling that these other small planets are detected by methods other than radial velocity (pulsar timing and gravitational microlensing). Time and again we see press releases for radial velocity planets that totally neglect the existence of planets detected by other methods, claiming “least massive planet” when they are not even close. And of course with the unknown quantity sin(i), it is possible that some of the previously-known radial velocity planets are less massive than this newly-found world. For example, HD 40307 b has m*sin(i)=4.2 Earth masses, so it could easily turn out to be less massive than HD 156668b.

  • Administrator January 8, 2010, 14:34

    I figured that would get a rise out of somebody — the pulsar planets have their advocates! I added a note on this in the text just now. I certainly agree, though, about previous detections that could be less massive than this one.

  • andy January 8, 2010, 14:56

    Well it appears that the exclusion of various exoplanets from consideration has moved beyond the pulsar planets into the realm of microlensing detections. There is no particular reason that these detection methods should be excluded from consideration. Usual reason cited is because the stars in question are unable to host habitable planets (in this case, the ignored planets orbit a pulsar and a brown dwarf*), but then again we should consider that none of the known planets around stars that would in principle be capable of hosting Earthlike planets are good candidates for being habitable either.

    * The possibility of potentially habitable conditions on terrestrial planets orbiting brown dwarfs (.PPT file) has not necessarily been ruled out though…

  • Carl January 8, 2010, 15:49

    http://planetquest.jpl.nasa.gov/index.cfm is my default browser homepage. The tally is one of the most satisfying things I see on the internet.

  • Ron S January 8, 2010, 17:05

    andy: PSR B1257+12A: 0.015 Earth masses (discovered in 1994!)

    Ahhh… but this Moon-mass object would now be classified as a dwarf planet, not a Real Planet (TM). ;-)

  • andy January 8, 2010, 18:10

    Ron S: haha. But actually, it probably wouldn’t. There’s no evidence this planet is actually located in an asteroid belt or that there is anything else sharing its orbit (given the results possible through pulsar timing this is actually quite a strong constraint).

    Also if you take the definition of unterplanets and überplanets in this paper as corresponding to the dwarf planet and planet categories respectively, we can get an idea of whether this object should be regarded as a planet. Plugging the numbers for PSR B1257+12A into equation 4, I get a value Λ≈400, safely in überplanet territory.

    So it seems quite safe that this object should be termed a planet, not a dwarf planet.

  • Ron S January 8, 2010, 20:31

    Thanks for that pointer — curious way to define a planet, but I can see the reasoning. Quite small bodies can be considered planets when there is little else around them to cause orbital deflections, and therefore have long durability (orbital stability) within the system. Λ ranges over ~13 orders of magnitude (from the paper you referenced) just for the largest bodies in our own solar system. Λ≈400 is a bit under the 900 value for lowest valued planet in our system (Pluto’s Λ is much, much lower), so still very stable.

    One thing I wonder about (if I’m not mistaken, andy, you made the following observation many months ago) is how that planet got there since, because it is orbiting a pulsar, there would have been a past nova event that blew off much of the stellar mass, and any extant planet would likely have departed the system due to its, now, hyperbolic trajectory. That possibly calls into question just what this object really might be.

  • Thomas January 9, 2010, 0:28

    @ Ron S

    The planets at PSR B1257+12 probably formed from debris after the nova.

  • spaceman January 9, 2010, 9:01

    An amazing finding indeed! The pace of extrasolar planet discovery has really started to pick-up, as a graph of the number of planets per year over the past decade that was presented at the recent AAS meeting nicely illustrates. However, I still wonder if there is a tension between the NASA/UC Eta-Earth results on super-Earths and the HARPS results (which still have yet to be published) on super-Earths. Does anyone know if these two surveys have reached similar conclusions regarding the abundance of low-mass planets?

  • andy January 9, 2010, 12:12

    There have been various studies of how to form the pulsar planets – the main contenders are disk formation by supernova fallback or alternatively by the tidal destruction of a companion star. The fallback disc currently seems to be the most plausible scenario (e.g. this paper) – furthermore such fallback discs have been observed, see for example the case of 4U 0142+61.

    Definitely there seems to be quite a bit of evidence that planet formation can occur later on in a system’s evolution. There is the possibility of “second-generation planets” formed in a disc produced by the capture of material from an evolved giant star by a companion, e.g. the disc around Mira B.

    The giant planet in the Gliese 86 system is a possible candidate second-generation planet. The system comprises a K-dwarf and a white dwarf, with the planet orbiting the K-dwarf. The problem is that the best-fit for the white dwarf progenitor mass implies that the original semimajor axis may have been sufficiently small to inhibit gas giant planet formation. One way to resolve the problem of the existence of the giant planet is to form it from a second-generation disc produced by mass loss from the white dwarf progenitor, as the semimajor axis of the binary system would increase and make it easier to form giant planets.

  • Ron S January 9, 2010, 12:23

    Thanks, Thomas. With that hint I am now reminded that I’ve heard that before but had somehow managed to completely forget all about it.

  • Terraformer (a.k.a Tobias Holbrook) January 9, 2010, 19:35

    Hmmm. Can brown dwarfs form from the ‘planetary nebulas’ left by dead stars?