XO-1b orbits in 4 days?

Status
Not open for further replies.
A

agnau

Guest
Ok, after reading the 5/23/2006 science tuesday I have a big question: How small is this star?<br /><br />"After observing XO-1 in June and July 2005, astronomers confirmed that the roughly 2 percent dips in the star's brightness are caused by a planet-sized object that eclipses the star every four days. Astronomers at the McDonald Observatory in Texas verified the object as a planet, named XO-16, and determined its mass was slightly less than that of Jupiter."
 
D

doubletruncation

Guest
It's a GIV star - meaning its mass and radius are very similar to the Sun. If you want to know more details about this system have a look at http://xxx.lanl.gov/abs/astro-ph/0605414<br /><br />There are actually 10 of these short-period transiting planets known, I think most of them actually have orbital periods shorter than this case even. <div class="Discussion_UserSignature"> </div>
 
Q

qso1

Guest
IMO, the transit method is actually more conclusive than radial velocity methods where confirmation of planets orbiting other stars are concerned. It is the closest we can get to a direct observation and in the case of this one, little doubt as to the orbital period since the dip in brightness should be repeatable and roughly the same every 2 days. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
D

doubletruncation

Guest
<blockquote><font class="small">In reply to:</font><hr /><p>IMO, the transit method is actually more conclusive than radial velocity methods where confirmation of planets orbiting other stars are concerned. It is the closest we can get to a direct observation and in the case of this one, little doubt as to the orbital period since the dip in brightness should be repeatable and roughly the same every 2 days.<p><hr /></p></p></blockquote><br /><br />For radial velocity you can still pin down the orbital period very well. I would agree with you though that the transit method is the most conclusive method (presuming that you have both radial velocity measurements and transit observations for the system - if you only have transit observations then the transit method is highly inconclusive since false positives like F dwarf-M dwarf pairings seem to be more than 10 times as likely than actual transiting planets). The key benefit that a transit adds to the radial velocity method for confirming that it is a planet is that you can measure the inclination this way. From the radial velocity you only get m*sin(i), knowing sin(i) from the transit allows you to actually determine the mass of the planet rather than just a lower limit on its mass. If all you have are radial velocities, then you may be able to tell from statistics that most of the objects are probably planets, and you can make an argument for what sin(i) might be from measuring the rotation velocity of the star (from the broadening of its absorption lines) and getting an estimate of the rotation period from the activity measured in the Ca H and K lines, but ultimately you never really *know* if that particular planet candidate is actually just a binary system in a nearly pole-on orbit. <div class="Discussion_UserSignature"> </div>
 
Q

qso1

Guest
Good points. In exoplanet huntings early days, radial velocity was still a rather suspect technique but currently it is a reliable method of planet detection. Its also probably better suited for multiple planet detection. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
J

jmilsom

Guest
Reading this thread raises a question that has been nagging me since the discovery of these rapidly orbiting gas giants.<br /><br />I must admit that I remain skeptical that a gas giant could possibly orbit its star in four days. Perhaps we have something wrong. Has anyone offered any alternative explanations for these dips? <div class="Discussion_UserSignature"> </div>
 
A

agnau

Guest
The thread has come full circle... It has always been my concern about the size of the star, distance to the star, and speed of the planet. That would be one small star, or polar-orbiting of the star.
 
D

doubletruncation

Guest
<font color="yellow">Reading this thread raises a question that has been nagging me since the discovery of these rapidly orbiting gas giants.<br /><br />I must admit that I remain skeptical that a gas giant could possibly orbit its star in four days. Perhaps we have something wrong. Has anyone offered any alternative explanations for these dips?</font><br /><br />I think we can say, with just about as much certainty as you can say anything in astronomy, that the transiting planets are in fact planet mass objects orbiting their stars with short periods. For these planets we have both light curves and radial velocity curves, together with the type of star inferred from its spectrum, that is enough information to pin down the mass and radius for the transiting object. For two of these transiting planets people have actually seen the secondary eclipse when the planet passes behind its star which further constrains the models on the things. People have also observed atmospheres for these planets during transit. How these things actually formed is certainly much less certain - the prevailing theory is that they did form like Jupiter but then migrated close to the star, there is very strong theoretical reasons to believe that planets should migrate a lot, in fact it's a bit of a problem to figure out how things stop or don't migrate toward the star. But, it's not totally impossible to say that these hot planets formed more like stars do (direct gravitational collapse). But we do know that these are planet mass objects orbiting right next to the star. <div class="Discussion_UserSignature"> </div>
 
J

jmilsom

Guest
Thanks for your explanation! I'll read up some more on the theories for the formation of such bodies. Any links to useful papers? <div class="Discussion_UserSignature"> </div>
 
D

doubletruncation

Guest
Oh, there are lots of papers <img src="/images/icons/wink.gif" />! I'm not really an expert on planet formation, so I don't know if the ones I'd recommend would really be the best ones (particularly as far as a general review of the subject goes). Ida and Lin have written a series of papers that are pretty big in the field currently (particularly because they actually make predictions about what the distribution of planet masses and semi-major axes should be):<br />Ida and Lin, 2004, ApJ, 604, 388<br />Ida and Lin, 2004, ApJ, 616, 567<br />Ida and Lin, 2005, ApJ, 626, 1045<br /><br />you can find them on ADS at:<br />http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004ApJ...604..388I&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=446aa1394107902<br />http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004ApJ...616..567I&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=446aa1394107902<br />http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ApJ...626.1045I&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=446aa1394107902<br /><br />(if you don't have access to an ApJ subscription, click on the arXiv e-print for the article which is free).<br /><br />These papers are really dense, the plot that is of key interest is figure 9 of the first paper which shows the predicted distributions of planet masses and semi-major axes from their t <div class="Discussion_UserSignature"> </div>
 
Q

qso1

Guest
jmilsom:<br />I must admit that I remain skeptical that a gas giant could possibly orbit its star in four days. Perhaps we have something wrong. Has anyone offered any alternative explanations for these dips?<br /><br />Me:<br />IIRC, there was an astronomer who questioned the 51 Pegasi discovery in the days or months after. He later verified to his satisfaction that there was a hot Jupiter as it became known, orbiting 51 Pegasi. But having mentioned that, it is still possible that some other explanation could account for these close gas giants. But given the number of them discovered since 51Pegasi, and the number of astronomers verifying the detections. It will be one of astronomys most embarrasing moments if these hot Jupiters should be proven to be accounted for by reasons other than being planets. This however, is one reason I mentioned transit photometry as a next to direct imaging, sure way to comfirm a planet. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
J

jmilsom

Guest
Thanks again for the detailed replies. Interesting articles too, I waded through what I could. I wonder what the life of a planet in this type of orbit is? Did you extract any speculation on this from the three (dense is an understatement!) papers? <div class="Discussion_UserSignature"> </div>
 
Status
Not open for further replies.

ASK THE COMMUNITY

Latest posts