interstellar object

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An interstellar object is an asteroid or comet not gravitationally bound to any star, arriving from interstellar space. It is distinguished by a hyperbolic orbit with eccentricity >1 relative to the Sun, indicating its extrasolar origin. Two such objects have been confirmed: 1I/ʻOumuamua (2017), a rocky body without a cometary coma but with non-gravitational acceleration, possibly elongated; and 2I/Borisov (2019), a comet with a typical coma and tail but extremely rich in carbon monoxide. It is expected that interstellar bodies pass through the Solar System regularly, but they are difficult to detect due to their faint brightness.

History

The existence of ejected bodies had been suspected for a long time, but evidence was obtained only in 2017 when ʻOumuamua was found using the Pan-STARRS telescope. The second object, 2I/Borisov, was discovered by amateur astronomer Gennady Borisov in 2019.

How it works

Imagine playing billiards: a strong hit with the cue can knock a ball from another table onto yours. That's how giant planets with their gravity eject small bodies from a system.

💡 ʻOumuamua means 'scout' or 'messenger from afar' in Hawaiian, and its speed was so great that it left for interstellar space just a few months after discovery.

An interstellar object (ISO) is a small Solar System body whose velocity at infinity (v_inf) exceeds the local escape velocity from the Sun, indicating a hyperbolic orbit and interstellar origin. The isotropic distribution of stellar velocities in the Galaxy gives a characteristic v_inf of about 25 km/s. Objects can be ejected from planetary systems as a result of gravitational interaction with giant planets. It is estimated that within the orbit of Neptune, there are about 10^4 interstellar bodies larger than 100 m at any given time. They provide a unique opportunity for in-situ study of the composition of planetesimals from other stars. ʻOumuamua exhibited brightness variations with an amplitude >2.5 magnitudes and a period of ~8 hours, interpreted as an extremely elongated shape (axis ratio up to 10:1) or as a flat disk. Non-gravitational acceleration is explained by outgassing or solar radiation pressure. 2I/Borisov showed a high CO/H2O ratio, distinguishing it from Solar System comets and indicating formation in a cold region of a protoplanetary disk.

History

The idea of interstellar comets was discussed by theorists (e.g., Jan Oort) as early as the mid-20th century. The first detection was on October 19, 2017, by the Pan-STARRS1 system in Hawaii — object C/2017 U1, later renamed 1I/ʻOumuamua. Its interstellar nature was confirmed by a hyperbolic orbit with v_inf ~ 26 km/s. The second object, 2I/Borisov, was discovered on August 30, 2019, by Gennady Borisov; cometary activity facilitated spectroscopy. Missions (e.g., ESA/JAXA's Comet Interceptor) are being developed to intercept future objects.

How it works

Objects form in protoplanetary disks of other stars as planetesimals; giant planets scatter them, imparting speeds above the parabolic limit relative to the star. Some escape into interstellar space. Upon approaching the Sun, orbital momentum and energy determine a hyperbolic trajectory. Observational manifestations: astrometry for orbit determination, photometry for shape and rotation, spectroscopy for composition. For ʻOumuamua, a radiation pressure model was applied to explain the non-gravitational acceleration. Laws: Newton's law of universal gravitation for the trajectory, the law of conservation of energy for a hyperbolic orbit, equations of motion with non-conservative forces. Gravitational lensing is not applicable (masses are small).

v_{\infty} = \sqrt{\frac{2E}{m}}

v_∞ — speed of the object at an infinitely large distance from the Sun (hyperbolic excess velocity, for ʻOumuamua ~ 26 km/s), E — total mechanical energy of the body (kinetic+potential), m — mass of the object

e = \sqrt{1 + \frac{2Eh^2}{\mu^2 m}}

e — orbital eccentricity (dimensionless), E — total mechanical energy, h — specific angular momentum, μ — gravitational parameter of the Sun (μ = G M_sun, G — gravitational constant (≈ 6.674×10⁻¹¹ m³/(kg·s²)), M_sun — mass of the Sun ≈ 1.989×10³⁰ kg), m — mass of the object (cancels out if h is specific)

Open problems & fun facts

Origin of ʻOumuamua's extreme shape: dynamic stretching or a fragment of a disrupted body.; Nature of ʻOumuamua's non-gravitational acceleration: outgassing of invisible volatiles or radiation pressure.; Typical influx rate of interstellar objects and their size distribution.; The possibility of distinguishing an interstellar object from an Oort Cloud body with a perturbed orbit.; Organization of a rapid mission to a passing object for detailed study.

💡 Upon discovery, ʻOumuamua was first thought to be a comet, then an asteroid, and then non-gravitational acceleration again hinted at a cometary nature — the object remains mysterious.

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interstellar object

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History

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History

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