Welcome our tiny family

By Phil Plait | June 2, 2008 10:55 am

News flash! The lowest mass planet yet found just so happens to orbit a very low mass star — so low mass, in fact, that it might not even really be a star.

Artist’s conception of the newly found system. Credit: NASA

OK, first, the planet. Called MOA-2007-BLG-192Lb — of course! — it has a mass of about three times the Earth, making it the lowest mass planet found so far. It orbits its parent star at about the same distance Venus orbits the Sun. However, that doesn’t make the planet terribly hot: the host star is itself may be a brown dwarf, in which case the planet may be as cold as Pluto!

The star is right on the borderline of what can be called star. By consensus, a star can maintain fusion in its core over long periods of time. Fusion is what powers the star, heating up the core and making the star shine. The Sun fuses hydrogen into helium (700 million tons every second!) which is what powers our star. But that’s maintained by the tremendous pressure at the Sun’s core. An object less than about 8% of the Sun’s mass won’t be able to squeeze hydrogen together hard enough to fuse it. In that case, it’s called a brown dwarf. It can stay warm for a few billion years just from the leftover heat of its formation, leaking out radiation slowly but never able to regenerate it.

The thing is, the exact mass of the newly found star is not known, so it may be just below or just above that limit. This makes a difference to the planet, certainly — its temperature depends directly on it! — but also on our theories. There are competing ideas of how brown dwarfs form, and being able to have a planet form nearby will certainly have people scratching their heads and trying to figure out how to manufacture a system like this.

There is a problem: the star (or whatever) and planet are 3000 light years away! They were not detected in the usual way by direct observation, but were found by gravitational lensing. The path a light ray takes bends when it passes a massive object. The gravity of the object can also amplify the light from stars behind it. In this case, both the planet and the dinky star were fund by this brightening of a background star. The amount of brightening can be used to determine the masses of the planet and the star, but not with perfect precision; hence the question on the stellar nature of the star. The planet’s mass is pretty secure, at least that it’s very low, only a few times that of the Earth.

In the end, we have at least one very cool thing about this announcement, and that’s the lowest mass planet yet found. At that mass, it’s almost certainly a rocky or icy body, and not a gas giant like Jupiter. And, if it does indeed orbit a brown dwarf, well, that’s pretty excellent too. But either way, say hello to our little friends.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (41)

  1. Christopher Ferro

    So, is this “Thee” announcement that the astronomy web community has been waiting for since last month?

    CJSF

  2. Duane

    Can we continually observe the pair, or is it just too faint and far away to study without the boost of a lensing event?

  3. Ken_g6

    Phil, don’t forget the very first extrasolar planets found – the ones around the pulsar, some or all of which were less than 1 Earth mass as I recall.

  4. Gnat

    “..will certainly have people scratching their heads and trying to figure out how to manufacture a system like this.”

    I know this is really talking about the math and physics, but it just struck me as funny…I want to manufacture a star system too!

  5. zeb

    Nice Scarface reference!

  6. What Bennett didn’t say at the press conference but what his 52-page paper does: The mass of this planet is very poorly defined, namely as 3.3 (+4.9/-1.6) Earth masses, and does not necessarily set a new low-mass record.

  7. David Bennett

    for Ken_g6:

    The first two pulsar planets have masses of 3.9 and 4.2 Earth masses (now that the inclination has been determined), but the 3rd is only 0.02 Earth masses. But these are thought to have formed after the supernova explosion. The formation of MOA-2007-BLG-192Lb is thought to be similar to the formation the planets in our Solar System.

    for Daniel Fischer

    You are correct that the error bars are such that they overlap with those of other low-mass exoplanets. But, this was also the case for the two previous low-mass “record holders”: Gl 581c and OGLE-2005-BLG-390Lb. This is simply the way the low-mass “records” are kept.

    Follow-up observations with the Hubble Space Telescope and the ESO VLT may allow us to improve the mass estimate.

  8. Sili

    How come it’s the planet’s mass that’s best known?

    I woulda thought it’s be easier to determine the bigger mass most precise. In particular is the orbital period of the planet is known.

  9. ChrisAnorve

    Hi !

    Why is the planet size important to see whether this is rocky/icy or gas planet?

    thanks!!

  10. Quiet_Desperation

    MOA-2007-BLG-192Lb

    Catchy. :-

    Then again I always did like the movie “Aliens” just calling the planet “LV-426″. And Stargate’s P***** numbering scheme.

  11. Irishman

    ChrisAnorve, gas giants are called that because the are giant. Our understanding of the physics behind them requires them to be extremely massive in order to hold that much light gas (particularly hydrogen). Ergo, since this body is in the mass range of Earth, it cannot be a gas giant. Most extrasolar planets found have been gas giants, which is why that is nifty.

    Of course, we can’t tell if it is rocky or icy yet, but given that it is Earth-sized, that suggests rocky rather than icy. But that’s only by virtue of what we know of the Solar system.

  12. Just a quick note that caused a brief moment of confusion:

    Phil, do you mean to say that this planet has the lowest mass of extra-solar planets?

    I’m nitpicking here, I realize, but I don’t often get to nitpick a PhD who isn’t a political theorist.

  13. MarshallDog

    If this system was observed using gravitational lensing, doesn’t that make it hard to do follow-up observations? Can you really determine the movement of the system well enough to know whether it’s going to pass by any other stars/galaxies/whatever? How long does a typical grav. lensing event last (or how long did this one last)? It’s a fascinating way to detect other worlds, but it seems like it has a few serious limitations… of course I ain’t no astronomer, so what do I know.

    I didn’t realize astronomers had questions about planets forming around brown dwarfs. Where they assuming they couldn’t only because nobody had found any?

  14. I didn’t mean to say that the planet’s mass is better known, it’s just that the star’s mass uncertainty was so much it wasn’t sure which side of the fusion line it was on. Unfortunately, I didn’t have time to read the actual professional paper; the error bars are interesting! But a lot of the low mass planets do have big uncertainties.

    And yes, I should have said it the lowest mass planet orbiting a (relatively) normal star. The pulsar planets still win.

  15. Esteban

    Beyond just residual heat of formation, I thought it was possible for Brown Dwarfs to fuse deuterium? I would assume that even if they were capable of fusing deuterium, this would still be secondary to the leftover heat.

    Are the limitations on getting mass values with gravitational lensing due to the uncertainties on the distances for both the “lens” and the brightened star? Are the GR formulas exact otherwise?

  16. Imrryr

    Hopefully, if there is life on that planet (fat chance, I know) the fact that we named it MOA-2007-BLG-192Lb won’t turn out to be the worse insult imaginable in their language.

  17. Hannu Siivonen

    “Hopefully, if there is life on that planet (fat chance, I know) the fact that we named it MOA-2007-BLG-192Lb won’t turn out to be the worse insult imaginable in their language.”

    Actually I hope the opposite. It would be so cool if MOA-2007-BLG-192Lb would mean “Your mom is so fat, she ate my trash can” in that planet ;-)

  18. MOA-2007-BLG-192Lb won’t turn out to be the worse insult imaginable in their language.

    At least we didn’t call it “Belgium”.

    Don’t forget your towel.

    J/P=?

  19. Nic

    Phil – what happens if a really small ‘star’ has just – barely – enough mass to fuse hydrogen? Does that fizzle out in a short time once the star has lost that little margin that let it fuse? What if it’s, say 10% over the threshold? Does it burn for an incredibly long time? I know the really really big ones burn out in a mere coupla million years so I guess that’s probably so..

    But the borderline case? Would it really just stop?

    Nic

  20. Imrryr

    Of course having such a large mother might be considered a great honor amongst the MOA-2007-BLG-192Lbians, Hannu :)

    And JP, you can rest assured that I would never travel without the most useful possession in the hitchhiker’s arsenal.

  21. Utakata

    Never understood the artist depicts the planet from behind, making it too dark to pick out the details. Instead, I would like to see it head on…especially one that is pink lit.

  22. Darth Robo

    >>>”Never understood the artist depicts the planet from behind, making it too dark to pick out the details. Instead, I would like to see it head on…especially one that is pink lit.”

    I’d say it’s probably because both the planet and the star are important (plus the fact they don’t know for sure what the planet looks like). Especially so in this case, as we’re not too sure of the exact size of the star. That’s why they painted two versions! Cool!
    :)

    Anyway, what’s wrong with the Dark Side?

  23. BMcp

    The path a light ray takes bends when it passes a massive object. The gravity of the object can also amplify the light from stars behind it. In this case, both the planet and the dinky star were fund by this brightening of a background star.

    Wouldn’t that be a foreground star, and the brown dwarfs were farther away and their light was redirected and brightened by that foreground star or am I misunderstanding gravitational lensing?

  24. Utakata

    Darth Roboon wrote inpart:

    “Anyway, what’s wrong with the Dark Side?”

    It’s not pink enough.

    /wink

  25. Imrryr

    Not to mention that if the artist just painted the light side of the planet people would be asking him why it was pink.

    It’s really cool stuff though. I’ve always admired Nasa’s space artists for their attention to detail and realism. Although, I like the not so realistic stuff too.

  26. “The discovery was made through a new Japanese-funded MOA telescope at Mt John Observatory in Canterbury, the world’s largest dedicated to gravitational microlensing” (http://www.stuff.co.nz/4569772a11.html)

    Woo hoo – go the Kiwi’s!

    Of course the Moa is an extinct large flightless bird, much like an emu. Does this mean all the inhabitants of this remote world are turkeys?

  27. Irishman

    The planet is pink because the light from the star is pink (magenta) because the light from some colors is absorbed by sodium and potassium.

    BMcp said:
    >Wouldn’t that be a foreground star, and the brown dwarfs were farther away and their light was redirected and brightened by that foreground star or am I misunderstanding gravitational lensing?

    From the press release: http://www.nd.edu/~bennett/moa07blg192/ND_PR_moa2007blg192_final.pdf

    Gravitational microlensing takes advantage of the fact that light is bent as the rays pass close to a massive object, like a star. The gravity from the mass of the intervening object, or lens star, warps surrounding space and acts like a giant magnifying glass. As predicted by Albert Einstein and later confirmed, this phenomena causes an apparent brightening of the light from a background “source” star. The effect is seen only if the astronomer’s telescope lies in almost perfect alignment with the source star and the lens star. Astronomers are then able to detect planets orbiting the lens star if the light from the background star also is warped by one or more planets.

    So in this case, the new star and planet are closer and causing the gravitational lensing, which makes a distant star brighter.

    MarshallDog said:
    > If this system was observed using gravitational lensing, doesn’t that make it hard to do follow-up observations? Can you really determine the movement of the system well enough to know whether it’s going to pass by any other stars/galaxies/whatever? How long does a typical grav. lensing event last (or how long did this one last)? It’s a fascinating way to detect other worlds, but it seems like it has a few serious limitations… of course I ain’t no astronomer, so what do I know.

    The press release states that this technique only gives observation time of less than 1 day.

    The primary challenge of the microlensing method is that the precise alignments needed for the planetary microlensing signals are quite rare and brief, often lasting less than a day. This new discovery was made possible by the new MOA-II telescope at New Zealand’s Mt. John Observatory, using the MOA-cam3 camera, which is able to image an area of sky 13 times larger than the area of the full moon in a single image. Bennett
    explains, “The new MOA telescope-camera system allows us to monitor virtually all of the known microlensing events for planetary signals. We would not have made this discovery without it.”
    The microlensing observations provided evidence that the host star has a mass of about
    6% of the mass of the Sun. This was confirmed by high angular resolution adaptive optics images with the Very Large Telescope (VLT) at the European Southern Observatory in Chile. These images confirm that the planetary host is either a brown dwarf or a very low-mass star.

    Utakata said:
    > Never understood the artist depicts the planet from behind, making it too dark to pick out the details. Instead, I would like to see it head on…especially one that is pink lit.

    Uh, you do realize this is an artist’s conception of what the planet might look like based solely upon an estimate of the mass of the planet and distance from it’s star? What exactly are you expecting the artist to draw? Beautiful landscapes? An ocean view? Skyscrapers?

  28. Utakata

    Imrryr wrote inpart:

    “Not to mention that if the artist just painted the light side of the planet people would be asking him why it was pink.”

    Couldn’t we have an absurdist artist conception of a planet?

    Irishman wrote:

    Uh, you do realize this is an artist’s conception of what the planet might look like based solely upon an estimate of the mass of the planet and distance from it’s star? What exactly are you expecting the artist to draw? Beautiful landscapes? An ocean view? Skyscrapers?

    Yes…

    …but beautiful landscapes, an ocean view and skyscrapers would be nice…especially with a pink sunset thrown in.

    /wink

  29. “There are competing ideas of how brown dwarfs form, and being able to have a planet form nearby will certainly have people scratching their heads and trying to figure out how to manufacture a system like this.”

    It’s obvious the Monolith had something to do with this.

  30. Anne

    The definition of “star” as something that undergoes fusion inside gets a bit messy at the end of their lives – neutron stars and white dwarfs are usually called stars in spite of not undergoing fusion. (Though black holes are not.)

  31. Christopher Ferro

    @ Celtic_Evolution, way at the top: Yes – you’re right, that is the announcement I was thinking of, not this… Thanks!

    CJSF

  32. jay

    Nice, we have a new member to the planetary family. I like the magenta colour. It would be interesting to find out the composition of the atmosphere of the planet with a very catchy name. MOA-2007-BLG-192Lb is an easy name to remember. I’ll call it Bob. Or maybe not.

  33. StevoR

    The BA noted :

    “An object less than about 8% of the Sun’s mass won’t be able to squeeze hydrogen together hard enough to fuse it. In that case, it’s called a brown dwarf. It can stay warm for a few billion years just from the leftover heat of its formation, leaking out radiation slowly but never able to regenerate it.

    The thing is, the exact mass of the newly found star is not known, so it may be just below or just above that limit.”

    What if this star (& please let’s give it a real name!) is exactly on the limit – what does that make it?

    Smallest red dwarf? Largest brown dwarf? What?

    Is there sucha thing as a grey dwarf? ;-)

    Neither black nor white nor red nor brown but something other,
    Have we found?

  34. Joker

    The Bad Astronomer said :

    “In the end, we have at least one very cool thing about this announcement, and that’s the lowest mass planet yet found.”

    Um .. isn’t the coolest thing going to be the likely temperature? Y’know the coolest star, a freezing planet …

    At that mass, it’s almost certainly a rocky or icy body, and not a gas giant like Jupiter. And, if it does indeed orbit a brown dwarf, well, that’s pretty excellent too. But either way, say hello to our little friends.

    Er … oka-ay..

    Hello!
    HELLO!
    HEEEEELLLLLLOOOOOOOO!!!

    Mm.. I don’t think it heard me! ;-)

    Or maybe its just being rude! ;-)

    Then again maybe its just the delay as myvoice travels through vacuum* to the star and for its reply toget get back .. my distant descendents are going to be very surprised in about 3000 years time! ;-)
    _____________________________________________
    * Sound in vacuum .. Oh. Yeah. That could explain why … Or maybe we’ll call it a telepathic ‘hello’ ..? Or not. ;-)

  35. StevoR

    Couple of extra points that some may find interesting :
    (If anyone’s still reading this so late in the piece)

    Regarding the smallest of the pulsar planets (& thx David Bennett too) from my article on ’26 Remarkable Exoplanets’ in the SA Astronomical Society’s newsletter ‘The Bulletin’ the info coming via various sources* :

    “PSR B 1257+12 : Discovered in 1991 these pulsar planets were the first ever found. Four very low mass worlds orbit a pulsar with the inner three spaced like a half-sized model of our inner solar system and the outermost just 1/5th Plutoâ??s mass at a distance equivalent to the asteroid belt in our system.”

    So we recognise exoplanets as small as 1/5th Pluto are planets! Now that sounds like Pluto (& Eris & perhaps also Ceres & Charon ) desertve to be termed planets to me! ;-)

    Secondly from the same source (which was compiled from other sources ..*) :

    “2M 1207b : The first exoplanet photographed â?? in 2004 on April 27th – by a European-American team using the Yepun telescope in Chile – although contending claims exist. This planet has 5 Jupiter masses and orbits 55 AU from a brown dwarf with 25 Jupiter masses with both objects having an estimated age of only 8 million years.”

    So that’s at least one other brown dwarf with planet combo known .. plus there’s the case of the brown dwarf Cha 110913-773444 whichis known to have a protoplanetary disk forming planets a discovery chronicled -complete with awesome artists comparion versus 55 Cancris in the Jan-Feb 2006 issue of ‘Sky & Space’ magazine (P.42,”Found :P lanets in the Making” by David Reneke.)

    So fantastic as this discovery is – & my congrats to the team who discovered it & the BA for his coverage here it’s not the first time planets or their makings have been found for brown dwarfs .. & I’m guessing there’ll be more too! 8)
    _______________________________________

    * Compiled from various sources? OK these would be :

    “Sources & further reading :

    Bad Astronomy : http://www.badastronomy.com/
    Exoplanets catalogue : http://exoplanets.org/
    Extrasolar planets modelling & discussion site : http://www.oklo.org/
    Kalerâ??s Planet Project : http://www.astro.uiuc.edu/~kaler/sow/pp.html
    NASA PlanetQuest site : http://planetquest.jpl.nasa.gov/index.cfm
    Space.com â?? astronomy & space exploration news : space.com
    Sol Station â?? Gliese 581 page : http://www.solstation.com/stars/gl581.htm

    Plus numerous articles & news items from magazines incl. â??Sky & Spaceâ??, â??Astronomyâ??, â??Sky at Nightâ??, â??Astronomy Nowâ??, â??Sky & Telescopeâ?? & various scientific papers avail. online.”

    ***

    Yeah, sorry I know that last part there is more than a little vague but there were just too many to include in the society newsletter – it would’ve taken up a few pages mostly of small “astro-news” type items and paper cites … :-(

  36. StevoR

    Yikes! The BA blog computer / server sure doesn’t seem to like the apostrophe!

    ‘ … Or at least my cut’n'paste version of it … :-(

    Sorry. I’m sure y’all get the gist of it anyhow.

    _______————______———___——–____—-

    Oh no, its an apostrophe!
    Don’t you mean a catastrophe?
    No, worse than that, an apostrophe. Catastrophes we can understand,
    apostrophes get everyone confused! ;-)

  37. Buzz Parsec

    JP -

    What do you think the BLG in the system’s name stands for?
    Obviously, it’s a euphemism for “B*****m”.

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