  The Earth Trojans are co-orbitals librating around the Lagrange points $L_4$
or $L_5$ of the Sun-Earth system. Although many numerical studies suggest that
they can maintain their dynamical status and be stable on timescales up to a
few tens of thousands of years or even longer, they remain an elusive
population. Thus far only one transient member (2010 TK$_7$) has been
discovered serendipitously. Here, we present a dynamical study of asteroid 2020
XL$_5$. With our meticulous followup astrometric observations of the object, we
confirmed that it is a new Earth Trojan. However, its eccentric orbit brings it
close encounters with Venus on a frequent basis. Based on our N-body
integration, we found that the asteroid was captured into the current Earth
Trojan status in the 15th century, and then it has a likelihood of 99.5% to
leave the $L_4$ region within the next $\sim$10 kyr. Therefore, it is most
likely that 2020 XL$_5$ is dynamically unstable over this timescale.
