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Transiting World at the Snow Line

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.


Comments on this entry are closed.

  • Astrocartician July 23, 2014, 12:50
  • FrankH July 23, 2014, 14:45

    I don’t think there’s enough data on this system (just two transits of the Neptune class planet) to make any far reaching statements about planetary formation other than “huh, gas giant just about at the frost line. Cool cool cool”.

    We don’t know if there are other gas giants further out that could have caused this planet to migrate inward. With just two transits (even if they’re deep) I’m not sure you can call this a confirmed planet – at least not without followup observations. It could be a giant starspot on a slowly rotating K dwarf.

  • Michael July 23, 2014, 15:15

    At around a 20 hour transit and a curve like that it favours a transit not a slowly revolving sun spot which would hang around there for weeks.

  • Tom Mazanec July 23, 2014, 15:35

    Well, if its a gas giant we can’t use radial velocity to study it, but perhaps we can at least rule out the “unlikely” model of a rocky planet.

  • andy July 23, 2014, 17:34

    Bearing in mind the caveats about the reliability of 2-transit detections, this does look like something of a bridge to the population of planets discovered by microlensing.

    Incidentally, depending on whether the light variations observed in 1981 were actually a transit, Beta Pictoris b might be the longest period transiting planet known, though this still awaits confirmation. It’s something of an indication of the changing emphasis within astronomy that the reaction in 1981 to the light variations was basically something along the lines of: well it looks like Beta Pictoris is not such a good standard star, let’s stop observing it so much.

  • Harry R Ray July 24, 2014, 9:34

    Two interesting points about this discovery. Fist, minus 135F is at the VERY outermost edge of the “OPTIMISTIC” habitable zone , but,in reality, Kepler 421b could only BE poentially habiable IF it were an “OCEAN world” with a rocky core with a mass of 50 earths,an ocean of 9earth mass, and a hydrogen envelope with a pressure of NO GREATER THAN the pressure at the bottom of the Challenger Deep( a PURE ROCKY WORLD would probably be TOO VOLCANIC to permit life, but a large INSULATING LAYRE of ice7 would prevent a RUNNAWAY interaction with the volcanoes and the liquid ocean on my above proposed planetary scenario). Second, to aleviate the “only two transits” issue, athird transit MUST BE OBSERVED! This can either be done with: Spitzer, if the transit occurs before October, Hubble STIS, or JWST( it MAY be bossible to observe on the ground at a 3sigma confidence level because the star is slightly smaller than the sun).

  • ljk July 24, 2014, 11:04

    July 24, 2014

    RELEASE 14-197

    Hubble Finds Three Surprisingly Dry Exoplanets

    Astronomers using NASA’s Hubble Space Telescope have gone looking for water vapor in the atmospheres of three planets orbiting stars similar to the sun — and have come up nearly dry.

    The three planets, known as HD 189733b, HD 209458b, and WASP-12b, are between 60 and 900 light-years away from Earth and were thought to be ideal candidates for detecting water vapor in their atmospheres because of their high temperatures where water turns into a measurable vapor.
    These so-called “hot Jupiters” are so close to their star they have temperatures between 1,500 and 4,000 degrees Fahrenheit, however, the planets were found to have only one-tenth to one one-thousandth the amount of water predicted by standard planet-formation theories.

    “Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside our solar system, and we can now say with much greater certainty than ever before that we’ve found water in an exoplanet,” said Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, England. “However, the low water abundance we have found so far is quite astonishing.”

    Madhusudhan, who led the research, said that this finding presents a major challenge to exoplanet theory. “It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially “hot Jupiters,” and investigate how they’re formed.”

    He emphasizes that these results may have major implications in the search for water in potentially habitable Earth-sized exoplanets. Instruments on future space telescopes may need to be designed with a higher sensitivity if target planets are drier than predicted. “We should be prepared for much lower water abundances than predicted when looking at super-Earths (rocky planets that are several times the mass of Earth),” Madhusudhan said.

    Full article here:


    To quote:

    “There are so many things we still don’t know about exoplanets, so this opens up a new chapter in understanding how planets and solar systems form,” said Drake Deming of the University of Maryland, who led one of the precursor studies. “The problem is that we are assuming the water to be as abundant as in our own solar system. What our study has shown is that water features could be a lot weaker than our expectations.”

    The findings are published July 24 in The Astrophysical Journal Letters.

  • ljk July 24, 2014, 11:08

    Exoplanets: Most accurate measurement yet of an alien world’s size

    Pasadena : CA : USA | Jul 24, 2014 at 1:55 AM PDT

    BY Robert Myles

    Aided by two NASA telescopes, astronomers have made the most precise calculation yet of the size of an exoplanet — a planet outwith our solar system.

    The measurement of the radius of planet, Kepler-93b, a so-called super-Earth orbiting a star 300 light years distant was calculated using NASA’s Kepler and Spitzer Space Telescopes. Scientists reckon they’ve pinned down the planet’s size with an uncertainty factor of just 74 miles (119 kilometers) on either side of the planetary body.

    The measurement of a planet lying roughly 60 times further away from us than our closest star system, Alpha Centauri, illustrates just how far astronomical telescopes have developed in less than 100 years.
    It was only in 1930 that astronomers first observed the ‘last planet’ of our solar system, Pluto. Since then, as telescopes have developed, Pluto has been demoted to the status of a dwarf planet — a “plutoid” — after it was found to be just one of several large icy bodies in the outer solar system.

    The measurement of Kepler-93b confirms it as a super-Earth sized body about 50 percent as big again as the Earth. As more and more exoplanets are catalogued, it’s become clear that super-Earths are common in our galaxy. In our own solar system, however, there are no super-Earths with planets being divided into two categories: small rocky worlds like Earth, Mars, Venus and Mercury and gas giants comprising Jupiter, Saturn, Uranus and Neptune.

    Exoplanets like Kepler-93b, therefore, are useful “laboratories” to study this major class of planet. Armed with more refined knowledge of the size and mass of such super-Earths, it’s possible for scientists to theorize about the make-up of these alien worlds.

    Full article here:


    To quote:

    “With Kepler and Spitzer, we’ve captured the most precise measurement to date of an alien planet’s size, which is critical for understanding these far-off worlds,” commented Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of the paper on the Kepler-93b findings.

    Putting this feat of measurement in perspective, Ballard added, “The measurement is so precise that it’s literally like being able to measure the height of a six-foot tall person to within three quarters of an inch — if that person were standing on Jupiter.”

  • ljk July 28, 2014, 10:44

    Astronomers Discover First Ice Giant Exoplanet Candidate In Long-Period Orbit

    By Leonidas Papadopoulos

    With new exoplanet discoveries announced almost at a monthly basis, it is no surprise that only those that involve potentially Earth-like, habitable worlds mostly manage to grab the headlines. Yet, as exoplanetary research has shown, even the ones that do not fit that bill are fascinating in their own right, offering a great insight into the processes that drive planetary formation and evolution. Such is the case with the discovery of the first Uranus-sized exoplanet candidate in a stable long-period orbit that was announced earlier this week, which could be similar to the ice giants of our own Solar System.

    Contrary to most of the exoplanetary systems that have been discovered to date, our Solar System can be described as a tidy and well-organized place, with all the eight major planets neatly grouped together in two different and distinct categories: four small rocky, terrestrial planets orbiting close to the Sun, are followed by four Jovians, or gas giants that are lying further out, with a large asteroid belt in between separating these two planetary groups.

    This assortment is not a product of chance however, but of physics and chemistry. In the inner part of the protoplanetary disk of material that gave rise to the planets, temperatures were such that all the volatile elements like water, ammonia, methane, hydrogen, nitrogen etc, boiled off due to the Sun’s intense heat, leaving behind only the microscopic solid particles of rock and metal that later clumped together to form the terrestrial planets. Further out in the protoplanetary disk beyond a certain point which is known as the frost or ‘snow line’, temperatures were low enough for all the volatile compounds to condense into solid ice grains, forming the building blocks from which the gas giant planets later emerged.

    Full article here:



    The discovery of Kepler-421b is a significant one, not only because it represents the first exoplanet in a long-period orbit ever to be discovered with the transit method, but also because of its similarity to the ice giants in our own Solar System. “We discovered the planet by detecting the tell-tale decrease in brightness of the parent star as Kepler-421b passed in front – a “transit” event”, explains Kipping at his personal webpage. “The amount of light blocked by the planet betrays her size and we find that Kepler-421b is nearly the same size as Uranus, which is about four times larger than the Earth. Using a special technique I developed called ‘Asterodensity Profiling’, we can also exploit the transit light curve shape to measure the orbital eccentricity of Kepler-421b, which works out to be 0.04, almost exactly the same as Uranus as well”. Furthermore, Kepler-421b’s distance from its host star was found to be approximately 110 million miles, or 1.2 Astronomical Units. Since orange dwarf stars like Kepler-421 are approximately 30 percent smaller and less bright than the Sun, that means that at its current distance, Kepler-421 orbits outside of its planetary system’s ‘frost line’ – the boundary beyond which ice grains condense to become solid, leading to the formation of gas and ice giant planets. “With a semi-major axis of 1.22 AU, Kepler-421b orbits closer to its parent star than the orbit of Mars (1.52 AU) around the Sun”, writes Kipping’s team. “Despite this smaller orbit, the lower luminosity of Kepler-421 causes Kepler-421b to receive just 64% of the insolation received by Mars. Comparing the incident insolation to the habitable-zone boundaries of [previous studies], Kepler-421b lies firmly outside the maximum greenhouse outer edge…Thus, Kepler-421b has an insolation roughly midway between the insolations of Mars and the asteroid Vesta”. Under these conditions, Kipping’s team also estimated the possible temperature on Kepler-421b to be a chilling -135 degrees Fahrenheit (-93 degrees Celsius).