Young Exoplanet is Cloudy With a Chance of Heat Waves

by Nancy Atkinson on August 30, 2010

Want to stay on top of all the space news? Follow @universetoday on Twitter

Keck II image of the young extrasolar planet HR 8799 b, seen as the point source in center of image.Credit: Brendan Bowler and Michael Liu, IfA/Hawaii


Back in 2008, the first multi-planet system of extrasolar planets was imaged, and further study of the planets in this very young system is yielding some puzzling results. Astronomers using the Keck Observatory have been able to obtain the spectrum of one planet, HR 8799 b, revealing the temperature, chemical composition, and atmospheric properties of the planet. The planet’s atmosphere is unlike that of any previously studied extrasolar planet, and it appears the planet is extremely cloudy, and also quite hot, even though it is very far from its host star.


“We are at a point where not only can we directly image planets around other stars, but we can begin to study the properties of their atmospheres in detail. Direct spectroscopy of exoplanets is the future of this field,” said Brendan Bowler, a graduate student at the University of Hawaii and the lead author of the study.

Although over 500 planets have been discovered around other stars, only six planets have been directly imaged.

Three exoplanets orbiting a young star 140 light years away are captured using Keck Observatory near-infrared adaptive optics. The planets are labeled and the two outer ones have arrows showing the size of their motion over a 4 year period.


HR 8799 b, is one of those imaged, and is one of three gas-giant planets orbiting the star HR 8799, located 130 light-years away from Earth in the constellation Pegasus. Bowler and his team said the properties of the planet’s atmosphere can’t be explained by current theoretical models of gas giant exoplanets, even those with what is considered a normal amount of thick or dusty atmospheres. From the new data on this planet, the astronomers believe that this exoplanet is extremely cloudy, and perhaps, all young gas-giant planets exhibit the same type of cloud cover in their atmospheres.

The technique the team used to determine the planet’s temperature relies on the chemistry of the planet’s atmosphere. Specifically, the presence or absence of gaseous methane can be used as a thermometer. The team found that HR 8799 b shows little or no methane in its atmosphere. Based on their spectrum and previously obtained images of the planet, and by comparing the observations to theoretical models of low-temperature atmospheres, they estimate the coolest possible temperature for the planet is about 1200 Kelvin (about 1,700 degrees Fahrenheit).

This planet is quite far from the star, 67 times the Earth-sun separation from the host star.

Current theoretical models predict HR 8799 b should be about 400 Kelvin cooler than they measured, based on the age of the planet and the amount of energy it is currently emitting. The team suspects the discrepancy arises because the planet is much more dusty and cloudy than expected by current models.

“Direct studies of extrasolar planets are just in their infancy. But even at this early stage, we are learning they are a different beast than objects we have known about previously,” said University of Hawaii astronomy professor Michael Liu, coauthor of the study.

The planets around HR 8799 are incredibly faint, about 100,000 times dimmer than their parent star. To obtain the spectrum of HR 8799 b, the team relied on the adaptive optics system of the Keck II Telescope, and focused on the star for several hours. Then they used the Keck facility instrument called OSIRIS, a special kind of spectrograph, to precisely separate the spectrum of the planet from the light of its parent star.

A paper describing the study will be published in the Astrophysical Journal later this year, but you can read the team’s abstract here.

There’s also a new paper out that suggests the these planets around HR 8799 could actually be brown dwarfs.

Source: Keck Observatory

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

IVAN3MAN_AT_LARGE August 30, 2010 at 8:24 PM

This planet is quite far from the star, at 67 times the Earth-sun separation from the host star.

The star HR 8799 has a luminosity of about 4.9 times that of the Sun; therefore, according to inverse-square law, the planet HR 8799 b, at the distance of 67 A.U. (Earth-Sun distance) from its host star, will receive approximately the same level of heat/light energy from HR 8799 as Neptune receives from the Sun.

(No need to thank me for that trivia information!) ;-)

Torbjorn Larsson OM August 30, 2010 at 10:46 PM

Ooh – shiny! … no, wait, that didn’t come out right. Ooh – radiant!

[Sorry. Still waiting on that Earth analogue. Any year now...]

Lawrence B. Crowell August 31, 2010 at 7:03 AM

I am presuming the heat of this planet is not from the central star, but due to the gravitational compression of the body.

LC

SweeneyTodd August 31, 2010 at 8:31 AM

is it 130 or 140 ly away ? :p

Anyway i cant’ wait to see images of this system with the JWST

IVAN3MAN_AT_LARGE August 31, 2010 at 11:30 AM

According to Wikipedia, HR 8799 is 129 ±4 light-years away.

Malcolm August 31, 2010 at 7:11 PM

Up to 500 exoplanets detected so far, none of them closely resembling earth and, it seems, all different from each other. That makes me think that maybe planets are as individually unique as fingerprints or snowflakes or DNA. The chances of ever finding an earth twin would be incredibly remote if that were the case.

vagueofgodalming September 1, 2010 at 5:08 AM

There’s also a new paper out that suggests the these planets around HR 8799 could actually be brown dwarfs.

Actually, that paper is arguing that they are planets. Admittedly the title (Could the planets around HR 8799 be brown dwarfs?) is somewhat misleading, but it’s a reply to another paper that did suggest that they are brown dwarfs.

The argument hinges on the age of the central star. If it’s young, the planets are young, and therefore bright for their size, i.e. small for their brightness. If it’s old, then the planets would have to be a whole lot larger still to be visible, i.e. brown dwarfs. They vote for a young star (100my), based on the dynamical stability of the system.

Nyx September 1, 2010 at 5:28 AM

Malcom, our current findings are easiest one: mostly gas giants and/or planets that are very close to their stars. Gas giants (if not brown dwarf star) are “easy” to detect because they are, well, giant. If you look at our solar system, from another star with the right equipment, you will see Jupiter, maybe Saturn at first. Then with improved instruments and technology advances you may discover Uranus and Neptune. To catch Earth, you need better instruments again. There are also techniques to detect a planet indirectly, in peculiar conditions, by observing a star and the luminosity variation as an orbiting planet passed in front of it. These are lucky conditions. Also you can measure the tiny gravitational influence on the star, but, again, the largest and most massive planets have more influence more than any Earth-sized (I mean mass) planet.
So this is the main reason. But, yeah, planets are not all alike. Just look at our solar system. We have a “twin”, Venus, that is large and massive almost like the Earth, but apart that, she is completely different.

Comments on this entry are closed.

Previous post:

Next post: