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Is 'Planet Nine' actually a grapefruit-sized black hole? Big new telescope could find out

Artist's conception of accretion flares resulting from the encounter of an Oort-cloud comet and a hypothesized black hole in the outer solar system.
Artist's conception of accretion flares resulting from the encounter of an Oort-cloud comet and a hypothesized black hole in the outer solar system.
(Image: © M. Weiss)

A coming sky survey will help test a wild idea — that a grapefruit-sized black hole lurks undiscovered in the outer solar system.

Over the past few years, researchers have noticed an odd clustering in the orbits of multiple trans-Neptunian objects (TNOs), which dwell in the dark depths of the far outer solar system. Some scientists have hypothesized that the TNOs' paths have been sculpted by the gravitational pull of a big object way out there, something five to 10 times more massive than Earth (though others think the TNOs may just be tugging on each other).

This big "perturber," if it exists, may be a planet — the so-called "Planet Nine," or "Planet X" or "Planet Next" for those who will always regard Pluto as the ninth planet. But there's another possibility as well: The shepherding object may be a black hole, one that crams all that mass into a sphere the size of a grapefruit.

Related: The evidence for 'Planet Nine' in our solar system (gallery)

Astronomers are already scanning the heavens for any sign of Planet Nine, and they should soon be able to hunt for the putative black hole as well, a new study reports. 

The highly anticipated Vera C. Rubin Observatory, a big telescope under construction in the Chilean Andes, is scheduled to begin a wide-ranging, decade-long survey of the southern sky called the Legacy Survey of Space and Time (LSST) in late 2022. 

The Rubin Observatory will be incredibly sensitive and scan large swaths of sky repeatedly, a combination that will provide an unprecedented wealth of data, scientists have said. For example, LSST data will allow astronomers to probe the nature of mysterious dark energy and dark matter, find and track large numbers of potentially hazardous asteroids and study our Milky Way galaxy's formation and evolution, among other things. 

The LSST observing program will also be able to spot a potential black-hole signature, the new study reports — "accretion flares" that result when black holes gobble up comets or other small objects.

"In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole," study lead author Amir Siraj, an undergraduate astronomy student at Harvard University, said in a statement. "Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole."

Such accretion causes radiation emission, flashes that briefly shine light on dark and mysterious objects.

"Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment," co-author Avi Loeb, the chair of Harvard's astronomy department, said in the same statement.

LSST data should be able to confirm or rule out the Planet-Nine-is-a-black-hole hypothesis within a year of the survey's commencement, according to the new study, which has been accepted for publication in The Astrophysical Journal Letters. (You can read a preprint of it for free at arXiv.org.)

And whatever LSST reveals will be of great interest to astronomers.

"The outskirts of the solar system is our backyard. Finding Planet Nine is like discovering a cousin living in the shed behind your home which you never knew about," Loeb said. "It immediately raises questions: Why is it there? How did it obtain its properties? Did it shape the solar system history? Are there more like it?"

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

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  • rod
    My note, arxiv report. 'Searching for Black Holes in the Outer Solar System with LSST', https://arxiv.org/abs/2005.12280
    "Planet Nine has been proposed to potentially be a black hole in the outer solar system. We investigate the accretion flares that would result from impacts of small Oort cloud objects, and find that the upcoming LSST observing program will be able to either rule out or confirm Planet Nine as a black hole within a year. We also find that LSST could rule out or confirm the existence of trapped planet-mass black holes out to the edge of the Oort cloud, indirectly probing the dark matter fraction in subsolar mass black holes and potentially improving upon current limits by orders of magnitude."

    The paper uses a BH mass ~ 5 earth masses and LSST efforts to find DM too in the outer solar system near the Oort cloud. A BH 5 earth masses, Schwarzschild radius about 4.3 cm or so. The diameter is about 8.9 cm. Postulated distance for Planet Nine is 400 - 800 AU in the report from the Sun. Using 400 AU distance from Earth, a BH 8.9 cm in diameter, the diameter can be resolved optical light near 3E-10 arcsec resolution :) This report also raises questions about primordial BH in the universe, just how many formed in the BB model, say within one second period after the BB?, https://phys.org/news/2020-07-scientists-planet-primordial-black-hole.html, Scientists propose plan to determine if Planet Nine is a primordial black hole
    Reply
  • AnubisxIII
    I really find it odd, and disturbing that we can take a picture of a black hole 55 light years away, yet don't know wether we have an extra planet or a grapefruit black hole in our solar system!!!
    Reply
  • rod
    AnubisxIII said:
    I really find it odd, and disturbing that we can take a picture of a black hole 55 light years away, yet don't know wether we have an extra planet or a grapefruit black hole in our solar system!!!

    FYI, I think you mean 55 million light-years distance for M87 imaged black hole. The point made is interesting.
    Reply
  • Jordan7497
    I am no expert on any of this, but wouldn't a black hole of this size radiate away very quickly due to Hawking radiation?
    Reply
  • Torbjorn Larsson
    AnubisxIII said:

    I really find it odd, and disturbing that we can take a picture of a black hole 55 light years away, yet don't know wether we have an extra planet or a grapefruit black hole in our solar system!!!

    They are hard to detect, mainly engulfing matter and radiation, hence the name - "black hole". The thermal Hawking radiation is very cold and diluted for a super massive black hole such as the imaged M87* lying ~ 54 million light years away https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole_M87* ]. It is instead imaged by painstakingly integrate radio emissions from its accretion disk - the rotating disk of infalling gas - by using interferometry and crunch the data in cloud computing.

    But most super massive black holes are observed by their jets that emerge when they have settled in galactic cores. Smaller black holes can be detected by other means, such as microlensing - distorting the light when passing between us and a star - or briefly with gravitational wave emissions when they result from merging stars or black holes. Only very small black holes that are more rapidly evaporating would give away less insignificant amount of heat.

    A planet lying hundreds of AU from Sun would of course absorb and emit little heat or other light as well. It is practically speaking much the same as nomad planets "lost in space", ejected from their original system - some seen in microlensing - that numbers one magnitude less than there are stars. That is why the first detection of the putative Planet Nine is by gravitational means.

    Remember that we didn't see asteroids for a long time, or the similar Kuiper debris belt (that is now used to observe disturbances in and near it).
    Reply
  • Torbjorn Larsson
    Jordan7497 said:

    I am no expert on any of this, but wouldn't a black hole of this size radiate away very quickly due to Hawking radiation?

    Good question!

    I'm no expert either, nor have I read the paper since it will take years for such results (while the search for a planet may result anytime soon) and the hypothesis is less likely. But one of the authors (Loeb) has studied primordial black holes - which seems to be the hypothesis, likely on account of numbers - and those seems to suggest new physics (hence less likely) as rod references as well as are mostly already rejected by observations such as microlensing https://en.wikipedia.org/wiki/Primordial_black_hole#Observational_limits_and_detection_strategies ].

    But assuming they exist, what are their evaporation times when they are roughly Earth massed? Since they are colder than the cosmic background radiation, they won't evaporate but likely grow as long as galaxies exist and/or until the universe expansion cools the cosmic background radiation below the black hole temperature https://en.wikipedia.org/wiki/Black_hole#Evaporation ]. "To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole would need a mass less than the Moon."
    Reply
  • rod
    FYI, the NASA ADS Abstract system has *many* papers published on PBH or primordial black holes. So it does not surprise that Planet Nine could be a 5 earth mass PBH in some models.

    'Constraining Primordial Black Holes with Dwarf Galaxy Heating', https://ui.adsabs.harvard.edu/abs/2020arXiv200702213L/abstract
    The attached report says, “Primordial black holes (PBHs) can form in the early Universe through a variety of mechanisms and can account for all or part of the dark matter (DM) (e.g. ). PBHs surviving until present can span many orders of magnitude in mass, from 10^15 g to well over 10^10 Msun, and they can account for the entirety of the DM in the mass window ~ 10^−16 – 10^−10 Msun, where there are no observational constraints . PBHs with sublunar masses can play a role in the synthesis of heavy elements, production of positrons, as well as other astrophysical phenomena . PBHs with larger masses can account for some of the gravitational wave events detected by LIGO as well as seed supermassive black holes . The mass window of 10 – 10^3 Msun is particularly interesting in connection with signals observed by LIGO . While a variety of constraints exist for this PBH mass range (see Ref. for review), they often rely on multiple assumptions and are subject to significant uncertainties. In this work, we set new constraints on PBH abundance based on the lack of gas heating from PBH interactions with the interstellar medium (ISM)… In summary, we have presented a new constraint on the abundance of PBHs in the intermediate ~ 10 – 10^3 Msun mass range, which is of great interest in connection with the LIGO gravitational waves events, as well as the lack
    of early seeds for supermassive black holes.”

    sciencecdaily is reporting on Planet Nine as a PBH too, 'Scientists propose plan to determine if Planet Nine is a primordial black hole', https://www.sciencedaily.com/releases/2020/07/200712105456.htm
    Reply
  • Michael Franks
    rod said:
    My note, arxiv report. 'Searching for Black Holes in the Outer Solar System with LSST', https://arxiv.org/abs/2005.12280
    "Planet Nine has been proposed to potentially be a black hole in the outer solar system. We investigate the accretion flares that would result from impacts of small Oort cloud objects, and find that the upcoming LSST observing program will be able to either rule out or confirm Planet Nine as a black hole within a year. We also find that LSST could rule out or confirm the existence of trapped planet-mass black holes out to the edge of the Oort cloud, indirectly probing the dark matter fraction in subsolar mass black holes and potentially improving upon current limits by orders of magnitude."

    The paper uses a BH mass ~ 5 earth masses and LSST efforts to find DM too in the outer solar system near the Oort cloud. A BH 5 earth masses, Schwarzschild radius about 4.3 cm or so. The diameter is about 8.9 cm. Postulated distance for Planet Nine is 400 - 800 AU in the report from the Sun. Using 400 AU distance from Earth, a BH 8.9 cm in diameter, the diameter can be resolved optical light near 3E-10 arcsec resolution :) This report also raises questions about primordial BH in the universe, just how many formed in the BB model, say within one second period after the BB?, https://phys.org/news/2020-07-scientists-planet-primordial-black-hole.html, Scientists propose plan to determine if Planet Nine is a primordial black hole

    There must be something out there as something is causing Pluto's fresh surfaces. I suspect that there is a large trans Neptune planet on an elliptical orbit which causes tidal friction on Pluto's core keeping it warm and liquid which results in Pluto having fresh young surfaces
    Reply
  • ty2010
    Admin said:
    A coming sky survey will help test a wild idea — that a grapefruit-d black hole lurks undiscovered in the outer solar system.

    Is 'Planet Nine' actually a grapefruit-d black hole? Big new telescope could find out : Read more
    more likely just a black body, very low albedo because there's no heat to bring liquids or gasses to surface to make crystals, no solar energy to "cook off" darker materials
    given it's likely position, material added would be dust and debris left over from comets after trips to the inner solar system stripped the lighter elements
    Reply