Figure 1: The number of planets per star as a function of the planet-star mass ratio as discovered by Kepler. Dark and light blue curves are for stars less massive than the Sun, while the pink curve is for more massive stars. From Pascucci et al. (2018).

Among the mind-blowing discoveries of the Kepler Mission is that the most common type of planet in the galaxy is the super-Earth/sub-Neptune. This unexpected type of planet straddles the boundary between Earth and Neptune in size and mass, and so they may be super-sized rocky planets or anemic gas-rich planets. We just can’t tell.

Stranger still is that this bizarre chimera orbits one out of every four stars in the Milky Way, but we don’t seem to have one in our solar system (unless the Planet Nine hypothesis pans out).

If that wasn’t enough to make you feel like a planetary outsider, a recent study suggests most planet systems have very different architectures from our own.

In their paper, Prof. Ilaria Pascucci of the University of Arizona and her colleagues studied the distribution of masses for planets discovered by the Kepler Mission. Unlike other recent work, though, they considered not just the planet masses but how they compare to the masses of their parent stars.

Naively, we might expect that more massive stars grow more massive planets, and there is good evidence that more massive stars host more massive dust disks. So as long as the processes that make planets are happy to operate around most any kind of star, then we can use the distribution of planet-star mass ratios to disentangle the influence of stellar mass from other effects.

And Pascucci et al. found exactly what we expected – big stars, small stars, they are all more likely to host planets more massive than Earth — but only up to a point. As shown in Figure 1, the number of planets per star increases with the planet-star mass ratio until it reaches about 0.00003 (or 10 Earth masses for one solar mass) — somewhere between the Earth and Neptune. Above that ratio, the occurrence rate drops, meaning more massive planets are less and less common.

But something even more interesting emerged when Pascucci et al. compared their results based on the Kepler Mission to other surveys. Because Kepler is good at finding planets close to their host stars and not at finding planets farther away, Pascucci’s results don’t tell us much about planets in orbits like Jupiter’s. But microlensing surveys, which use a totally different planet-finding technique, *are* good at finding these more distant planets.

Figure 2: The number of planets around each star with a given planet-star mass ratio for planets close to their host star (as found by Kepler) and planets farther away (as found by microlensing surveys). From Pascucci et al. (2018).

Comparing mass ratios from the two kinds of surveys, Pascucci et al. showed more distant planets found by microlensing also show a preference, but for a ratio several times larger than what’s preferred by closer-in planets — somewhere between Neptune and Jupiter in mass, as shown by the figure above.

What does this all mean? Apparently, the universe seems to like make planets about ten thousand times less massive than their host stars, whether the stars are tiny red dwarfs or massive F-stars. And the farther away the planet is from its star, the more likely it is to be more massive.

Standard planet formation theory predicts this trend: protoplanetary disks (from which planets form) have more solid, icy material farther from the host star, and more solids means more planet.

But these results also make our solar system look even stranger than before. In most solar systems, orbits like Earth’s (and closer) are occupied by planets somewhere between Earth and Neptune in mass, and more distant orbits like Jupiter’s are occupied by planets between Neptune and Jupiter.

It’s tempting to speculate that things that make the solar system unique in one way (in this case, the type of planets we have) are related to other unique things (like, the fact that there’s life here). We don’t know much about super-Earths, but if they turn out to be more Neptune- than Earth-like, it’s not hard to imagine they wouldn’t be good places for life to get started.

Life as we know it needs a planet with a solid surface and lots of liquid water, and so maybe it’s not surprising we haven’t found life elsewhere in the galaxy yet: most of the planets are lifeless, gas balls.

With the very first discovery of an exoplanet around a Sun-like star (51 Peg b), astronomers were introduced to hot Jupiters. These totally unexpected planets resemble Jupiter in mass, composition, and size, but they have orbits that nearly skim the surfaces of their host stars. Some of them are even losing their atmospheres under the apocalyptic glare of their host stars.

How their lives began remains a mystery, but we have a pretty good idea of how their lives will end – they will be engulfed or torn apart by their host stars. That’s because hot Jupiters are big and close enough that they can actually raise a tidal bulge on the stars (and we can actually see the bulge in a handful of cases).

This tidal interaction can cause the planets to spiral downward toward the stars, and at the same time, it causes the spins to spin faster until the planet is destroyed by the star. The same tidal effect, just in reverse, is driving the Moon away from the Earth, while slowing down the Earth’s spin. But here’s the key: we don’t know how quickly the planets are spiraling in.

Tidal decay of planetary orbital period over billions of years (Gyrs). From Penev et al. (2018 –

Enter Prof. Kaloyan Penev of UT Dallas Physics Dept. On Valentine’s Day last week, he and his colleagues published an academic love note exploring planetary tidal decay. To do this, they modeled the evolution of planetary orbits and stellar spins under the influence of tides. The tracks in the figure at left show how a planet’s orbital period (or distance from its star) might shrink over billions of years, thanks to tides. The clump of spaghetti noodles in the figure shows that evolution for a range of assumptions about the rate of decay.

By comparing the stellar spin rate and planetary orbit predicted by their model to those we actually observe for each system, Penev and colleagues showed that the tidal decay rates might actually slow down as the planets approach their stars. So perhaps instead of an reckless death dive into the star over a few million years, the planets make like Zeno’s tortoise and tiptoe closer and closer without plunging in.

Upcoming surveys such as the TESS mission and the Large Synoptic Survey Telescope may soon allow us to test whether planets do or do not plunge into their stars. Theoretically, we expect stars that eat their planetary children dramatically brighten up by a factor of 10,000 over a few days – faster than a supernova brightens but nowhere near as bright. These surveys might able to see stars engaged in this act of cosmic infanticide.

Comparing the TRAPPIST-1 system to the solar system. From

In astronomy, when you look for evidence supporting a hypothesis and don’t find it, that’s called a “null result“. The null result is usually not all that exciting, but last week, an attempt to detect the atmospheres of potentially habitable exoplanets came up null, and that, as it turns out, may mean the planets are habitable.

In a recent study published last week in Nature Astronomy, Dr. Julien de Wit of MIT and colleagues observed the transits of four of the planets in the TRAPPIST-1 system. The discovery of this system was announced last year and generated a lot of interest — it comprises seven Earth-sized planets orbiting a red-dwarf star, and at least four of the planets orbit in the star’s habitable zone. So astronomers are scrambling to determine the climatic conditions on these planets and find out whether they host life.

Key to those conditions is the composition of the planets’ atmospheres, and the best way to probe atmospheres lightyears distant is to detect the colors of the planets’ shadows as they pass in front of their host stars, i.e. as they transit. When light from the star passes through a planet’s atmosphere, the cool gases can imprint spectral signatures, which we can then detect using a very sensitive telescope — de Wit used the Hubble Space Telescope.

Now, even though the TRAPPIST planets are Earth-sized, that doesn’t mean their atmospheres are Earth-like — astronomers have founds lots of weird planets in the last several years. The atmospheres could be hydrogen-rich like Jupiter, hydrocarbon-rich like Neptune, or rich in nitrogen and oxygen like Earth.

In principal, each kind of atmosphere would give a distinct spectrum, but in practice, atmospheres rich in hydrocarbons or nitrogen, potentially good atmospheres for life, are difficult to detect because they are weighty and drape over the planet like a heavy blanket. By contrast, a hydrogen-rich atmosphere, although probably not great for life, can be light and fluffy, relatively easy to detect.

When de Wit and colleagues analyzed transit data they collected from Hubble showing transits for planets TRAPPIST-1 d, e and f, they found no atmospheric signals down to their detection limits. The spectra below show this lack of atmospheric coloration and what they would have detected if the atmosphere was hydrogen-rich.

Now, of course, this non-detection does NOT mean the planets are habitable or even Earth-like. As the figure shows, their atmospheres could still be radically different from the Earth’s (drenched in water vapor or carbon dioxide-rich like Venus), but it rules out the possibility that they are Jupiter-like — potentially good news for life there.

Very likely, when the James Webb Space Telescope finally launches next year, the TRAPPIST-1 system will be one of its first targets. JWST’s vastly improved sensitivity will help reveal not just a potent null result for the TRAPPIST-1 system but may also reveal the glimmer of distant Earth-like worlds.

Spectra for TRAPPIST-1 d, e, f, and g. From de Wit et al. (2018) –

I’m a participant this academic year in the Boise State Teaching Scholars, a program to promote the scholarship of teaching and learning. As part of that program, participants are asked to give presentations on various topics, and I led a discussion last Friday on inclusive teaching. In case the presentation and the links provided are useful to anyone else, I’ve posted it below.

Total lunar eclipse on 2015 Sep 27. From wikipedia.

As a follow-on to our spectacular total solar eclipse last August, North America will be treated to a total lunar eclipse early on Wednesday (Jan 31) morning. The eclipse will start about 5:51am MST and will end just after 7a. No special equipment needed to observe the eclipse. If you can see the Moon, you’ll be able to see the eclipse.

A lunar eclipse occurs when the Moon passes through the Earth’s shadow (as opposed to a solar eclipse, when the Earth passes through the Moon’s shadow). Because red light from the Sun is refracted around the Earth by the atmosphere, during eclipse the Moon can appear murky red (similar to why sunsets appear red).

In fact, Wednesday’s eclipse is triply special — not only will the Moon pass through Earth’s shadow. It will also pass near perigee (the closest point in its orbit to the Earth), making the Moon appear especially large and bright in the sky (about 14% larger and 30% brighter). It will also be the second full moon in the month of January, making it an unofficial blue moon.

These three celestial events aren’t all that rare individually — lunar eclipses happen roughly every six months, supermoons happen every few months, and blue moons happen every two to three years. The confluence of all three is also not all that rare — the last time was in Dec 1982. So while Wednesday’s sky show may be impressive, it will not be the harbinger of doom claimed by some false prophets.

For Boise, though, another more mundane event may spell doom — we’re slated for clouds that morning, and no sky = no eclipse.

Enhanced color image of the thick bands of ice (blue) have been spotted in steep cliff faces. NASA/JPL/UNIVERSITY OF ARIZONA/USGS

At last week’s journal club, we discussed a recent paper that reports the discovery of ancient glaciers on Mars.

Dr. Colin Dundas of the USGS’s Astrogeology Group based in Flagstaff spotted these buried ice cliffs during his daily scan of the regularly collected images taken by the HiRISE camera onboard Mars Reconnaissance Orbiter (MRO) currently circling Mars. (The camera itself is pretty stunning – it produces orbital images of Mars at high enough resolution that you could almost read the headline on a martian newspaper, assuming they had newspapers.)

In scanning through the daily haul of images, Dundas spotted striking blue strata in the walls of steep cliffs just a few meters below the dusty martian surface that sure look like water ice. Follow-up spectral observations by CRISM instrument on MRO confirmed the cliffs were indeed almost completely pure water ice, with no more than with less than 1% dust.

Ice on Mars isn’t particularly surprising – astronomers have known (or at least suspected) there is water ice at the poles of Mars for more than 100 years, and a mountain of data has indicated vast stores of ice in Mars’ subsurface, especially near the poles. But key questions about this ice have persisted: Was the ice recently deposited, and how much dust is mixed in?

Since these newly discovered cliffs are so pure, though, Dundas and colleagues suggest that they were probably deposited as snow before being buried. Mars’ current climate isn’t really conducive to water snow, and so the ice was probably deposited millions of years ago, when Mars’ axis had a very different tilt¬†resulting a very different climate from now. The fact that the ice cliffs occur much nearer to the equator than might be expected also points to formation during a previous climatic epoch.

The implications of these cliffs for Mars’ climate history aren’t entirely clear, but their importance for exploration of Mars is hard to overstate. As Dundas et al. say in their paper, the cliffs would very likely serve as a resource for future human visitors. The water could be combined with gases in the martian atmosphere to make rocket propellent and even oxygen.

So there are large deposits of ice in the subsurface of Mars? Maybe “Total Recall” wasn’t so much science fiction as science prophesy.