BBC NEWS | Science/Nature | Puffy planet poses pretty puzzle

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width="629"> <a name="startcontent"></a><span class="ds"><span class="lu">Last Updated: </span>Friday, 15 September 2006, 15:55 GMT 16:55 UK</span> <img height="2" hspace="0" vspace="0" border="0" width="2" alt="" src="http://newsimg.bbc.co.uk/shared/img/o.gif" /> <div class="mvb"><img height="1" hspace="0" vspace="0" border="0" width="629" alt="" src="http://newsimg.bbc.co.uk/nol/shared/img/v3/dot_629.gif" /></div> <div class="mvtb"><table cellspacing="0" width="416" border="0" cellpadding="0"><tr> <td width="213"><a onClick="popUpPage('http://newsvote.bbc.co.uk/mpapps/pagetools/email/news.bbc.co.uk/2/hi/science/nature/5346998.stm','status=no,scrollbars=yes,resizable=yes,width=370,height=445','Mailer')" href="http://newsvote.bbc.co.uk/mpapps/pagetools/email/news.bbc.co.uk/2/hi/science/nature/5346998.stm" class="epl" target="Mailer"> <img height="11" hspace="3" align="left" vspace="0" border="0" width="17" alt="" src="http://newsimg.bbc.co.uk/nol/shared/img/v3/email.gif" /> E-mail this 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new world (David A. Aguilar CfA) " src="http://newsimg.bbc.co.uk/media/images/42088000/jpg/_42088440_planet_cfa_203l.jpg" /> <div class="cap">An artist's impression of the strange new world, HAT-P-1b</div> </div> </td></tr> </table>   <B>Astronomers have found a strange new world that has them pondering again the essential properties of a planet.</B> <P> This new object, designated HAT-P-1b, orbits one member of a pair of stars 450 light-years away in the constellation Lacerta. <P> Although its radius is about 1.38 times that of our own Jupiter, it has a mass that is only half that of Jupiter. <P> This makes it much bigger and lower in density than planets are usually, raising questions about how it formed.  <P> The mathematical equations describing planetary structure do not fit. <P>  <table cellspacing="0" cellpadding="0" width="208" align="right" border="0"> <tr> <td width="5"><img height="1" hspace="0" vspace="0" border="0" width="5" alt="" src="http://newsimg.bbc.co.uk/shared/img/o.gif" /></td> <td class="sibtbg"> <div> <div class="mva"> <img height="13" border="0" width="24" alt="" src="http://newsimg.bbc.co.uk/nol/shared/img/v3/start_quote_rb.gif" /> <b>It would float in a bathtub if you could find a tub big enough to hold it</b> <img height="13" align="right" vspace="0" border="0" width="23" alt="" src="http://newsimg.bbc.co.uk/nol/shared/img/v3/end_quote_rb.gif" /><br clear="all"/> </div> </div> <div class="mva"> <div>Gaspar Bakos</div> </div> </td> </tr> </table>  "This planet is about one-quarter the density of water," said Gaspar Bakos, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). <P> "In other words, it's lighter than a giant ball of cork! Just like Saturn, it would float in a bathtub if you could find a tub big enough to hold it, but it would float almost three times higher." <P> HAT-P-1b is one of the 200 or so planets that have been detected outside our Solar System. It has the largest radius yet measured. <P> Like many of these extrasolar bodies, it orbits close to its parent star, revolving around it in just once every 4.5 Earth days. <P> <B>New HAT?</B> <P> HAT-P-1b is too far away to be imaged directly, but scientists know it is there because of the way light from its parent star dims as the planet passes in front. <P> Scientists know of one other extrasolar planet, HD 209458b, which is also puffed up about 20% bigger than predicted by theory. HAT-P-1b is 24% larger than expected. <P> Several ideas had been proposed solve the puzzle but none seemed to quite fit, said the CfA research team, which has detailed its discovery in a paper submitted to the Astrophysical Journal. <P> "Until we can find an explanation for both of these swollen planets, they remain a great mystery," said co-researcher Dimitar Sasselov. <P> The astronomers used a network of telescopes in the states of Arizona and Hawaii to discover the planet. <P> The double-star system, of which HAT-P-1b's parent forms a part, is called ADS 16402 and is visible through binoculars. <P> </font> <br /><br clear="all" /> <div class="mvtb"><table cellspacing="0" width="416" border="0" cellpadding="0"><tr> <td width="213"><a onClick="popUpPage('http://newsvote.bbc.co.uk/mpapps/pagetools/email/news.bbc.co.uk/2/hi/science/nature/5346998.stm','status=no,scrollbars=yes,resizable=yes,width=370,height=445','Mailer')" 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src="http://newsimg.bbc.co.uk/shared/img/o.gif" /></td> <td width="203" valign="top"> <div class="seeAlsoH"> SEE ALSO </div> <div class="arr"> <a href="/2/hi/science/nature/5344892.stm"> Astronomers name 'world of chaos' </a> <br /> <span class="sad"> 14 Sep 06 |  Science/Nature </span> </div> <div class="arr"> <a href="/2/hi/science/nature/5317906.stm"> A view from the cosmic shoulder </a> <br /> <span class="sad"> 06 Sep 06 |  Science/Nature </span> </div> <div class="arr"> <a href="/2/hi/science/nature/5010936.stm"> Planet shine 'to aid life search' </a> <br /> <span class="sad"> 24 May 06 |  Science/Nature </span> </div> <div class="arr"> <a href="/2/hi/science/nature/4801842.stm"> Planet hunters find 'super-Earth' </a> <br /> <span class="sad"> 14 Mar 06 |  Science/Nature </span> </div> <div class="arr"> <a href="/2/hi/science/nature/4647142.stm"> Smallest Earth-like planet found </a> <br /> <span class="sad"> 25 Jan 06 |  Science/Nature </span> </div> <img height="10" width="1" 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