Kinds of Exoplanets

[lpetrich]

I was looking for stuff in arxiv.org about desert exoplanets, because I was hoping to find something on what a low-water-content Earth-sized planet might be like. But I found something different.

• [1602.07843] Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes -- there is a gap between high-mass and low-mass planets that gets greater and greater for a shorter and shorter period.
• [1604.05220] Hot super-Earths stripped by their host stars -- no very close super-Earths known
• MASS-LOSS EVOLUTION OF CLOSE-IN EXOPLANETS: EVAPORATION OF HOT JUPITERS AND THE EFFECT ON POPULATION - IOPscience
• Extreme radiation can strip the atmospheres from exoplanets | Astronomy.com -- "The runaway effect of mass loss results in a dichotomy of populations: hot Jupiters that retain their envelopes and super Earths whose envelopes are completely lost."
• Hot Earths may be stripped-down Jupiters | Astronomy.com
• Young star appears to be stripping away layers of close-orbiting ‘hot Jupiter’ – Astronomy Now -- [1606.02701] H-alpha Variability in PTFO8-8695 and the Possible Direct Detection of Emission from a 2 Million Year Old Evaporating Hot Jupiter

So this dearth of in-between planets is likely from their going through that phase relatively fast as they get stripped by their stars -- they don't have much mass left to strip, and even with a lower cross-sectional area, that mass will go relatively fast.

'Hot Jupiter' moons unlikely to exist › News in Science (ABC Science) -- because of orbital instability. I have decided to address this issue empirically, by looking at the farthest known moons of the Solar System's planets. For planet orbit period Pp and moon orbit period Pm, I found:

• Direct: Pp/Pm >~ 9 (Jupiter's moon LXII Valetudo)
• Retrograde: Pp/Pm ~ 5 (Jupiter's moon XIX Megaclite)

If Jupiter had the period of 51 Pegasi b, about 4.3 days, then it could only hold on to its 2 or 3 or maybe 4 innermost moons, and certainly not its four big moons.

I chose 51 Pegasi b because it is the first exoplanet of an ordinary sort of a star that was reliably discovered.

 

[lpetrich]

[1806.11234] Survival Function Analysis of Planet Size Distribution with GAIA Data Release 2 Updates

Applying the survival function analysis to the planet radius distribution of the Kepler confirmed/candidate planets, we have identified two natural divisions of planet radius at 4 Earth radii and 10 Earth radii. These divisions place constraints on planet formation and interior structure model. The division at 4 Earth radii separates small exoplanets from large exoplanets above. When combined with the recently-discovered radius gap at 2 Earth radii, it supports the treatment of planets 2-4 Earth radii as a separate group, likely water worlds. For planets around solar-type FGK main-sequence stars, we argue that 2 Earth radii is the separation between water-poor and water-rich planets, and 4 Earth radii is the separation between gas-poor and gas-rich planets. We confirm that the slope of survival function in between 4 and 10 Earth radii to be shallower compared to either ends, indicating a relative paucity of planets in between 4-10 Earth radii, namely, the sub-Saturnian desert there. We name them transitional planets, as they form a bridge between the gas-poor small planets and gas giants. Accordingly, we propose the following classification scheme: (<2 Earth radii) rocky planets, (2-4 Earth radii) water worlds, (4-10 Earth radii) transitional planets, and (>10 Earth radii) gas giants.

GAIA is an astrometric satellite, one for precision measurement of stars' positions. So far, it has made measurements of over a billion stars, measurements that have enabled measuring distances to them with parallaxes. These stars include many of the Kepler-planet stars, and these distances enable improved size estimates for those stars, and thus for their planets.

As to what a survival function is, I will quote the paper:

Definition of Survival Function:
SF (Survival Function) = 1 - CDF (Cumulative Distribution Function) = 1 - Integral of PDF (Probability Density Function)
Differentiate SF, one gets the PDF (Probability Density Function).

Here, it is the number of planets greater than some radius.

With Kepler's observations, enough planets were found to identify kinks in this distribution, kinks that the authors conclude are boundary lines between different kinds of planets.

For reference, the Solar System:

Ceres: 0.0472, Pluto: 0.186, Moon: 0.2724, Mercury: 0.383, Titan 0.404, Ganymede: 0.413, Mars: 0.532, Venus: 0.949, Earth: 1, Neptune: 3.88, Uranus: 4.01, Saturn: 9.45, Jupiter: 11.21

So the Solar System has two planets near the 4-Earth-radius kink, and two planets near the 10-Earth-radius kink.

 

[lpetrich]

NASA's TESS has found the first-ever ‘ultrahot Neptune’ exoplanet | Science News

It's a missing link between very hot Jupiter-sized planets and very hot Earth-sized ones.

Name: LTT 9779b
Star type: Sunlike
Distance: 260 lyr
Discoverer: TESS satellite
Period: 19 hours
Radius: 4.6 RE
Mass: 29.3 ME
Equil Temp: 2000 K

At about 2.5 million kilometers from its star, LTT 9779b may be about the closest a planet can physically get before the star gobbles up all of the atmosphere. If so, it could be a bridge between exoplanets called hot Jupiters, which are gas giants like Jupiter but have much closer-in orbits, and smaller, scorched rocky worlds, Jenkins suggested. The new planet is much smaller than a hot Jupiter, but still has a thick atmosphere that makes up about 9 percent of its mass, he said.

The next step is to measure how quickly LTT 9779b is losing mass, Adams says. If it’s rapid, that could explain why no other ultrahot Neptunes have been discovered: They shift from gas giant to rocky core too quickly. Finding one mid-transition may have been a stroke of luck.