The Hunt for Young Exoplanets

While there is a great deal of excitement and effort in the hopes of finding small, terrestrial sized exoplanets, another realm of exoplanet discovery that is often overlooked is that of ones of differing ages to explore how planetary systems can evolve. The first discovered exoplanet orbited a pulsar, showing that planets can be hardy enough to survive the potential violent deaths of their parent stars. On the other end, young planets can help astronomers constrain how planets form and a potential new discovery may help in those regards.

Historically, astronomers have often avoided looking at stars younger than about 100 million years. Their young nature tends to make them unruly. They are prone to flares and other eccentric behaviors that often make observations messy. Additionally, many young stars often retain debris disks or are still embedded in the nebula in which they formed which also obscures observations.

Despite this, some astronomers have begun developing targeted searches for young exoplanets. The age of the exoplanet is not independently derived, but instead, taken from the age of the host star. This too can be difficult to determine. For isolated stars, there are precious few methods (such as gyrochronology) and they generally have large errors associated with them. Thus, instead of looking for isolated stars, astronomers searching for young exoplanets have tended to focus on clusters which can be dated more easily using the main sequence turn off method.

Through this methodology, astronomers have searched clusters and other groups, such as Beta Pictoris which turned up a planet earlier this year. The Beta Pic moving group boasts an age of ~12 million years making it one of the youngest associations currently known.

Trumpler 37 (also known as IC 1396 and the Elephant Trunk Nebula) is one of the few clusters with an even younger age of 1-5 million years. This was one of several young clusters observed by a team of German astronomers led by Gracjan Maciejewski of Jena University. The group utilized an array of telescopes across the world to continuously monitor Trumpler 37 for several weeks. During that time, they discovered numerous flares and variable stars, as well as a star with a dip in its brightness that could be a planet.

The team cautions that the detection may not be a planet. Several objects can mimic planetary transit lightcurves such as “the central transit of a low-mass star in front of a large main-sequence star or red giant, grazing eclipses in systems consisting of two main-sequence stars and a contamination of a fainter eclipsing binary along the same line of sight.” Due to the physics of small objects, the size of brown dwarfs and many Jovian type planets are similar leading difficulty in distinguishing from the light curve alone. Spectroscopic results will have to be undertaken to confirm the object truly is a planet.

However, assuming it is, based on the size of the dip in brightness, the team predicts the planet is about twice the radius of Jupiter, and about 15 times the mass. If so, this would be in good agreement with models of planetary formation for the expected age. Ultimately, planets of such age will help test our understanding of how planets form, whether it be from a single gravitational collapse early on, or slow accretion over time.

Tight Binaries are ‘Death Stars’ for Planets

This plot of data from NASA's Spitzer Space Telescope tells astronomers that a dusty planetary smashup probably occurred around a pair of tight twin, or binary, stars. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA


Astronomers studying double star systems where the two stars are extremely close have found a pattern of destruction. While there probably isn’t a Star Wars-like Death Star roaming the Universe, tight binary systems might provide the equivalent of Darth Vader’s favorite weapon. “This is real-life science fiction,” said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. “Our data tell us that planets in these systems might not be so lucky — collisions could be common. It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Using the Spitzer Space Telescope, Drake and his team spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs, that might be the aftermath of tremendous planetary collisions.

Drake is the principal investigator of the research, published in the Aug.19 issue of the Astrophysical Journal Letters.

The particular class of binary stars in the study are extremely close together. Named RS Canum Venaticorums, or RS CVns for short, they are separated by only about 3.2-million kilometers (two-million miles ), or two percent of the distance between Earth and our sun. The binaries orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

These stars are familiarly like our own Sun – about the same size and probably about a billion to a few billion years old — roughly the age of our sun when life first evolved on Earth. But these stars spin much faster, and, as a result, have powerful magnetic fields, and giant, dark spots. The magnetic activity drives strong stellar winds — gale-force versions of the solar wind — that slow the stars down, pulling the twirling duos closer over time.

This is not a good scenario for planetary survival.

As the stars cozy up to each other, their gravitational influences change, and this could cause disturbances to planetary bodies orbiting around both stars. Comets and any planets that may exist in the systems would start jostling about and banging into each other, sometimes in powerful collisions. This includes planets that could theoretically be circling in the double stars’ habitable zone, a region where temperatures would allow liquid water to exist. Though no habitable planets have been discovered around any stars beyond our sun at this point in time, tight double-star systems are known to host planets; for example, one system not in the study, called HW Vir, has two gas-giant planets.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” said Marc Kuchner, a co-author from NASA Goddard Space Flight Center. “And it’s a future that’s messy and violent.”

The temperatures around these systems measured by Spitzer are about the same as molten lava. The astronomers says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something — most likely planetary collisions — must therefore be kicking up the fresh dust. In addition, because dusty disks have now been found around four, older binary systems, the scientists know that the observations are not a fluke. Something chaotic is very likely going on.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, lead author of the paper, also from Harvard-Smithsonian said, “The skies there would have two huge suns, like the ones above the planet Tatooine in ‘Star Wars.'”

The research was published in the Aug.19 issue of the Astrophysical Journal Letters.

Source: JPL

How Water Protected Our Molecules

One would think that crafting a shield out of water wouldn’t do much good (not in medieval combat re-enactments, anyways). But that’s precisely what the molecules in the early Solar System – some of the same ones that you are made out of today, perhaps – may have done. In their case, protection from broadswords wasn’t as much of a concern as the effects of ultraviolet radiation from the Sun.

UV light is pretty hard on molecules because it readily breaks them up into their constituent parts. Larger organic molecules that coalesced in the dusty disk out of which our planets formed billions of years ago would have been broken apart by the Sun’s rays, but calculations by two astronomers at the University of Michigan show that thousands of oceans worth of water present in a protoplanetary disk can shield other molecules from being broken up.

Edwin (Ted) Bergin and Thomas Bethell, both of the Department of Astronomy at the University of Michigan, calculated that in Sun-like systems the abundance of water early on can absorb much of the ultraviolet light from the central star. By shielding other molecules from being broken up, they continue to persist in the later stages of the disk’s development. In other words, these molecules hang around until the formation of planetesimals and planets, and this mechanism could have been guarded the constituents of life from the ravages of the Sun in our own Solar System.

Circumstellar disks modeled by Bergin and Bethell in their paper include DR Tau, AS 205A and AA Tau.

Bergin told Universe Today, “At present there have been upwards of 4 systems with water vapor observed.  All are consistent with our model. I understand that there are numerous other detections of water vapor by Spitzer but these have yet to be published. The water vapor that we see is continually replenished by high temperature chemistry in these systems, so you would not see any degradation.”

In systems like the Solar System, planets form out of a disk of dust and gas that surrounds the young star. This large, flat disk later solidifies into planets, comets and asteroids. Near the center of the disk, between 1 and 5 astronomical units, warm water vapor in the disk could “protect” molecules inside this layer from being broken apart by UV light.

H2O breaks down when exposed to UV light into hydrogen and hydroxide. The hydroxide can be further broken down into oxygen and hydrogen atoms. But water, unlike other molecules, reforms at a quick pace, replenishing the shield of water vapor.

Smaller dust grains within the disk capture some of the UV radiation in the early formation periods of a protoplanetary disk. Once these dust grains start to snowball into bigger pieces, though, the UV light filters through and breaks apart molecules in the inner portions of the disk, where planets are in their early stages of formation.

The previous model for how organic molecules persisted past this point suggested that comets from the outer portion of the disk somehow fall into the center, releasing water to absorb the harmful radiation. But this model didn’t explain the hydroxide measurements for the disks so far observed.

If enough water is present, which seems to be the case in a handful of disks observed by the Spitzer Space Telescope, these other molecules remain intact, and as a bonus the water present in the interior portions of the disk also sticks around.

Bergin told Universe Today, “There are other molecules that can shield themselves – CO and H2 – but these cannot shield other molecules as well (because they capture only a fraction of the spectrum of light). Water is the only one with a strong formation that can compensate for destruction. It then provides the full shielding for other species. It is unlikely that another molecule will do this.”

This mechanism would only protect water vapor and other molecules in the inner part of the disk, closest to the star.

“This will likely be active in the inner few AU — at some point say between 5-10 AU it will become inactive and things will be inhospitable for various species [of molecule],” Bergin said.

So, where does all of the water go once the planets form? The vapor closest to the star – within about 1 AU – eventually gets broken down by the starlight into hydrogen and oxygen. At about 3 AU from the star, the water could constitute part of the planets and asteroids that form in that region. It may have been such asteroids that carried water to the surface of the Earth during its early formation, filling up our oceans. Outside of this region, H2O is broken down into hydrogen and oxygen and blown into space, said Bergin.

When asked whether this protective shield of water was present in our own Solar System, Bergin answered, “When we say that there were thousands of oceans of water vapor in the habitable zone, we mean around Sun-like stars.  Presumably this was present around our Sun as well.”

Source: Physorg, Science, email interview with Ted Bergin

Spitzer Watches Planet-Forming Disk Change Quickly

This artist's conception shows a lump of material in a swirling, planet-forming disk. Image credit: NASA/JPL-Caltech

Something strange is going on around a young star called LRLL 31. Astronomers have witnessed a swirling disk of gas and dust which is changing rather quickly; sometimes weekly. This is likely a planet forming disk, however, planets take millions of years to form, so it’s rare to see anything change on time scales we humans can perceive. Another object appears to be pushing a clump of planet-forming material around the star, and this region is offering astronomers with the Spitzer Space Telescope a rare look into the early stages of planet formation.

Astronomer are seeing the light from this disk varying quite frequently. One possible explanation is that a close companion to the star — either a star or a developing planet — could be shoving planet-forming material together, causing its thickness to vary as it spins around the star.

“We don’t know if planets have formed, or will form, but we are gaining a better understanding of the properties and dynamics of the fine dust that could either become, or indirectly shape, a planet,” said James Muzerolle of the Space Telescope Science Institute, Baltimore, Md. Muzerolle is first author of a paper accepted for publication in the Astrophysical Journal Letters. “This is a unique, real-time glimpse into the lengthy process of building planets.”

One theory of planet formation suggests that planets start out as dusty grains swirling around a star in a disk. They slowly bulk up in size, collecting more and more mass like sticky snow. As the planets get bigger and bigger, they carve out gaps in the dust, until a so-called transitional disk takes shape with a large doughnut-like hole at its center. Over time, this disk fades and a new type of disk emerges, made up of debris from collisions between planets, asteroids and comets. Ultimately, a more settled, mature solar system like our own forms.

Before Spitzer was launched in 2003, only a few transitional disks with gaps or holes were known. With Spitzer’s improved infrared vision, dozens have now been found. The space telescope sensed the warm glow of the disks and indirectly mapped out their structures.

Muzerolle and his team set out to study a family of young stars, many with known transitional disks. The stars are about two to three million years old and about 1,000 light-years away, in the IC 348 star-forming region of the constellation Perseus. A few of the stars showed surprising hints of variations. The astronomers followed up on one, LRLL 31, studying the star over five months with all three of Spitzer’s instruments.

The observations showed that light from the inner region of the star’s disk changes every few weeks, and, in one instance, in only one week. “Transition disks are rare enough, so to see one with this type of variability is really exciting,” said co-author Kevin Flaherty of the University of Arizona, Tucson.

Both the intensity and the wavelength of infrared light varied over time. For instance, when the amount of light seen at shorter wavelengths went up, the brightness at longer wavelengths went down, and vice versa.

Muzerolle and his team say that a companion to the star, circling in a gap in the system’s disk, could explain the data. “A companion in the gap of an almost edge-on disk would periodically change the height of the inner disk rim as it circles around the star: a higher rim would emit more light at shorter wavelengths because it is larger and hot, but at the same time, the high rim would shadow the cool material of the outer disk, causing a decrease in the longer-wavelength light. A low rim would do the opposite. This is exactly what we observe in our data,” said Elise Furlan, a co-author from NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

The companion would have to be close in order to move the material around so fast — about one-tenth the distance between Earth and the sun.

The astronomers plan to follow up with ground-based telescopes to see if a companion is tugging on the star hard enough to be perceived. Spitzer will also observe the system again in its “warm” mission to see if the changes are periodic, as would be expected with an orbiting companion. Spitzer ran out of coolant in May of this year, and is now operating at a slightly warmer temperature with two infrared channels still functioning.

“For astronomers, watching anything in real-time is exciting,” said Muzerolle. “It’s like we’re biologists getting to watch cells grow in a petri dish, only our specimen is light-years away.”

Source: JPL