IELTS Reading 21: The Neptune That Orbits Backwards

Adapted from Science X / arXiv (June 2026). IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://sciencex.com/news/2026-06-neptune-sized-world-orbits-hinting.html

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Reading Passage

A. In a well-ordered planetary system, all the planets orbit the star in the same direction — the direction in which the star itself spins — and their orbits lie close to a single plane. This neat arrangement reflects the conditions under which planets form: they coalesce from a flat, rotating disk of gas and dust around a young star. Exoplanet research has repeatedly shown, however, that nature does not always prefer neatness. Some planets have wildly tilted orbits; some orbit their stars backwards; some are flung into eccentric paths that swing close and then far. The latest such oddity, announced in June 2026, is a Neptune-sized world circling a star about 150 light-years away on a tilted, backwards orbit. Its existence, researchers say, is evidence that a hidden — as yet unseen — giant planet is gravitationally manipulating it. Our own system, with its nearly circular, coplanar orbits, long looked like the template against which all others would be judged. Exoplanet surveys have steadily dismantled that expectation: tilted, flipped and wildly eccentric worlds turn out to be the rule rather than the exception, each one a clue that the disk from which a system forms is only the beginning of its story.

B. The planet, designated TOI-1710 b, was identified by NASA's TESS mission from the regular dips in starlight that mark its transits. Follow-up observations with ground-based spectrographs measured the star's radial-velocity wobble, confirming the planet's mass and orbit. What surprised astronomers was the angle of that orbit. Using the Rossiter-McLaughlin effect — a measurement of how the planet's transit distorts the spectrum of the rotating star — the team determined that the planet orbits in the opposite direction to the star's spin, and that its orbit is tilted at a significant angle to the star's equator. Such a configuration cannot have been produced in the orderly disk from which the planet formed. Something must have tilted it. The Rossiter-McLaughlin measurement is clever because it does not image the planet at all. Instead it watches, during the transit, how the planet blocks light first from the approaching side of the rotating star and then from the receding side. The order and strength of that shadow reveal the angle at which the planet's orbit crosses the star's face.

C. The leading explanation is gravitational interaction with a more distant, more massive companion. The star TOI-1710 is already known to have two other companions: a closer giant planet and a small M-dwarf star orbiting far away. Theorists have long proposed that an outer giant or a stellar companion can, through a mechanism known as the Kozai-Lidov effect, periodically tilt and flip the orbit of an inner planet. The effect works like this: the distant companion exerts a gentle gravitational torque on the inner planet over millions of years, pumping its orbit's inclination up and down while preserving the orbit's overall size. At the peaks of these cycles, the inner planet's orbit can flip completely, leaving it tilted or even retrograde. TOI-1710 b's backwards orbit, the researchers argue, is the signature of such a dance. The Kozai-Lidov effect was first described for comets perturbed by the outer planets, and it has since become astronomers' default explanation for anything orbiting at a strange angle. The mechanism is gentle and slow, operating over millions of orbits rather than in a single collision, which is why a system can end up with a neatly formed inner world whose orbit has been quietly remodelled from the outside.

D. The case is not yet closed. The team's model requires the outer M-dwarf companion to be both massive enough and close enough to produce the observed tilt within the star's lifetime — conditions that, while plausible, have not been directly measured. An alternative explanation holds that the inner giant planet itself is responsible, and that the two planets interacted gravitationally in the system's early history. A third possibility — that the planet was captured from interstellar space — is considered unlikely but not formally excluded. Distinguishing among these hypotheses will require more precise measurements of the outer companion's orbit and mass, which future observations with larger telescopes should provide. The researchers are careful to describe their retrograde finding as a "hint" of the hidden giant rather than a definitive discovery. This caution is not evasion. Inferring an unseen planet from the disturbance of another is an old and treacherous exercise — history is littered with supposed companions that turned out to be instrumental artefacts. Until the outer object is seen directly, or its mass is pinned down by decades of precise position measurements, the claim rests on inference rather than observation.

E. The broader significance lies in what such oddities reveal about planetary system architecture. When exoplanets were first discovered in the 1990s, astronomers assumed that our own solar system — orderly, coplanar, prograde — was the norm. Two decades of TESS and Kepler data have shown the opposite: systems with tilted, retrograde or eccentric orbits are common, and their existence implies that unseen companions are not rare. TOI-1710 b is a particularly clean example because its orbit is well measured and its host star is relatively nearby. It demonstrates that even systems that look ordinary — a single star, a few planets, nothing remarkable in the visible data — can hide a distant, unseen partner whose gravity has shaped the inner worlds for billions of years. The next time a planet is found doing something unexpected, the response may be less "how strange?" and more "what's pulling it?" The implication is practical as well as theoretical. A tilted inner world no longer needs to be explained by a chaotic early collision; it can be read as a fingerprint of a wider architecture. For the growing catalogue of exoplanets, that makes each oddball less of a curiosity and more of a detective clue — a small disturbance in the inner system advertising the presence of something large in the dark beyond it.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. How the planet's backwards orbit was measured ii. The Kozai-Lidov mechanism and the hidden giant iii. Uncertainties and alternative explanations iv. Why retrograde orbits are impossible v. What the discovery reveals about planetary systems vi. A history of the TESS mission vii. How Neptune was discovered

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What is unusual about TOI-1710 b? A. It is the largest exoplanet ever found. B. It orbits its star in the opposite direction to the star's spin. C. It has no atmosphere. D. It orbits two stars at once.

6. How was the orbit's tilt measured? A. By photographing the planet directly. B. Using the Rossiter-McLaughlin effect on the star's spectrum during transit. C. By timing the planet's transits over decades. D. By measuring the star's distance from Earth.

7. What is the Kozai-Lidov effect? A. A collision between two planets. B. A gravitational torque from a distant companion that periodically tilts an inner orbit. C. The formation of a planet from a gas disk. D. The ejection of a planet from its system.

8. How do the researchers describe their conclusion? A. They have definitively discovered the hidden giant. B. They describe the retrograde orbit as a hint of an unseen companion. C. They believe the planet was captured from interstellar space. D. They have ruled out all alternative explanations.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Planets in a newly formed system typically orbit in the same direction as their star's spin.
10. TOI-1710 b is located about 1,500 light-years from Earth.
11. TOI-1710 b is known to possess a system of icy rings.
12. The researchers consider interstellar capture to be unlikely.
13. Most exoplanetary systems discovered so far are as orderly as our solar system.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

TOI-1710 b orbits in a (14) __________ direction relative to its star's spin, suggesting that a (15) __________ companion is gravitationally influencing it.

Answers & Explanations

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