Universe Today – Asteroid Hygiea is Round Enough That it Could Qualify as a Dwarf Planet, the Smallest in the Solar System.
Within the Main Asteroid Belt, there are a number of larger bodies that have defied traditional classification. The largest among them is Ceres, which is followed by Vesta, Pallas, and Hygeia. Until recently, Ceres was thought to be the only object in the Main Belt large enough to undergo hydrostatic equilibrium – where an object is sufficiently massive that its gravity causes it to collapse into a roughly spherical shape.
However, it now seems that there is another body in the Main Belt that has earned the designation of “dwarf planet”. Using data from the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large Telescope (VLT), an international team of astronomers found compelling evidence that Hygeia is actually round, making it the smallest dwarf planet in the Solar System.
According to estimates, Hygiea measures just over 430 km (~270 mi) in diameter, where Ceres measures a comparatively robust 950 km (590 mi) in diameter and Pluto measures 2400 km (1490 mi).
Astronomer Mike Brown considers a large number of trans-Neptunian bodies, ranked by estimated size, to be “probably” dwarf planets. He did not consider asteroids, stating “In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round”. If all asteroids 270 miles and larger are dwarf planets then there would be over 350 dwarf planets detected in the solar system so far.
Vesta is larger than Hygiea. Vesta could have been classified as a dwarf planet if it had retained a spherical shape, and it has other qualities that lead to the thought it could be a protoplanet. The only thing that knocked it out of the category of a dwarf planet was the formation of two large impact basins at its southern pole. Pallas is also larger than Hygia. Pallas shape departs significantly from an ellipsoid.
The terms for varying degrees of likelihood for dwarf planets are:
Near certainty: diameter estimated/measured to be over 900 kilometers (560 mi). Sufficient confidence to say these must be in hydrostatic equilibrium, even if predominantly rocky.
Highly likely: diameter estimated/measured to be over 600 kilometers (370 mi). The size would have to be “grossly in error” or they would have to be primarily rocky to not be dwarf planets.
Likely: diameter estimated/measured to be over 500 kilometers (310 mi). Uncertainties in measurement mean that some of these will be significantly smaller and thus doubtful.
Probably: diameter estimated/measured to be over 400 kilometers (250 mi). Expected to be dwarf planets, if they are icy, and that figure is correct.
Possibly: diameter estimated/measured to be over 200 kilometers (120 mi). Icy moons transition from a round to irregular shape in the 200–400 km range, suggesting that the same figure holds true for KBOs. Thus, some of these objects could be dwarf planets.
Probably not: diameter estimated/measured to be under 200 km. No icy moon under 200 km is round, suggesting that the same is true for KBOs. The estimated size of these objects would have to be in error for them to be dwarf planets.
As of Fri Sep 13 2019 there are:
10 objects which are nearly certainly dwarf planets,
27 objects which are highly likely to be dwarf planets,
68 objects which are likely to be dwarf planets,
130 objects which are probably dwarf planets, and
741 objects which are possibly dwarf planets.
Universe Today – The Lowest Mass Black Hole has Been Found, only 3.3 Times the Mass of the Sun.
A recent discovery where a team of astronomers observed a black hole that is just over three Solar masses, making it the smallest black hole discovered to date. The Journal Science study is titled A noninteracting low-mass black hole–giant star binary system. The team responsible was led by astronomers from Ohio State University and included members from the Harvard-Smithsonian Center for Astrophysics, The Observatories of the Carnegie Institution for Science, the Dark Cosmology Center, and multiple observatories and universities.
A black hole hiding in a binary star
As material falls toward a black hole, it heats up and emits x-rays. Almost all black holes are discovered by this x-ray emission. Thompson et al. observed light from a giant star that is Doppler shifted, indicating an orbit around a binary companion. The companion object must weigh more than 2.6 solar masses, but it emits no light, including x-rays. This indicates the presence of a black hole that is not currently consuming any material. There may be a population of similarly hidden black holes that have been missed by x-ray observations.
Black hole binary systems with companion stars are typically found via their x-ray emission, generated by interaction and accretion. Noninteracting binaries are expected to be plentiful in the Galaxy but must be observed using other methods. We combine radial velocity and photometric variability data to show that the bright, rapidly rotating giant star 2MASS J05215658+4359220 is in a binary system with a massive unseen companion. The system has an orbital period of ~83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicating star spots and tidal synchronization. Constraints on the giant’s mass and radius imply that the unseen companion is 3.3+2.8−0.7 solar masses, indicating that it is a noninteracting low-mass black hole or an unexpectedly massive neutron star.
The Hill – Who will be first on the moon? NASA or SpaceX?
Jeff Bezos, the CEO of Amazon.com and an aerospace company called Blue Origin, recently announced his plan to return humans to the moon at the International Astronautical Conference. Blue Origin would serve as the prime contractor, leading a coalition of old-line aerospace companies, including Lockheed Martin, Northrop Grumman and Draper Industries, to develop a three-stage lunar lander.
SpaceX president and COO Gwynne Shotwell revealed her company’s plan to return to the moon. SpaceX’s plans revolve around the Starship rocket, a massive spacecraft being built both at the company’s facility in Boca Chica, south Texas, and at the Kennedy Space Center. Shotwell laid out an aggressive timeline that included the first orbital flight within a year, an un-crewed landing on the lunar surface in 2022, a trip around the moon in 2023 and a crewed lunar landing in 2024.
The Blue Origin team is not the only one that will bid for a NASA lunar lander contract. Boeing is putting together a team that includes Houston-based Intuitive Machines. SpaceX may bid on the contract as well.
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