Among the eight major planets, Mercury has both the most elliptical and the most tilted orbit. But if you expand the question to include dwarf planets, comets, and other small bodies, the extremes get far more dramatic. The answer depends on which category of solar system object you’re asking about.
Mercury Leads the Major Planets
Mercury’s orbit has an eccentricity of 0.206, meaning it deviates noticeably from a perfect circle. For comparison, most other planets have eccentricities well below 0.1, hugging much closer to circular paths. At its closest approach to the Sun, Mercury is about 46 million kilometers away; at its farthest, that distance stretches to nearly 70 million kilometers. No other major planet experiences that kind of variation.
Mercury also wins for orbital tilt. Its orbit is inclined 7.0 degrees relative to the ecliptic, the flat plane defined by Earth’s orbit around the Sun. Venus comes in second at 3.4 degrees, followed by Saturn at 2.5 degrees, Mars at 1.85 degrees, and Neptune at 1.8 degrees. Jupiter and Uranus hug the ecliptic most closely, at 1.3 and 0.8 degrees respectively. Earth’s inclination is zero by definition, since the ecliptic is literally the plane of Earth’s orbit.
These numbers might seem small, and they are. The major planets all formed from the same spinning, flattened disk of gas and dust around the young Sun, so they naturally ended up orbiting in roughly the same plane with roughly circular paths. Mercury’s relatively high values reflect its proximity to the Sun and a history of gravitational interactions that nudged its orbit into a more extreme shape and tilt than its neighbors.
Dwarf Planets Push the Extremes Further
Once you step beyond the eight major planets, orbits get much wilder. Pluto’s eccentricity is about 0.25, and its orbit is tilted 17 degrees to the ecliptic. That tilt is more than double Mercury’s, and the elliptical shape of its path actually brings Pluto closer to the Sun than Neptune for part of its 248-year orbit.
Eris, another dwarf planet roughly the same size as Pluto, has an eccentricity of about 0.44 and an orbital inclination of 44 degrees. That means Eris swings far above and below the plane where the major planets travel, tracing a path that looks nothing like the neat, flat orbits of Jupiter or Earth.
Then there’s Sedna, a distant minor planet with one of the most elongated orbits known. Its eccentricity is 0.85, meaning its orbit is extremely stretched out. Sedna takes roughly 11,900 years to complete a single trip around the Sun, swinging from about 76 AU at its closest to nearly 900 AU at its farthest. (One AU is the distance from Earth to the Sun.) What pulled Sedna into such an extreme orbit remains an open question, with explanations ranging from a passing star in the Sun’s early history to the gravitational influence of a hypothetical distant planet.
Comets and Asteroids With Extreme Orbits
Comets routinely have the most elliptical orbits of any solar system objects. Most periodic comets have eccentricities between 0.2 and 0.7, but the well-known examples are far more extreme. Halley’s Comet has an eccentricity of 0.967, meaning its orbit is almost a completely flattened ellipse. It swings from just inside Venus’s orbit out past Neptune before looping back. Comet Hale-Bopp’s eccentricity is 0.995, even closer to a parabola. And Comet McNaught (C/2006 P1) technically has a hyperbolic orbit with an eccentricity of 1.00001, though it still returns roughly every 100,000 years because it remains gravitationally bound to the Sun.
Among asteroids, most follow relatively circular, low-tilt orbits within the asteroid belt. A notable exception is Pallas, one of the largest asteroids, whose orbit is inclined a striking 34 degrees to the ecliptic with an eccentricity of 0.23. Pallas also has an unusual axial tilt of 84 degrees, meaning it essentially rolls along its orbital path rather than spinning upright.
What Makes Orbits Elliptical or Tilted
Gravity from other bodies is the main sculptor of orbits over time. Jupiter and Saturn, the two most massive planets, exert the strongest influence. Their gravitational pull causes the shape of other planets’ orbits to shift gradually between more circular and slightly more elliptical over thousands of years. This same mechanism also causes orbits to wobble and precess, slowly changing their orientation in space.
For smaller and more distant objects like Sedna or long-period comets, the story is different. Close gravitational encounters with giant planets can fling objects into highly elongated orbits. A comet drifting inward from the outer solar system might pass close to Jupiter and get hurled into a path that takes it far beyond Pluto before gravity pulls it back. Objects can also be scattered into high-inclination orbits through these encounters, ending up on paths sharply angled to the ecliptic. The more distant and smaller an object is, the more susceptible it is to these gravitational kicks, which is why the outer solar system is full of bodies on wild, tilted, stretched-out orbits while the major planets remain comparatively well-behaved.