Category: Extrasolar Planets

One of the biggest news stories of the year was an Earth-sized planet orbiting Gliese 581. Even more importantly, this terrestrial planet is orbiting within the star’s habitable zone, where any water will likely be in liquid form. But it takes more than just water to encourage life, you need a nice, stable star. And according to a new survey, Gliese 581 fits the bill there too.

The survey was done by Canada’s MOST telescope, nicknamed the Humble Space Telescope. It’s a suitcase-sized space observatory with the ability to watch for changes in brightness with incredible sensitivity. MOST focused on Gliese 581 for 6 weeks, watching for any flareups, or drops in light.

According to University of British Columbia researcher, Jaymie Matthews, the brightness of the star only changed a few tenths of a percent during their observations. That means its radiation output remains very stable over time.

So, Gliese 581c has the possibility of liquid water and stable warming from the star. Good news for potential life on this distant planet.

Original Source: UBC News Release

Another new planet has been announced this week that crosses the line between planet and brown dwarf. This time, the planet is called XO-3b, and it was discovered through a collaboration between amateur and professional astronomers.

XO-3b contains 13 times the mass of Jupiter, and orbits its parent star every 4 days. It was first discovered using a low budget telescope, part of the XO project, located on the Haleakala summit on Maui, Hawaii. The telescope is actually made up of two commercially available 200-millimetre telephoto lenses which watch stars for the characteristic dimming as a planet passes in front. When a suspected transit is seen, larger telescopes are brought in to confirm the findings.

Brown dwarfs are failed stars, lacking the mass to ignite fusion, but they do have enough mass to fuse deuterium. Astronomers categorized them as any object between 13 and 80 times the mass of Jupiter. With 13-Jupiter masses, XO-3b sits right at the dividing line between planet and brown dwarf.

Original Source: Rice University News Release

When you look at the ground beneath your feet, you’re looking at matter created in the heart of stars at the end of their lives. Some of the heavier elements were fashioned in the supernovae explosions of massive stars. And in many cases, these elements went through several generations of stars. So it was a tremendous surprise this week when astronomers discovered planets orbiting a metal poor star.

The discovery was made by a team of researchers from the University of Texas using the 9.2-metre Hobby-Eberly Telescope at McDonald Observatory. They found a system of two Jupiter-like planets orbiting a star that’s so low in metals that it shouldn’t have planets at all.

But there they are.

The star is known as HD 155358, and the planets were discovered using the radial velocity method, where the gravity from the planets pull the star back and forth with a velocity we can detect here on Earth. This allows astronomers to calculate their mass and the length of their orbit.

One planet has an orbital period of 195 days and has 90% the mass of Jupiter. The other takes 530 days and has 50% the mass of Jupiter. They actually orbit so close to one another, that they must gravitationally interact. They push each other around.

A commonly-held model of planetary formation requires that there are large quantities of heavy metals present in the protoplanetary disk. Finding such a low metal star, but still with two planets around it, is an impressive find, and will give astronomers reason to reconsider their theories.

Original Source: UT Austin News Release

One of the most dramatic extrasolar planetary discoveries of the year was announced this week; unfortunately, with little fanfare. Planet hunters uncovered a Neptune-sized planet orbiting a nearby star. This planet is close enough to its parent star that it’s extremely hot – above 250 degrees Celsius. And yet the intense pressure from gravity forces large quantities of liquid water into solid ice.

The planet was discovered orbiting the nearby M-dwarf star GJ 436 using the planetary transit technique. This is where a sensitive instrument called a photometer measures the periodic dimming and brightening of a star as a planet passes in front. In August 2006, astronomers captured the first hint of the planet using the Observatoire Francois-Xavier Bagnoud (OFXB) observatory in St-Luc Switzerland. It was then confirmed using the Euler 1.2m telescope at La Silla Observatory in Chile.

The announcement was made in the paper Detection of transits of the nearby hot Neptune GJ 436 b, which has been accepted for publication in the journal Astronomy and Astrophysics Letters.

With more traditional planet hunting techniques, very little information can be found about the planet, other than its mass. But planetary transits offer a wealth of data. Since the light from the star dims, and the chemical composition of the light changes, astronomers can determine the planet’s atmosphere by subtracting it from the star. They can measure both the mass, and the size of the planet, and measure the temperature of its surface.

According their calculations, GJ 436 b is approximately 50,000 km across; 4 times the radius of Earth, and approximately the size of Neptune. This makes is the smallest planet ever discovered using the planetary transit technique, and brings the possibility of uncovering Earth-sized planets tantalizingly closer. But unlike frigid Neptune, it orbits much closer than the orbit of Mercury, completing an orbit in just a few days. Even through the dwarf star it orbits is less luminous than our Sun, the planet orbits so close that it’s heated above 250 degrees Celsius. This makes it the first “hot Neptune” ever discovered.

A planet with this amount of water ice must have formed outside the star’s “snow line”, where the protoplanetary disc is cool enough for water to condense. Some process must have brought it gradually closer to the parent star, to its current position today. Once the planet got close enough to the star its outer envelope of hydrogen and helium would have evaporated away, leaving the smaller icy core.

Original Source: Arxiv

Of the extrasolar planets astronomers have discovered, it’s the hot jupiters that really boggle the imagination. These worlds can be much larger than Jupiter, but tear around their parent star in just a few hours. Or in the case of newly discovered TrES-3, in just 31 hours.

This latest discovery, titled TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31-Hour Orbit, will be published in an upcoming issue of the Astrophysical Journal Letters.

As with many planetary discoveries, this was a team effort, made by astronomers and observatories around the world. The story begins when astronomers measured a periodic dimming around parent star GSC 03089-00929 – a G-dwarf star 90% the mass of the Sun. Since the discovering instruments were part of the Trans-atlantic Exoplanet Survey (TrES) network, this is how the planet got its name.

Once the astronomers had a candidate, other astronomers performed follow up observatories using the Hungarian Automated Telescope Network, the Fred L. Whipple Observatory, the Submillimeter Array atop Mauna Kea, and a handful of other instruments around the world. By the time they were done, hundreds of observations had been made in many wavelengths.

It’s currently estimated to have a mass of approximately 1.92 the mass of Jupiter. As mentioned, it orbits its parent star once every 31 hours – at a distance of only 0.0226 AU (1 AU = the distance from the Earth to the Sun). That sounds fast, and it is. The current record is OGLE-TR-56b, which orbits every 29 hours. So TrES-3 comes in a close second.

Based on their observations, TrES-3 is not a grazing transit. In other words, the planet passes directly between the star and the Earth, fitting its disk entirely within the disk of the star at mid-transit. The transit itself takes only 1.3 hours to complete. This provides astronomers with a useful testbed for theoretical models of gas giants. Astronomers want to understand what happens to planets this close to their parent star; how their orbits decay, and rate of thermal evaporation from being so close.

And here’s the mystery. At this close distance to its star, TrES-3 must be going furious evaporation of its gas. Astronomers are wondering if a planet could form this close, and still be around billions of years later. Another possibility is that it formed further out, and was pulled in close over millions of years. TrES-3 is so close that its gravity tidally effects the star, modifying the rate at which its orbit decays, and confusing the calculations.

The astronomers are planning follow up observations of TrES-3 with the powerful Spitzer Space Telescope, since it’s an ideal candidate for attempts to detect reflected starlight. This will allow astronomers to measure the planet’s albedo, or reflectiveness.

Original Source: Arxiv Release

Thanks to Dr. Pamela Gay for helping me puzzle through some of the scientific language.

How’s the weather? Hot enough for you? Well, if you’re living on extrasolar planet HD 189733b, you’d really want to be anywhere else. That’s because the high noon temperatures reach 926 degrees C (1700 degrees F). How do we know what the weather’s like on this distant planet? Just thank Spitzer.

Astronomers working with NASA’s Spitzer Space Telescope have produced a rough map of the weather systems on HD 189733b. Over the course of 33 hours of observations, they gathered together more than 250,000 data points measuring the planet’s brightness. These data points were then mapped onto the planet, to show its global temperatures.

HD 189733b orbits its parent star at a distance of only 4.8 million km (3 million miles); it completes an orbit every 2.2 days. In terms of mass and size, it’s a little larger than Jupiter. This close proximity to its parent star puts it into the “Hot Jupiter” category, of extrasolar planets.

One interesting surprise: the hottest spot on the planet doesn’t directly face the star. Instead it’s offset about 30 degrees longitudinally. The researchers speculate that powerful weather systems redistribute the heat across the planet, and into these pockets of heat.

Original source: CfA News Release