exoplanets Archives

December 10, 2009December 24, 2015 by Jon Voisey

Forming Planets Around Binary Stars

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Fanciful science fiction and space art frequently depict the lovely visage of a twin sunset where a pair of binary stars dips below the horizon (think Star Wars). While it has been established that planets could exist in such a system by orbiting in resonances, that only holds true for fully formed planets. Can forming star systems even support an accretion disk from which to form planets? This is the question a new paper by M. G. Petr-Gotzens and S. Daemgen of the European Southern Observatory with S Correia from the Astronomiches Institut Potsdam attempts to answer.

Observations of single young stars with disks have revealed that the main force causing the dispersion of the disk is the star itself. The stellar wind and radiation pressure blow the disk away within 6 to 10 million years. Predictably, more massive and hotter stars will disperse their disks more quickly. However, “many stars are members of a binary or multiple system, and for nearby solar-like stars the binary fraction is even as high as ~60%.” Could gravitational perturbations or the added intensity from two stars strip disks before planets could form?

To explore this, the team observed 22 young and forming binary star systems in the Orion Nebula to look for signs of disks. They used two primary methods: The first was to look for excess emission in the near infrared. This would trace accretion disks as they radiate away absorbed energy as heat. The second was to look spectroscopically for specific bromine emission that is excited as the magnetic field of the young star pulls this (and other) elements from the disk onto the stars surface.

When the results were analyzed they found that as much as 80% of the binary systems had an active accretion disk. Many only contained a disk around the primary star although nearly as many contained disks around both stars. Only one system had evidence of an accretion disk around only the secondary (lower mass) star. They authors note, “[t]he under-representation of active accretion
disks among secondaries hints at disk dissipation working faster on (potentially) lower mass secondaries, leading us to speculate that secondaries are possibly less likely to form planets.”

However, the average age of the stars observed was only ~1 million years. This means that, even though disks may be able to form, the study was not comprehensive enough to determine whether or not they would last. Yet a survey of the currently known extra-solar planets reveal that they must. The authors comment, “[a]lmost 40 of all the extra-solar planets discovered to date reside in wide binary systems where the component separation is larger than 100AU (large enough that planet formation around one star should not strongly be inuenced [sic] by the companion star).”

Strangely, this seems to stand at odds with a 2007 paper by Trilling et al. which studied other binary systems for the same IR excess indicative of debris disks. In their study, they determined “[a] very large fraction (nearly 60%) of observed binary systems
with small (<3 AU) separations have excess thermal emission.” This suggests that such close systems may indeed be able to retain disks for some time. It is unclear on whether or not it can be retained long enough to form planets although it seems unlikely since no exoplanets are known around close binaries.

November 16, 2009December 24, 2015 by Nicholos Wethington

Second Exoplanet with Retrograde Orbit Discovered

The exoplanet HAT-P-7b has been observed to have a very curious orbit. It either has a highly tilted orbit – passing almost over the poles of its parent star, HAT-P-7 – or a retrograde orbit; that is, orbiting in the opposite direction of its parent star. Two teams of researchers, both using the Subaru Telescope in Japan, have published papers on the bizarre properties of this planet, the second exoplanet ever observed to have a retrograde orbit.

In our Solar System, the planets calmly rotate in the same direction as that of their parent star, in our case the Sun. This is called a prograde orbit, and the Earth has the most inclined orbit with regard to the equator of the Sun, of 7.15 degrees. The planet HAT-P-7b, however, has an orbit that is the opposite of the rotation of its parent star but in the same plane as the equator (effectively a 180 degree incline). This is called a retrograde orbit. It may also be the case that it is inclined to at least 86 degrees of the equator of its Sun, so as to have almost a polar orbit. The researchers have yet to determine the true rotation of the star HAT-P-7, and thus which scenario is true for the exoplanet.

“There is a large range of uncertainty because we have not measured the true angle between the orbit and the stellar equator. Instead we can only measure the angle that we see from our perspective on Earth,” said Winn in a MIT press release.

HAT-P-7b is about 1.4 times as wide and 1.8 times as massive as Jupiter, and lies approximately 1,000 light years from the Earth.

A Japanese collaboration led by Norio Narita of the National Astronomical Observatory of Japan, and a team led by MIT assistant professor of physics Joshua Winn both published papers detailing their studies of HAT-P-7b. These studies were published in the Publications of Astronomical Society of Japan Letters October 25, 2009 and the Astrophysical Journal Letters for October 1, 2009, respectively. The paper by the Japanese team is available for your perusal on Arxiv here.

Both research teams used the Subaru Telescope’s High Dispersion Spectrograph instrument to observe the star HAT-P-7. The spectrograph allowed the researchers to monitor the redshift or blueshift of light as the planet orbited the star. In planets with a prograde orbit, their transit in front of the star blocks the blue shifting of the light from the star first, then blocks the redshift of the light, making the star appear to move more that it actually is.

In the case of HAT-P-7b the effect was reversed – that is, the redshifted light appeared bluer, then the blueshifted light appeared redder, making it apparent that the orbit of the planet was not in the same direction of that of HAT-P-7. This effect is called the Rossiter-McLaughlin effect, illustrated below.

The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons

The odd orbit of HAT-P-7b could have been caused by a number of different factors, and theorists that model the formation of exoplanetary systems will not have to “go back to the drawing boards”. The general consensus is that planets form out of a large disk of material orbiting the star, and thus all orbit in the same direction as the disk out of which they formed.

Multiple planets could have formed in an unstable configuration around the star, and their proximity to each other could have caused a rather chaotic series of gravitational billiards to boot HAT-P-7b into its current orbit. Another explanation is the presence of a third object in the system, such as another massive planet or companion star, that is tilting the orbit of HAT-P-7b due to what’s known as the Kozai effect.

The announcement of the retrograde orbit of HAT-P-7b came only one day after the announcement on August 12th, 2009 that the planet WASP-17b orbits opposite its parent star. HAT-P-7b is also one of the first exoplanets to be studied by the Kepler mission, which studied the planet’s orbit over 10 days. Kepler will take further images of the star during its mission, and by observing the rotation of spots on the surface of the star, nail down the orbital direction, after which we’ll know whether HAT-P-7b is orbiting “backwards” or around the poles of the star.

Source: Subaru Telescope, MIT

February 16, 2009October 10, 2016 by Nicholos Wethington

The Milky Way Could have Billions of Earths

Exoplanets like the Earth might be more common than we think. Image Credit: ESO

With the upcoming launch in March of the Kepler mission to find extrasolar planets, there is quite a lot of buzz about the possibility of finding habitable planets outside of our Solar System. Kepler will be the first satellite telescope with the capability to find Earth-size and smaller planets. At the most recent meeting of the American Association for the Advancement of Science (AAAS) in Chicago, Dr. Alan Boss is quoted by numerous media outlets as saying that there could be billions of Earth-like planets in the Milky Way alone, and that we may find an Earth-like planet orbiting a large proportion of the stars in the Universe.

“There are something like a few dozen solar-type stars within something like 30 light years of the sun, and I would think that a good number of those — perhaps half of them would have Earth-like planets. So, I think there’s a very good chance that we’ll find some Earth-like planets within 10, 20, or 30 light years of the Sun,” Dr. Boss said in an AAAS podcast interview.

Dr. Boss is an astronomer at the Carnegie Institution of Washington Department of Terrestrial Magnetism, and is the author of The Crowded Universe, a book on the likelihood of finding life and habitable planets outside of our Solar System.

“Not only are they probably habitable but they probably are also going to be inhabited. But I think that most likely the nearby ‘Earths’ are going to be inhabited with things which are perhaps more common to what Earth was like three or four billion years ago,” Dr. Boss told the BBC. In other words, it’s more likely that bacteria-like lifeforms abound, rather than more advanced alien life.

This sort of postulation about the existence of extraterrestrial life (and intelligence) falls under the paradigm of the Drake Equation, named after the astronomer Frank Drake. The Drake Equation incorporates all of the variables one should take into account when trying to calculate the number of technologically advanced civilizations elsewhere in the Universe. Depending on what numbers you put into the equation, the answer ranges from zero to trillions. There is wide speculation about the existence of life elsewhere in the Universe.

To date, the closest thing to an Earth-sized planet discovered outside of our Solar System is CoRoT-Exo-7b, with a diameter of less than twice that of the Earth.

The speculation by Dr. Boss and others will be put to the test later this year when the Kepler satellite gets up and running. Set to launch on March 9th, 2009, the Kepler mission will utilize a 0.95 meter telescope to view one section of the sky containing over 100,000 stars for the entirety of the mission, which will last at least 3.5 years.

The prospect of life existing elsewhere is exciting, to be sure, and we’ll be keeping you posted here on Universe Today when any of the potentially billions of Earth-like planets are discovered!

Source: BBC, EurekAlert