Rocky Extra-solar Planets
The Distinction between Rocky and Gas-enveloped planets (Weiss & Marcy 2014)
Rocky surfaces are important for supporting life as we know it. Many planets are small, but which small planets are rocky? Our analysis of the masses and radii of 65 small exoplanets revealed that the rocky planets are smaller than 1.5 times the radius of Earth. This was an unexpected discovery that arose during our empirical determination of a one-to-one relationship between the masses and radii of small exoplanets (see figure, adapted from Weiss & Marcy 2014). We found that for planets larger than 1.5 Earth radii, the planet density decreases as planet radius increases, which is consistent with the planets having a gaseous envelope. However, for planets smaller than 1.5 Earth radii, planet density increases as planet size increases, in a manner consistent with the slight compression of rock. Our empirical relationship has been widely used to predict the masses of small planets both above and below the rock-to-gas transition and has guided NASA in its definition of potentially habitable planets.
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Jupiter-like Siblings to Earth-like Planets
Patterns in Planetary Systems
Planets Form in Patterns (Weiss et al. 2018a)
Planets in the same system tend to have similar sizes and regular orbital spacing. That is, a planet's size is very well predicted by the size of its nearest neighbor, and the orbital period ratio of a neighboring pair of planets is well predicted by the orbital period ratio of another pair of adjacent planets in the same system. The regular sizes and spacing of planets is a remnant of planet formation. New theories that reproduce the observed patterns in multi-planet systems will illuminate how a substantial fraction of planetary systems assembled.
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The Kepler Planets were likely Born in Multi-planet Systems (Weiss et al. 2018b)
Many of the Kepler planetary systems only have one transiting planet. Were these planets born solo, or do they have siblings?
We compared the host star properties of the systems with just one versus two or more transiting planets and found that the stellar masses, metallicities, and rotation velocities are indistinguishable between these two types of planetary architectures. Also, the planet radii from the two samples were indistinguishable*. The Kepler singles overwhelmingly have planet radii and host star properties that are indistinguishable from the properties of planets in multi-planet systems, and so it seems likely that these planets are indeed all formed in multi-planet systems. Whether the Kepler singles still have their siblings is a matter for future investigation!
Full article [link]
(*with the exception of hot Jupiters, which are known to be single, c.f. Steffen et al. (2012), Bryan et al. (2016)).
Individual planetary systems as planet formation laboratories
The present-day orbital and physical properties in individual systems open a window into the past, allowing us to test theories of planet formation. I have done several projects combining multiple datasets, including radial velocities (RVs) and transit timing variations (TTVs), to form comprehensive portraits of planet compositions and orbital configurations.
The multi-planet systems I have studied in detail include WASP-47 (top figure from Weiss et al. 2017), which has two Jupiter-mass planets and two sub-Neptunes and is the only known system in which a hot Jupiter has nearby small planets. I have also studied the Kepler-11 system, in which RVs confirmed the gas-rich compositions of the six transiting planets discovered through TTVs. In Kepler-10, a combined RV + TTV analysis revealed the likely presence of a third planet, in addition to the known rocky planet and sub-Neptune. Our use of a large RV dataset resulted in a revised composition estimate for the sub-Neptune sized planet (from "solid" to gas-enveloped).
TOI-561 is a multi-planet system containing a rocky planet with an orbital period of less than half a day. At an age of 10 billion years, the host star is one of the oldest stars in the Milky Way galaxy, making this one of the oldest rocky planets yet discovered (artistic rendition by Adam Makarenko / W. M. Keck Observatory).