Interstellar Objects

A rock from another star, passing through

For all of recorded history, every comet and asteroid we tracked belonged to the Sun. They orbit on ellipses, return on a schedule, and stay gravitationally bound to our star. Then on 19 October 2017, the Pan-STARRS survey on Maui caught a faint, fast-moving point of light already on its way out — and when astronomers fit an orbit, it would not close. The object had arrived with more energy than the Sun could ever reclaim. It came from another star, threaded the inner Solar System, and was leaving forever. We named it 1I/'Oumuamua, Hawaiian for "a messenger from afar arriving first."

An interstellar object is exactly that: a planetesimal — a comet nucleus or an asteroid-like body — that formed around some other star, was flung out of its home system by a gravitational kick, and has been drifting through the Galaxy for hundreds of millions or billions of years. When one happens to pass close to the Sun, our star's gravity bends its trajectory into a single hyperbolic flyby. The defining feature is not its composition (which can look much like a Solar System comet) but its motion: it is unbound, and it leaves.

Bound, parabolic, unbound: the role of eccentricity

Every orbit under an inverse-square force is a conic section, classified by its eccentricity e. The single number e tells you whether an object stays or goes:

e = 0          circle        bound
0 < e < 1      ellipse       bound — returns
e = 1          parabola      marginally unbound (escape, zero leftover speed)
e > 1          hyperbola     UNBOUND — escapes with energy to spare

The physics behind the cut is the specific orbital energy ε (energy per unit mass), which combines kinetic and potential energy:

ε = v²/2 − GM☉/r

ε < 0   bound (elliptical)
ε = 0   parabolic escape
ε > 0   unbound (hyperbolic)

A native long-period comet from the Oort Cloud falls in on an orbit so nearly parabolic that e is a hair below 1 (say 0.9999) — it is bound by a whisker and will return in millions of years. An interstellar object crosses the line: it has ε > 0, e > 1, and a real, positive leftover speed at infinity. That is the signature surveys hunt for.

Hyperbolic excess velocity: the speed it keeps forever

The cleanest way to describe an unbound visitor is its hyperbolic excess velocity, v_∞ — the speed it retains relative to the Sun once it has climbed back out to infinite distance, after the Sun has reclaimed everything it can. For a hyperbola the orbital energy is set entirely by this single number:

ε = v∞² / 2        (specific orbital energy of a hyperbola)

v∞ = √(GM☉ / |a|)  where a < 0 for a hyperbola

Here a is the semi-major axis, which is negative for a hyperbola — a useful bookkeeping convention. The relationship to eccentricity at perihelion distance q is:

q = a(1 − e)       (a < 0, e > 1, so q > 0)

speed at perihelion:  v_peri = √( GM☉ (1 + e) / q )

'Oumuamua came in with v_∞ ≈ 26 km/s; 2I/Borisov with v_∞ ≈ 32 km/s. These are not arbitrary — they sit squarely inside the 10–60 km/s velocity dispersion of stars in the Solar neighbourhood. An object that has shared the Galaxy's stellar motions for billions of years should approach the Sun at exactly such speeds, and from a direction statistically biased toward the Solar apex (the point on the sky toward which the Sun itself is moving through the local star field). That kinematic match is the strongest argument that these bodies are genuinely interstellar and not Solar System escapees.

The first three confirmed visitors

As of 2026, three interstellar objects have been confirmed, each strikingly different from the others.

Object	Discovered	Eccentricity e	v∞ (km/s)	Size	Character
1I/'Oumuamua	Oct 2017	≈ 1.20	≈ 26	~100–200 m, elongated	Dry, no visible tail, tiny non-grav push
2I/Borisov	Aug 2019	≈ 3.36	≈ 32	~0.4–1 km nucleus	Active comet, CO-rich coma and tail
3I/ATLAS	2025	> 1 (strongly hyperbolic)	tens	km-scale, active	Cometary, third confirmed visitor

1I/'Oumuamua was the puzzle. It showed no coma or dust tail, yet exhibited a small non-gravitational acceleration as it departed — a gentle radial push away from the Sun. Its light curve varied by a factor of ten over its rotation, implying a wildly elongated or flattened shape with an axis ratio near 6:1. The leading natural explanations are outgassing of nearly invisible hyper-volatiles (molecular hydrogen or nitrogen frozen in the body) and a fragment of a tidally shredded planetesimal.

2I/Borisov, by contrast, was an unambiguous comet — an active, dusty coma and tail, and a spectrum showing water, cyanogen (CN), and an unusually high abundance of carbon monoxide. The CO-rich chemistry hinted that it may have formed beyond the CO snow line of its parent star, giving us a literal chemical sample of another planetary system's outer disk. Its eccentricity of 3.36 is the most extreme of the three, making it the most obviously unbound.

Where they come from and how they get ejected

Planetary systems are messy nurseries. As giant planets form and migrate, they gravitationally scatter the leftover icy and rocky planetesimals around them. A fraction get thrown inward, a fraction rain onto the star, and a large fraction are flung clean out of the system on hyperbolic orbits — exactly the inverse of the capture problem. Models of our own Solar System suggest the early Sun ejected a mass of comets comparable to many Earth masses during the era of giant-planet migration. Every other planetary system does the same.

Multiply that ejecta budget by the hundreds of billions of stars in the Galaxy and you get a Galaxy seeded with free-floating planetesimals. Estimates of the local space density cluster near

n ≈ 0.1 interstellar objects per cubic AU   (order-of-magnitude)

which sounds sparse until you integrate it over the volume of the Sun's neighbourhood: roughly 10¹⁵ such bodies passing through the Sun's neighbourhood at any given moment, with one drifting within ~1 AU of the Sun every year or so. They are everywhere; they are simply too faint to see unless they come close.

Quantified picture: speeds, distances, timescales

A few concrete numbers anchor the scale of the phenomenon.

Quantity	Value	Comparison
'Oumuamua perihelion	0.255 AU	Inside Mercury's orbit (0.39 AU)
'Oumuamua perihelion speed	~87 km/s	~3× Earth's orbital speed; set by GM☉/q
Approach v∞	26–32 km/s	Matches local stellar dispersion (10–60 km/s)
Detection window	weeks to months	Bright only near perihelion, then gone
Time drifting in the Galaxy	10⁸ – 10⁹ yr	Possibly older than the Solar System
Local space density	~0.1 per AU³	~10¹⁵ inside the Sun's neighbourhood
Rubin Observatory yield	~1 to several / yr	vs. 1 per ~2 years pre-Rubin

The key timescale is that an interstellar object is observable for only a brief window. It brightens as it approaches perihelion, where solar heating drives any sublimation, and then dims rapidly on its way out — never to return. 'Oumuamua was already past perihelion and fading when discovered; we had only weeks of usable data, which is why so many of its properties remain debated.

What 2I/Borisov told us about another solar system

Because 2I/Borisov was a fully active comet, spectroscopy could probe its volatiles directly. The headline result was a CO/H₂O production-rate ratio at the high end of, but overlapping with, the range seen in Solar System comets. Carbon monoxide is hyper-volatile, freezing only in the very coldest outer regions of a protoplanetary disk, so a CO-rich body likely assembled far from its star, beyond the CO snow line. In effect, Borisov delivered a chemical postcard: it told us that planetesimal formation in at least one other system reached the same cold, ice-rich outer-disk conditions that built our own Oort Cloud comets.

'Oumuamua, lacking a visible coma, gave a more indirect chemical clue — its non-gravitational acceleration with no dust implies an extremely volatile, dust-poor surface, consistent with a hydrogen- or nitrogen-ice-rich composition. The two visitors together hint at a diverse population: some icy and active, some dry and inert.

Misconceptions and edge cases

• "A hyperbolic orbit alone proves interstellar origin." Not quite. Planetary perturbations and non-gravitational forces can push a native comet to a marginally hyperbolic orbit with e just above 1. The case is convincing only when the hyperbolic excess is large and well-measured — 'Oumuamua's e ≈ 1.20 and Borisov's e ≈ 3.36 carry far more energy than any Solar System mechanism can supply.
• "They were captured by the Sun." Almost never. A clean two-body flyby conserves energy: a body that arrives unbound leaves unbound. Capture needs a third body (typically Jupiter) to remove the excess energy in the same passage — an event so rare that for any given visitor, departure is essentially certain.
• "'Oumuamua was alien technology." The non-gravitational push and odd shape inspired the idea, but faint outgassing of hyper-volatiles reproduces the acceleration naturally, and tidal-fragment or nitrogen-shard models reproduce the shape. The scientific consensus is a natural, if unusual, small body.
• "Interstellar comets are chemically alien." 2I/Borisov's volatiles overlapped the range of Solar System comets. Planetesimal chemistry appears broadly universal; the differences are statistical and dynamical, not exotic.
• "They're extremely rare." They're extremely faint. The inferred space density implies they are common; we simply lacked surveys deep and fast enough to catch them. Vera C. Rubin Observatory is expected to turn a trickle into a steady stream.