X-ray Study Shows Older Stars May be More Supportive to Life

A study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to how hospitable a star system could be. Credit: NASA/CXC/M.Weiss

Astronomers have long understood that there is a link between a star’s magnetic activity and the amount of X-rays it emits. When stars are young, they are magnetically active, due to the fact that they undergo rapid rotation. But over time, the stars lose rotational energy and their magnetic fields weaken. Concurrently, their associated X-ray emissions also begin to drop.

Interestingly, this relationship between a star’s magnetic activity and X-ray emissions could be a means for finding potentially-habitable star systems. Hence why an international team led by researchers from Queen’s University Belfast conducted a study where they cataloged the X-ray activity of 24 Sun-like stars. In so doing, they were able to determine just how hospitable these star systems could be to life.

This study, titled “An Improved Age-Activity Relationship for Cool Stars Older than a Gigayear“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Rachel Booth, a PhD student from the Astrophysics Research Center at Queen’s University Belfast, the team used data from NASA’s Chandra X-ray Observatory and the ESA’s XMM-Newton to examine how the X-ray brightness of 24 Sun-like stars changed over time.

This artist’s impression shows the magnetar in the very rich and young star cluster Westerlund 1. Credit: ESO/L. Calçada

To understand how stellar magnetic activity (and hence, X-ray activity) changes over time, astronomers require accurate age assessments for many different stars. This has been difficult in the past, but thanks to mission like NASA’s Kepler Space Observatory and the ESA’s Convection, Rotation and planetary Transits (CoRoT) mission, new and precise age estimates have become available in recent years.

Using these age estimates, Booth and her colleagues relied on data from the Chandra X-ray observatory and the XMM-Newton obervatory to examine 24 nearby stars. These stars were all similar in mass to our Sun (a main sequence G-type yellow dwarf star) and at least 1 billion years of age. From this, they determined that there was a clear link between the star’s age and their X-ray emissions. As they state in their study:

“We find 14 stars with detectable X-ray luminosities and use these to calibrate the age-activity relationship. We find a relationship between stellar X-ray luminosity, normalized by stellar surface area, and age that is steeper than the relationships found for younger stars…”

In short, of the 24 stars in their sample, the team found that 14 had X-ray emissions that were discernible. From these, they were able to calculate the star’s ages and determine that there was a relationship between their longevity and luminosity. Ultimately, this demonstrated that stars like our Sun are likely to emit less high-energy radiation as they exceed 1 billion years in age.

And while the reason for this is not entirely clear, astronomers are currently exploring various possible causes. One possibility is that for older stars, the reduction in spin rate happens more quickly than it does for younger stars. Another possibility is that the X-ray brightness declines more quickly for older, more slowly-rotating stars than it does for younger, faster ones.

Regardless of the cause, the relationship between a star’s age and its X-ray emissions could provide astronomers and exoplanet hunters with another tool for gauging the possible habitability of a system. Wherever a G-type or K-type star is to be found, knowing the age of the star could help place constraints on the potential habitability of any planets that orbit it.

Further Reading: Chandra, MNRAS

Another Exoplanet Hunting Mission Ends: CoRoT Spacecraft Can’t be Recovered

The COROT spacecraft. Credits: CNES/D. Ducros

More bad news on the exoplanet-hunting front: While the final fate of the Kepler spacecraft remains unknown, the CoRoT (Convection, Rotation and Planetary Transits) satellite has now been officially shut down. CoRoT suffered a computer failure on November, 2, 2012 and although the spacecraft is capable of receiving navigational commands, the French Space Agency CNES reports it can no longer retrieve data from its 30-centimeter telescope. After a valiant effort to try and restore the computer, CNES announced this week that the spacecraft has been retired. CoRoT’s journey will come to a fiery end as it will be deorbited and it will burn up on re-entry in Earth’s atmosphere.

While it’s always hard to see the end of successful mission, we can’t be too sad about CoRoT, however. The mission lasted twice as long as expected and it gathered a remarkable haul of exoplanets. CoRoT looked for planetary transits — a dimming in brightness of the host star as a planet crossed in front. CoRoT was the first mission to find a planet using the transit method.

In all, CoRoT has spotted 32 confirmed planets and at least 100 more are awaiting confirmation. The mission also allowed astronomers to study the stellar physics and the interior of stars.

This is not the first computer failure for the mission. CoRoT launched in December of 2006, and in 2009 the main computer failed and has since been running on the backup computer. When the second computer failed in November, engineering teams have tried to reboot both computers, with no success.

But space radiation is tough on spacecraft, and after enduring 6 years of intense bombardment by high-energy particles in space, both computers have been deemed unrecoverable.

CNES said a series of operations will be performed to lower CoRoT’s orbit and conduct some technology experiments before passivating and deorbiting the satellite. Its journey will end as it burns up on re-entry in Earth’s atmosphere.

Family portrait of the first 15 CoRoT planets. Credit: Patrice Amoyel (CNES)
Family portrait of the first 15 CoRoT planets. Credit: Patrice Amoyel (CNES)

CoRoT discovered a diverse array of planets, mostly gas giants. Some of the planets discovered, like CoRoT-7b, orbit their star in less than 24 hours and have a blistering hot surface, while others like CoRoT-9b have an orbital period of 95 days and is one of very few known “warm” transiting exoplanets.

CoRoT was also the first to obtain measurements of the radius of brown dwarves, intermediate objects between a planet and a star, and literally opened up a whole new field of study of temporal analysis of the micro-variability of stars by measuring the frequencies and amplitudes of stellar vibrations with unprecedented precision.

CNES did not provide a timetable for CoRoT’s demise, but we’ll keep you posted.

Source: CNES

Weird Collection of Worlds in the Latest Cache of CoRoT Expoplanets

Family portrait of the first 15 CoRoT planets. Credit: Patrice Amoyel (CNES)


The CoRoT (Convection, Rotation and Transits) spacecraft has been busy, and using this exoplanet-finding-machine astronomers recently found six new extrasolar planets, which contain an odd assortment of new worlds. They include shrunken-Saturns to bloated hot Jupiters, as well a rare brown dwarf with 60 times the mass of Jupiter. “Each of these planets is interesting in its own right, but what is really fascinating is how diverse they are,” said co-investigator Dr Suzanne Aigrain from Oxford University’s Department of Physics. “Planets are intrinsically complex objects, and we have much to learn about them yet.”

CoRoT is dedicated to looking for planets orbiting other stars, and finds them when they transit, or pass in front of their stars. CoRot now has found 15 of the total 461 exoplanets.

Once CoRoT detects a transit, additional observations are made from the ground, using a number of telescopes all over the world. Although astronomers cannot see the planets directly, they use the space- and ground-based data to measure the sizes, masses, and orbits of these new planets precisely. This is why, among all known exoplanets, those with transits yield the most complete information about planet formation and evolution.

‘Every discovery of an extrasolar planetary system is a new piece in the puzzle of how these systems do form and evolve. The more systems we uncover, the better we can hope to understand the processes at play,’ said Magali Deleuil, researcher at the Laboratoire d’Astrophysique de Marseille (LAM) and head of the CoRoT exoplanet program.

The six new planets are:

CoRoT-8b: the smallest in this batch: At about 70% of the size and mass of Saturn, CoRoT-8b is moderately small among the previously known transiting exoplanets. Its internal structure should be similar to that of ice giants, like Uranus and Neptune, in the Solar System. It is the smallest planet discovered by the CoRoT team so far after CoRoT-7b, the first transiting Super-Earth.

CoRoT-10b: the eccentric giant: The orbit of CoRoT-10b is so elongated that the planet passes both very close to and very far away from its star. The amount of radiation it receives from the star varies tenfold in intensity, and scientists estimate that its surface temperature may increase from 250 to 600°C, all in the space of 13 Earth-days (the length of the year on CoRoT-10b).

CoRoT-11b: the planet whose star does the twist: CoRoT-11, the host star of CoRoT-11b, rotates around its axis in 40 hours. For comparison, the Sun’s rotation period is 26 days. It is particularly difficult to confirm planets around rapidly rotating stars, so this detection is a significant achievement for the CoRoT team.

CoRoT-12b, 13b and 14b: a trio of giants: These three planets all orbit close to their host star but have very different properties. Although CoRoT-13b is smaller than Jupiter, it is twice as dense. This suggests the presence of a massive rocky core inside the planet. With a radius 50% large than Jupiter’s (or 16 times larger than the Earth’s), CoRoT-12b belongs to the family of `bloated hot Jupiters’, whose anomalously large sizes are due to the intense stellar radiation they receive. On the other hand, CoRoT-14b, which is even closer to its parent star, has a size similar to Jupiter’s. It is also massive, 7.5 times the mass of Jupiter, which may explain why it is less puffed up. Such very massive and very hot planets are rare, CoRoT-14b is only the second one discovered so far.

CoRoT-15b: the brown dwarf: CoRoT-15b’s mass is about 60 times that of Jupiter. This makes it incredibly dense, about 40 times more so than Jupiter. For that reason, it is classified as a brown dwarf, intermediate in nature between planets and stars. Brown dwarfs are much rarer than planets, which makes this discovery all the more exciting.

Source: Oxford University

Finally, a “Normal” Exoplanet

Artist’s impression of Corot-9b. Credit: ESO/L. Calçada


Chalk up another exoplanet discovery for the CoRoT satellite. But this planet, while a gas giant, could have temperatures cool enough to host liquid water. Corot-9b orbits a sun-like star at a distance similar to Mercury – one of the largest orbits of any extrasolar planet yet found, and may have an interior that closely resembles Jupiter and Saturn. “This is a normal, temperate exoplanet just like dozens we already know, but this is the first whose properties we can study in depth,” said Claire Moutou, who is part of the international team of 60 astronomers that made the discovery. “It is bound to become a Rosetta stone in exoplanet research.”

Corot-9b (unofficial nickname Carrot Nimby) regularly passes in front of its star, located 1,500 light-years away from Earth towards the constellation of Serpens (the Snake), allowing astronomers to view the planet for 8 hours at a time. The transits occur every 95 days.

“Our analysis has provided more information on Corot-9b than for other exoplanets of the same type,” says co-author Didier Queloz. “It may open up a new field of research to understand the atmospheres of moderate- and low-temperature planets, and in particular a completely new window in our understanding of low-temperature chemistry.”

The star Corot-9b orbits is slightly cooler than our sun, so the astronomer estimate that Corot-9b’s temperature could lie somewhere between -23°C and 157°C.

Corot-9b has a radius around 1.05 times that of Jupiter but only 84% of the mass. This leads to a density of 0.90 g/cc, or 68% that of Jupiter.

More than 400 exoplanets have been discovered so far, 70 of them through the transit method. Astronomers say Corot-9b is special in that its distance from its host star is about ten times larger than that of any planet previously discovered by this method. And unlike all such exoplanets, the planet has a temperate climate. The temperature of its gaseous surface is expected to be between 160 degrees and minus twenty degrees Celsius, with minimal variations between day and night. The exact value depends on the possible presence of a layer of highly reflective clouds.

“Like our own giant planets, Jupiter and Saturn, the planet is mostly made of hydrogen and helium,” said team member Tristan Guillot, “and it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures.”

This team’s findings were published in this week’s edition of the journal Nature. (“A transiting giant planet with a temperature between 250 K and 430 K”), by H. J. Deeg et al.”)

Sources: ESO, EurekAlert

Planet of Lava a Former Gas Giant

Matryoshka dolls are a popular novelty for tourists going to Russia to bring home for their children. These dolls, which are hollow wooden bowling pin-shaped representations of a Russian woman (or babushka), are nested inside of each other, each doll smaller than the one that encases it.

In a perfect model of planetary matryoshka dolls, the exoplanet Corot 7-b – which is currently one of the exoplanets that is closest in size and mass to the Earth – used to be nestled inside a much larger version of itself. Corot 7-b was formerly a gas giant with a mass of 100 Earths, which is about that of Saturn. Its mass now: 4.8 times that of our planet.

How this rocky, lava-covered world got to its current state was presented at the American Astronomical Society’s meeting last week in Washington, DC by Brian Jackson of NASA’s Goddard Space Flight Center. Corot 7-b was discovered in February of 2009 by the ESA’s planet-hunting satellite, Convection, Rotation and planetary Transits(CoRoT), and has since been the subject of intense study.

The planet is about 1.7 larger in diameter than the Earth, and a little shy of five times as massive. Its star is about 1.5 billion years old, a third that of our Sun. It orbits very close to its star, which is much like our own Sun, only taking 20.4 hours to circle the star. The system lies in the constellation Monoceros, and is about 480 light-years away.

This tight orbit makes the planet extremely hot, as in 3,600 degrees Fahrenheit (1,982 degrees Celsius). That’s hot enough that the crust of the planet facing the star is an ocean of lava. Since Corot 7-b is tidally locked to its star, only one side of the planet faces the star at all times (just like we only see one side of the Moon from the Earth). On the opposite side of Corot 7-b from its star, the surface temperature is estimated to be a chilly negative 350 degrees F (negative 210 degrees C).

It rains on Corot 7-b just like it does here, though you wouldn’t want to be caught out in it. The rain on Corot 7-b is made of rock, so even the heaviest umbrella wouldn’t do much for you, and the very thin atmosphere is composed of rock vapor. In other words, we aren’t looking to Corot 7-b for signs of life. What we are looking there for is signs of planetary formation and evolution.

Jackson et al. modeled the orbit of the planet backwards, and showed that the star blew off much of the material that made up the planet in its previous incarnation as a gas giant. It previously orbited about 50 percent further out than it currently is. The stellar wind – a constant flow of charge particles from the star – interacted with the gassy atmosphere of the planet, blowing away the atmosphere.

“There’s a complex interplay between the mass the planet loses and its gravitational pull, which raises tides on the star,” Jackson said.

As it was pulled in closer to the star due to the process of tidal migration, more and more of the gas evaporated, and the orbital change of the planet slowed to the distance at which it currently orbits. Once the planet got closer to the star, it also heated up, and this heating process contributed to the mass loss of Corot 7-b. This evaporative process left only the rocky core of the planet.

“CoRoT-7b may be the first in a new class of planet — evaporated remnant cores. Studying the coupled processes of mass loss and migration may be crucial to unraveling the origins of the hundreds of hot, earthlike planets space missions like CoRoT and NASA’s Kepler will soon uncover,” Jackson said.

Many of the extrasolar planets discovered early on were gas giants that orbited close to their stars, so-called “hot Jupiters”. It’s possible that many of them will experience the same or similar fate as Corot 7-b, as we wrote about in an article last April.

Corot 7-b will likely lose more mass because of the proximity to its star, though not at the rate seen previously. What the next planetary matryoshka of Corot 7-b will look like is anyone’s guess. My prediction: turtles all the way down.

Source: NASA press release