Oops, TW Hydrae b Isn’t a Planet; Just a Sunspot


You gotta love this about science; someone is always checking your work. Early this year a new exoplanet discovery was announced: TW Hydrae b, a huge planet about ten times as massive as Jupiter. Astronomers thought the planet was in a super-tight orbit around its host star (TW Hydrae), circling in only 3.56 days at a distance of about 6 million kilometers, which is about 4 percent of the distance from the Sun to the Earth. However, another group of astronomers decided to analyze some new optical and infrared data to confirm the radial velocity signal of the planet. Something didn’t seem right, so they ran a few more tests and computer models and determined what they were seeing wasn’t a planet. It was a big sunspot. “Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54 deg can reproduce the reported RV variations,” the astronomers reported in their paper. The rest of the astronomical world must agree with the new determination, as TW Hydrae b has now been dropped from the Planet Quest New Worlds Atlas (a fun site to peruse.) But nature doesn’t like a void, — and astronomers have been working hard in the planet-search department, — so, three new extra solar planets have been discovered and added to the atlas, for a current planet count of 309.

GJ 832 b is about half the size mass of Jupiter and orbits 3.4 AU from its tiny host star. The star is a yellow, sun-like G star, about 16 light years from Earth. It was found with the Anglo-Australian Telescope. Astronomers say it has the largest angular distance from its star among radial velocity detected exoplanets, which makes it a potentially interesting target for future direct detection.

HD 205739 b was also just announced:. This exoplanet is 1.37 times the size mass of Jupiter, and orbits about .9 AU from its star, a blue to white star, which is 1.22x the size of the sun, and 294 light years from Earth. It has an eccentric orbit, and astronomers believe there may be an additional planet in this system, because of how the planet orbits.

Another planet found by the same astronomical team is HD 154672 b. This is a biggie, at about five times the size mass of Jupiter, but only about .6 AU distant from its star, which is just about sun-size, and about 213 light years from Earth. The planet has an orbital period of 163.9 days.

These last two planets were found using the N2K Doppler planet search program with the Magellan telescopes.

Sources: arXiv (here, here and here) and Twitter, PlanetQuest

18 Replies to “Oops, TW Hydrae b Isn’t a Planet; Just a Sunspot”

  1. Aodhhan, the links provided by Nancy make it clear that mass is what’s being talked about. We don’t know what the dimensions of these planets are, because they’ve been detected by their gravitational pull.

    One of the best things about UT is that they always provide the link to the source so that if you have a question like that, you can look it up.

  2. Nancy, wouldn’t it be more correct to talk about “mass” instead of “size”? Or are you really speaking about diameters here?

  3. Typically when dealing with “Size” in this context they are referring to actual spherical (area) displacement in space, as opposed to “mass” which more closely relates to weight and density.
    Which one you use is dependent on what information you have and what you are trying to convey.
    In this article, “size” appears to be the correct means of measurement. Also, most people relate to ‘size’ much better than the mass of an object since what you see is what you get. With mass, this isn’t the case; you can have something small in size yet be very massive (like a black hole) or something large in size and not very massive (like Saturn).

  4. Not all exosolar planets are detected by gravitational pull, some are detected by occultation of the star, as long as the planet passes in front of the star along the line of sight from observers on Earth.

    For this case, it is more likely that it was the occultation that was detected, since the outcome ultimately was a large sunspot, which would not have caused a gravitational pull.

  5. Vagueofxxxxxxx…
    Ahh…Kudos, mass should have been transposed. Bad me for not looking at the reference first, and for not paying close enough attention to the question misreading what the REAL question is.

    Studies really aren’t done by just watching the “gravitational pull” as you put it. You might initially find it this way, but you don’t study that ‘per se’. After all…what if there is more than one planet pulling on the star?

    Common method is using spectral and doppler to plot positions of the star and subject for a period of time; then these figures are put together to come up with a radial velocity curve (where velocity is on one axis, and time is on the other). From this you can come up with quite a few things… such as orbital periods, orbital amplitude, semi major axis and probable mass. I won’t bore you with the trigonometry behind it… Actually the data is pushed into computer models and they do the RV comps!

    In short.. it’s done by both watching the star and probable subject movment and spreading out the light from the star. So both you and John are correct.

  6. Talk about someone checking your work! Mass is correct. I pulled “size” from Planet Quest’s Twittter feed, which might not be technically accurate. I’m sticking with “sun-sized”, however because that’s relative.

    Thanks guys,

  7. Another way to find extrasolar planets may also include direct imaging. I refer to the planetary candidate orbiting the K7V star 1RXS J160929.1-210524, posted at arXiv:0809.1424. Spectroscopy & direct imaging suggest an 8 Jupiter mass object( type L4 ) orbiting this young low-mass star in the upper Scorpius association. Spectra suggest a temp of ~1800 K , but this might be expected of a young(hot) planetary-mass object. I’m familiar with earlier claims of planetary mass objects (which have mostly been inconclusive or just in error). These authors caution that further observations are needed to determine if this object is actually orbiting this star (or may possibly be a free-floating planet) but the evidence they present seems pretty interesting. I’d be interested to see what subsequent observations of this candidate planet may reveal.

  8. @Kevin F

    Star spots have been observed on many stars. BY Draconis variables are stars that are variable due to heavy sunspot activity, for example. In rare occasions, a transiting planet covers up a star spot, causing a brief rise in the apparent brightness of the system–very noticeable in the star’s light curve.

  9. This makes a lot more sense.

    I was always a little bit leery of giant exoplanets (and even binary stars) so close to their star that they whipped around it in a couple days. Some, IIRC, have been discovered that supposedly orbit in only few hours, which really strains credulity.

    Makes me wonder how many more of the more far-out theories of astronomy need to be re-visited like this, with Occam’s Razor firmly in hand.

  10. Kevin F.–Sunspots (or should that be starspots?) have previously been detected on Betelgeuse. I thought I heard that somewhere, and Wikipedia confirms it.

    Science–self-correcting by definition!

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