Which of the following planets formed within the snow line

Astronomers have, for the first time, directly imaged a snow line at another star. Using the new Atacama Larger Millimeter/submillimeter Array (ALMA) telescope in Chile, they obtained radio-wavelength images of the carbon monoxide snow line around TW Hydrae, a young star 175 light-years away from Earth. TW Hydrae, in the constellation Hydra, is believed to be our closest infant solar system.

“We’ve had evidence of snow lines in our own solar system, but now we’re able to see one with our own eyes. That is exciting,” said Edwin Bergin, professor of astronomy in the University of Michigan College of Literature, Science and the Arts. Bergin is co-author on a paper on the results published in Science Express on July 18.

Different chemical compounds freeze at different distances from a central star. In our own solar system, water freezes at about five times the distance from the Earth to the sun, between the orbits of Mars and Jupiter. Various chemical compounds’ snow lines may be linked to the formation of specific kinds of planets. The carbon monoxide line in our system corresponds to the orbit of Neptune, and it could also mark the starting point where smaller icy bodies like comets and dwarf planets like Pluto would form, according to the National Radio Astronomy Observatory.

Until now, snow lines have only been detected by their spectral signature. They have never been imaged directly, so their precise location and extent could not be determined.

“ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of our own solar system,” said co-author Chunhua “Charlie” Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old.”

Snow lines have been difficult to image because they only form in the relatively narrow central plane of a planet-forming disk. Above and below this region, radiation from the central star keeps the gases warm.

An outer cocoon of hot gas prevents astronomers from peering inside the disk where the gas is frozen. Instead, they hunted for a different molecule called diazenylium. Carbon monoxide gas destroys diazenylium, so it is only detectable in regions where the gas is frozen. It shines brightly in the millimeter portion of the electromagnetic spectrum, which can be detected by radio telescopes like ALMA.

By tracing the distribution of diazenylium, astronomers identified a boundary approximately 30 astronomical units from TW Hydrae. An astronomical unit is the average distance between the Earth and the sun.

“Using this technique, we were able to create, in effect, a photonegative of the carbon monoxide snow in the disk surrounding TW Hydrae,” said Karin Öberg, also with Harvard but who was with the University of Virginia at the time of the observation. “With this, we could see the snow line precisely where theory predicts it should be—the inner rim of the diazenylium ring.”

Öberg also points out that this snow line is particularly interesting since carbon monoxide ice is needed to form methanol, which is a building block of more complex organic molecules essential for life. Comets and asteroids could then ferry these molecules to newly forming Earth-like planets, seeding them with the ingredients for life.

ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA construction and operations are led on behalf of Europe by the European Southern Observatory, on behalf of North America by the National Radio Astronomy Observatory and on behalf of East Asia by the National Astronomical Observatory of Japan. The Joint ALMA Observatory provides the unified leadership and management of the construction, commissioning and operation of ALMA.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc.

It’s 9000 times easier to find a ‘hot Neptune’ than a Neptune out around the ‘snow line,’ that region marking the distance at which conditions are cold enough for ice grains to form in a solar system. Thus says David Kipping (Harvard-Smithsonian Center for Astrophysics), who is lead author on the paper announcing the discovery of Kepler-421b, an interesting world about which Kipping has been sending out provocative tweets this past week. Kepler-421b draws the eye because its year is 704 days, making it the longest orbital period transiting planet yet found. The intriguing new world is located about 1000 light years from Earth in the direction of the constellation Lyra.

The transit method works by detecting the characteristic drop in brightness as a planet moves across the face of the star as seen from Earth. What’s unusual here is that Kepler-421b moved across its star only twice in the four years that the Kepler space telescope monitored it. As Kipping explains on this CfA web page, the further a planet is from its host star, the lower the probability that it will pass in front of the star as seen from Earth. Kepler-421b should have had, by Kipping’s calculations, a tiny 0.3% chance of being observed in a transit. We can be happy for the discovery while also considering how tricky it will be to find worlds like it by transit methods.

Image: Transit light curve of Kepler-421b. Blue and red points denote the two different transit epochs observed, offset in time by 704 days. Credit: David Kipping et al.

Also known as the ‘frost line,’ the snow line in our own Solar System is the divider between the rocky inner planets all the way out to Mars, and the outer gas giants. The kind of planet you get depends in part on whether, during the early period of planet formation, the emerging planet is inside or outside the snow line. According to our current formation models, gas giants form beyond the snow line, where the temperatures are low enough that water condenses into ice grains. The planetary embryos that become the gas giants should have abundant ice grains sticking together to create worlds rich in ice and water compared to the inner system.

That has major implications, of course, because we have discovered a large number of ‘hot Jupiters’ and Neptune analogues that orbit far inside the snow line in their respective systems. That makes for migration scenarios where gas giants forming in the outer system move inward as the result of likely gravitational encounters with other worlds. Kepler-421b, however, orbits its K-class primary at a distance of about 177 million kilometers, a gas giant that may never have migrated, and the first example of such ever found using the transit method.

The snow line moves inward over time as the young planetary system evolves, and Kipping and team’s calculations show that when this system was about three million years old, early in the era of planet formation, its snow line should have been at about the same distance as Kepler-421b’s present location. The planet is roughly the size of Uranus, about four times the size of Earth, which may be an indication that it formed late in the planet formation era, at a time when not enough material was left in the system to allow it to become as large as Jupiter.

But is Kepler-421b truly an ice giant or could it actually be a large, rocky world? The evidence strongly favors the former. From the paper (internal citations deleted for brevity):

Although calculating detailed formation scenarios for Kepler-421b is outside the scope of this work, simple arguments suggest Kepler-421b is an icy planet which formed at or beyond the snow line. With a radius of roughly 4 R⊕ and a mass density of at least 5 g cm-3, a rocky Kepler-421b has a mass of at least 60 M⊕. Growing such a massive planet requires a massive protostellar disk with most of the solid material at 1-2 AU. Among protoplanetary disks in nearby star-forming regions, such massive disks are rare. Thus, a rocky Kepler-421b seems unlikely.

And as to the place of formation:

For Kepler-421b, in situ formation is a reasonable alternative to formation and migration from larger semi-major axes. Scaling results from published calculations, the time scale to produce a 10-20 M⊕ planet is comparable to or larger than the median lifetime of the protoplanetary disk. Thus formation from icy planetesimals is very likely. If significant migration through the gas and leftover planetesimals can be avoided, Kepler-421b remains close to the ‘feeding zone’ in which it formed.

To place the planet in context, consider that Mars orbits the Sun every 780 days, as compared to Kepler-421b’s 704 day orbit (around, as mentioned above, a K-class star that would be cooler and dimmer than the Sun). The researchers’ calculations indicate a temperature of about -135 Fahrenheit (180 K). At least one recent paper, cited by Kipping and colleagues, suggests that planets near the threshold of the snow line may be common, but finding them by transit methods will be difficult because of the low transit probability. As for radial velocity detection, the planet poses what the paper calls “a significant challenge to current observational facilities,” but determining the mass of worlds like this could help us understand the relationship between mass and radius as we move further from the parent star.

The paper is Kipping et al., “Discovery of a Transiting Planet Near the Snow-Line,” accepted by The Astrophysical Journal and available as a preprint online.