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Transiting Super-Earth Detected Around Naked Eye Star

One of the first known stars to host an extrasolar planet, was that of 55 Cancri. The first planet in this system was reported in 1997 and today the system is known to host at least five planets, the inner most of which, 55 Cnc e, was recently discovered to transit the star, giving new information about this planet.

55 Cnc is an interesting system in many respects. Being a mere 41 lightyears from the Earth, the system is composed of a primary, yellow dwarf star in a wide binary orbit (1,000 AU) with a red dwarf. The planetary system lies within this orbit. The primary star is just brighter than 6th magnitude meaning it is visible to the naked eye under good viewing conditions.

One of these planets, 55 Cnc e, was discovered in this system via radial velocity measurements in 2004. At that point, the planet was reported to have a period of 2.8 days, and a minimum mass of 14.2 times the mass of the Earth. However, in 2010, Rebekah Dawson and Daniel Fabrycky from the Harvard-Smithsonian Center for Astrophysics argued that gaps in the observational period skewed the statistics and the true period the planet should be a short 0.7365 days.

One of the results of this was that the planet would have to orbit closer to the parent star. In turn, this increased the likelihood that the planet could transit the star from 13% to 33%. A team led by Joshua Winn from the Massachusetts Institute of Technology went searching for this faint transit and report its detection in a recent paper. But while the star itself is one of the brightest stars in our sky to harbor known extrasolar planets, the eclipse is far from visible without precise observations, changing by only 0.0002%, one of the smallest changes known. The timing of the eclipses confirms that correction by Dawson and Fabrycky and adds new information about the body.

Given the radius determined as well as the mass, the team was able to estimate the structure of the planet and report that the mass is 8.57 ± 0.64 Earth masses. The reported radius is 1.63 ± 0.16 times that of Earth, and the density is 10.9 ± 3.1 g cm-3 (the average density of Earth is 5.515 g cm-3). This places the planet firmly into the categories of a rocky super-Earth.

The team also explores whether or not the planet could retain an atmosphere in such a close orbit (only three times the radius of the star itself). At this close range, the planet would likely be tidally locked and with an albedo typical of rocky planets, the planet would likely have an average temperature of nearly 2970 K (5,000° F). If the planet were able to redistribute the heat, it may be as low as 2100 K (3,300° F). Either way, a planet of such mass would have difficulty retaining any primordial, gaseous atmosphere. However, the team reports that it may be possible for volcanic activity to create a thin atmosphere of high molecular weight components.

While this new report adds precious little in the grand scheme of the rapidly growing body of knowledge of exoplanets, the authors close with the note that, “there is some pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets.”

About 

Jon is a science educator currently living in Missouri. He is a high school teacher and does outreach with the St. Louis Astronomical society as well as presenting talks on science and related topics at regional conventions. He graduated from the University of Kansas with his BS in Astronomy in 2008 and has maintained the Angry Astronomer blog since 2006.
For more of his work, you can find his website here.

Comments on this entry are closed.

  • Sirius_Alpha_1 May 1, 2011, 10:36 PM

    “The most recent planet, 55 Cnc e, was discovered in this system via radial velocity measurements in 2004″

    The most recent planet was 55 Cnc f, made public in 2007. Remember the order, b c d e f, and you already mentioned there were five planets ;)

    • Jon Voisey May 2, 2011, 12:53 AM

      Good call. I have a hell of a cold that’s preventing me from thinking clearly. I was trying to double check that in my head and only got as far as “e is the 5th letter of the alphabet”, but didn’t get so far as “but we skip A since that’s the star”.

  • Greg May 1, 2011, 10:54 PM

    Nice bit of detective work there. This is a very interesting system so far that defies some comonly held beliefs about the formation of planetary systems around other stars. Of note this star is nearly identical in mass and diameter to our sun. Of particular importance however is that it has twice the metallicity of the sun. As such, one would expect to find larger planets orbiting around it and indeed that is the case here. It also has a E-K like belt at 27 to 44 AU. There is a distantly orbiting Jupiter sized planet at 5 AU. So far so good, but then the inner part of the system is unlike ours. There is a Saturn sized planet orbiting in the habitable zone (.73 AU), potentially with moon(s) with liquid water on the surface. Then there are two more Saturn-sized objects packed in between .1 and .2 AU with slightly eccentric orbits. And then finally this super-Earth in a torch-orbit. Many of the current theories that I have read would rule out the possibility of the two innermost planets remaining in stable orbits, yet this is a middle-aged yellow dwarf star estimated to be 4-5 by old. I would suppose the best explanation is that this system exhibited inward migration of gas giants after they formed and at some point the process stopped leaving the current planets where they are. Still a difficult puzzle to piece together given what we currently know.

    • postman1 May 2, 2011, 12:26 AM

      Could it be that they are still migrating inward and we are near observing the end of 55CncE as it dives into the star? Might be interesting to keep an eye on this one. The possible moons in the habitable zone do sound extremely interesting. Also, thanks for reporting the estimated temps in Fahrenheit.

    • Jon Voisey May 2, 2011, 12:55 AM

      As I understand it, the belt you’re talking about is pretty speculative and followup studies have suggested the detection may have been background noise.

  • IVAN3MAN_AT_LARGE May 1, 2011, 11:26 PM

    …, the planet would likely have an average temperature of nearly 5,000° F. If the planet were able to redistribute the heat, it may be as low as 3,300° F.

    AARRGGHH!! Fahrenheit!

    5,000 °F = 2,760 °C or 3,033 K.

    3,300 °F = 1816 °C or 2,089 K.

    In future, please state the given temperature in Celsius (°C) and/or kelvin (K), and Fahrenheit (if you must!) within parenthesis for the old-fashioned lot out there.

    • norssk May 2, 2011, 12:55 AM

      Kelvin – logical common sense approach
      Celsius – logical common sense approach

      Fahrenheit…? Maybe somewhere it is logical.

      • Torbjorn Larsson OM May 2, 2011, 12:28 PM

        All of them are logical if you think of them as measurements. Fahrenheit used common references (brine & horses; the later due to large body mass), Celsius did too.

        It is later that there has been a separation between the need for comparisons (say F=m*a so kg*m/s^2 = (kg)*(m/s^2)) and references, mainly because it has become easier to establish and maintain the later through standards et cetera.

        What is not logical is continuing to use old and moldy units, but that is a social issue. … oh, I see now, yes; no logic whatsoever! :-D

    • Andrew James May 2, 2011, 1:28 AM

      If you were really pedantic (no me) you could use °Ra of °R , also known as the Rankine or Réaumur scale. (The latter is great for making cheese!)

      My favourite temperature scale is the Delisle °D, where 0°D is the boiling point of water and 150°D is freezing. Typically French, I like it the best only because M.Joseph-Nicolas Delisle was an astronomer. It is based on the contraction of the element Mercury in hundred thousandths.
      It is alleged that it was used in Russia for over 100 years, because it gives the illusion that is hotter than it really is in mid-winter in St. Petersburg!
      1°D = 0.33333…°C or 1.2°F

    • Torbjorn Larsson OM May 2, 2011, 12:19 PM

      If your were really, really pedantic (not me too) you could adjust the precision after conversion:

      5,000 °F = 2,800 °C or 3,000 K.
      3,300 °F = 1800 °C or 2,100 K.

      And If your were really, really, really pedantic (still not me) Celsius doesn’t make much sense for very hot or cold objects. It is convenient only for everyday temperatures.

  • CrazyEddieBlogger May 2, 2011, 3:32 AM

    oh wow… says wiki:

    ” The Celsius scale, likewise, originally ran from zero for boiling water down to 100 for freezing water. This was reversed to its modern order some time after his death, in part at the instigation of Daniel Ekström, the manufacturer of most of the thermometers used by Celsius.”

    And here I was, thinking that inches and yards and miles are awkward.

    This is like a linear measurement where zero distance is labeled 300, then the king’s arm is labeled zero, and hilarity truly ensues.

  • tripleclean May 2, 2011, 4:55 AM

    Here’s an easy to explain the temperture on 55 cnc e, about twice as hot a molten steel. How can SETI hear or see anything through heliopauses?

    • Torbjorn Larsson OM May 2, 2011, 3:29 PM

      Steel is a term for huge set of various alloys and various melts, I think you mean iron to narrow it down.

      “Hear and see” is much the same thing here, radio and light being photons. And whatever the Sun heliopause is* (and its associated magnetopause) the property relevant here is that it isn’t much visible what I know of, either as blocking or as a source.

      * We are guaranteed that somewhere the influence of the solar wind momentum will be dispersed by the interstellar media and/or meeting winds. That boundary is the idealized heliopause. Apparently the 2008-10 IBEX measurements has shown that it isn’t as simple a phenomena as once thought.

  • HeadAroundU May 2, 2011, 10:42 AM

    Looking at the pic, upper right is a 5th planet or the companion star with 3 planets?

    Also, would be great to have more info about other planets.

  • Navneeth May 2, 2011, 2:54 PM

    [T]he star itself is the brightest star in our sky to harbor known extrasolar planets, brighter than any other host by two full magnitudes…

    Jon,
    Isn’t Fomalhaut nearly 4 magnitudes brighter than 55 CNC?

    • Jon Voisey May 2, 2011, 11:04 PM

      Indeed it is. I’d taken that information directly from the paper so I’m laying the blame for that one at the feet of the researchers.

      Fixing it here though.

      • Andrew James May 2, 2011, 11:20 PM

        Jon
        May I suggest you keep at hand the W. Lyra paper “Naming the extrasolar planets” http://arxiv.org/abs/0910.3989, which has a table of extrasolar planets by constellation.
        Cheers

      • Andrew James May 2, 2011, 11:24 PM

        Also just an idea.
        A UT article somewhere on the naming system of extra solar planets might help too. I.e. The recent paper by Hessman, F.V., et.al. “On the naming convention used for multiple star systems and extrasolar planets.” http://arxiv.org/abs/1012.0707
        Might help with all those a’s and b’s, etc.; and be entertaining at the same time!

      • Navneeth May 3, 2011, 5:30 AM

        Hi Jon. Going through the paper I noticed that the authors say that 55 Cnc is brightest star in the sky hosting planets discovered by the transit method.

        On page 4, just below Fig. 4,

        55 Cnc e has a uniquely bright host star, towering above the
        other super-Earth hosts and nearly 2 mag brighter than any
        other transit host.

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