close-in exoplanets

All posts tagged close-in exoplanets

The TESS Spacecraft.

At our journal club last week, we discussed the discovery of a new warm Jupiter from the TESS Mission.

TESS is the successor to the wildly successful Kepler/K2 Mission and is designed to find exoplanets using the same technique as Kepler – looking for their shadows as planets pass in front of their host stars, i.e. the transit technique.

Sadly, the Kepler spacecraft was officially shut down two weeks ag0 because it ran out of fuel, but TESS, launched last March, is off and running, having already discovered about half a dozen new planets.

One of those planets, we discussed in journal club on Friday – a planet orbiting the star HD1397. The gas giant planet is about the same size as Jupiter but half the mass, making it significantly less dense than Saturn.

The planet also has an unusually eccentric or stretched-out orbit that swings very near its host star, passing to within 8 stellar radii from its star at its closest point. By contrast, the Earth is 200 stellar radii away from the Sun.

If this planet had been discovered 20 years ago, it would have completely stumped astrophysicists, and many would likely have doubted its existence. Nowadays, though, such strange planets are practically the norm in exoplanet astronomy.

So with HD1397 b’s discovery, the exoplanet train rumbles on, and we should expect thousands upon thousands more bizzarities from TESS that will, like Kepler’s discoveries, again re-write the planetary rulebook.


At our research group meeting, we also discussed the art of scientific presentations. I’ve pasted the example presentation I gave below.

Artist’s impression of planet alignment in 2016. From here.

Anyone who’s done some stargazing has probably noticed that the Sun and the Moon appear along nearly the same arc in the sky. This Sun’s arc, called the ecliptic, corresponds to the plane of the Earth’s orbit. Since all planets in the solar system share nearly the same orbital plane, they likewise hew close to this arc. It turns out that the ecliptic also coincides closely with the Sun’s equator.

The near alignment of all planetary orbits in the solar system is one of the most important clues to their formation – the solar system originated billions of years ago from a thin disk of gas and dust girding the young Sun’s belly like a hula hoop, an idea going back at least to Immanuel Kant in the 1700s called the Nebular Hypothesis.

Once it was accepted, this idea was so successful at explaining and predicting features of the solar system, astronomers believed all planetary systems in our galaxy would resemble our own – with small, rocky planets close to their stars and large, gassy planets farther away, but all sharing the same orbital plane.

The discoveries of thousands of exoplanets have turned all that on its head – planets around other stars have orbits oriented every which way. For example, the Upsilon Andromeda system has three Jupiter-like planets, all on orbits that are widely misaligned.

Although these topsy-turvy planetary orbits were initially puzzling, astronomers are starting to tease out the explanations for these systems. Planets probably do start out in well-aligned orbits, but, like kids in the backseat on a long car trip, jostling between the planets (due to mutual gravitational tugs) soon upsets this delicate arrangement and upends the orbits. In the case of Upsilon Andromeda, planets may even have been ejected from the system.

A recent study from Fei Dai and colleagues explored connections between orbital misalignment and the origins of one puzzling class of exoplanet – small, short-period planets. These planets range in size (and probably composition) from Neptune-like to smaller than Earth but inhabit orbits very close to their host stars, some taking only hours to circle the star. Many of these short-period planets also have sibling planets farther out, and the arrangement of these orbits might tell us how the planets got so close to their stars.

As for the Upsilon Andromeda system, the mutual inclination between the orbits, if its big, may point to a history of violence in the system. Such violence may explain how the short-period planets got so close to their stars – they could have started out far away and been thrown by their siblings toward the star. By contrast, a small mutual inclination could mean the system has always been relatively quiescent, and the short-period planets may have gently migrated inward from farther out.

By analyzing the transit light curves of the planets as observed by the Kepler spacecraft, Dai and colleagues found a pattern in the mutual inclinations for these systems. From their paper, the figure below shows that when the distance of the shortest-period planet in a system a/R* is larger, the mutual inclination ΔI between orbits tends to be less widely distributed.

Figure 3 from Dai et al. (2018).

What does this result mean? Since the short-period planets closest to their stars (small a/R*) also seem to have a very wide range of mutual inclinations, maybe they experience the same kind of gravitational jostling that took place in Upsilon Andromeda, while planets farther out, they were moved in more gracefully.

Taking a wider perspective, evidence is mounting that, while planetary systems are common in the galaxy, our own solar system is unique in many ways – there’s really no place like home.

This artist’s impression shows the super-Earth exoplanet orbiting the nearby star GJ 1214.

Eve Lee and Eugene Chiang of Berkeley Astronomy just posted a very interesting paper about the origins of super-Earths in ultra-short-period orbits.

The topic I’ve been interested in most in recent years is the origin and fate of these ultra-short-period planets. These little guys orbit very close to their host stars, taking, in some cases, only a few hours to circle their host stars. In other words, the year for some of these planets is shorter than a feature-length movie.

Such planets were completely unexpected before astronomers began discovering them, and it’s not at all clear where they came from – naïvely, we’d expect that they can’t form where we find them. And many of them are so small (less massive than the Earth in some cases) that tidal interactions, which can cause bigger planets to death spiral into their stars, probably don’t have much effect.

In their paper, Lee and Chiang explored the origins of short-period super-Earths, planets somewhat, but not much, bigger than Earth. This population declines the closer you get to the host star – there are more super-Earths with periods of several days (short-periods) than of several hours (ultra-short-periods), which probably tells us something about the planets’ origins.

It was thought that such planets might originate via gas disk migration. This is the gravitational give-and-take between a nascent planet and the maternal gas disk from whence it arises.

Lee and Chiang found that, surprisingly, this migration on its own would not have made enough ultra-short-period planets but too many short-period planets. Next, they tried to include tidal interactions, which made enough ultra-short-period planets but too many short-period planets.

Instead, Lee and Chiang found that they could explain the short-period super-Earths if they assumed the planets formed near where we find them (and included a little tidal migration). That’s a little surprising since the standard model of planet formation posits that the grains of dust and ice that eventually coalesce to form planets cannot exist within a few days of their host star.

So, if Lee and Chiang are right, these super-Earths, instead of growing up from tiny grains, may have grown from the collisions of 1000-km planetesimals that themselves migrated close to the host star. In this case, the origins of short-period super-Earths may have been particularly violent.

The radii Rp and periods P of KELT-16 b, along with lots of other short-period planets. From Oberst et al. (2017).

There’s a new paper from the KELT Survey and led by Prof. Thomas Oberst from Westminster College announcing the discovery of another exoplanet in a very small orbit, nearly skimming the surface of its host star: KELT-16 b is a highly irradiated, ultra-short period hot Jupiter nearing tidal disruption.

These planets have been something of a puzzle since the first was discovered back in 1995. Like Jupiter, they are mostly made out of hydrogen and helium gas, but unlike Jupiter, they orbit very close to their host star, which probably means they didn’t form where we see them today.

Even among hot Jupiters, though, KELT-16 b is an outlier. It’s one of a handful of hot Jupiters with orbital periods less than 1 day (as compared to Jupiter’s orbital period of 12 years), so whatever processes led to its origin are cranked up to 11 for KELT-16 b and its ultra-short period siblings.

The mystery of its origins aside, its short period means KELT-16 b is probably a good candidate for follow-up observations of its atmosphere, particularly by the James Webb Space Telescope. But tidal interactions with its host star means it may get eaten by its host star in less than a million years, so we need to get those observing proposals submitted soon.