In 2005, a District Court judge accepted pleas of guilt by the defendant to two counts of assault and battery. In 2012, the judge allowed the defendant’s motion to vacate the convictions. The defendant contended that the pleas had resulted from the ineffective assistance of counsel because his attorney *736had failed to advise him of the exposure to deportation created by the pleas as required by Padilla v. Kentucky, 559 U.S. 356 (2010) {Padilla). The Commonwealth appealed upon the ground that the judge’s failure to conduct an evidentiary hearing and to provide supporting findings of fact and reasoning constituted an abuse of discretion and error of law. We agree. We now vacate the order and remand the case for further action.

Background. 1. Defendant’s guilty pleas. The Commonwealth charged the defendant with assault and battery by means of a dangerous weapon, G. L. c. 265, § 15A(b) (count one); and assault and battery on a child causing injury, G. L. c. 265, § 13J(Z?) (count two). The charges arose from allegations that the defendant had physically abused his nine year old daughter. The parties engaged in plea discussions and made recommendations to the judge for disposition. The “Tender of Plea” form, otherwise known as the “green sheet,” shows that the defendant accepted the following disposition: (1) on count one, a finding of guilty and a sentence of one year of confinement, suspended for two years, upon certain probationary terms; and (2) on count two, a finding of guilty and a sentence of one year of confinement concurrent with the first sentence, also suspended for two years, but with no additional probationary terms.

The green sheet includes the following information concerning the potential deportation consequences of the guilty pleas: (1) the defendant’s signature underneath his acknowledgment that “conviction of this offense may have the consequence of deportation . . . pursuant to the laws of the United States”; (2) plea counsel’s certification that he had explained to the defendant the “above-stated provisions of law ... so as to enable [him] to tender his . . . plea of guilty . . . knowingly, intelligently and voluntarily”; and (3) the judge’s certification that he had informed the defendant that conviction of the charges “may have the consequences of deportation, exclusion from admission to the United States, or denial of naturalization.” On July 7, 2005, the defendant pleaded guilty to both counts.

2. Defendant’s motions to vacate his guilty pleas. In September of 2011, the defendant through new counsel moved to vacate his guilty pleas. By affidavit the defendant alleged that plea counsel had “never explained to [him] the immigration con*737sequences of pleading guilty.” By affidavit plea counsel reported that he had no record or independent memory of the case, but that his practice had been “to go over the alien warnings on the green sheet with the client” but not to “provide specific advice based on the charges.” Motion counsel argued that plea counsel’s ineffective assistance had prevented a tender of pleas knowingly, voluntarily, and intelligently.1 On November 7, 2011, the motion judge, who had been the plea judge, endorsed the motion as “[djenied.”

On January 20, 2012, the defendant submitted to the court the same motion (with its original September 1, 2011, date); the affidavits of the defendant and plea counsel; a copy of the criminal complaint, green sheet, and police report; and a memorandum of law. These papers appear to have been identical to the original motion papers. On February 2, 2012, before the Commonwealth had filed a response, the same judge endorsed the motion as “[ajllowed.” No hearing had occurred.

On February 16, 2012, the Commonwealth filed a comprehensive motion for reconsideration. It included a copy of the green sheet and challenged the integrity of the defendant’s affidavit. The Commonwealth pointed out also that it had not received a sufficient opportunity to respond to the defendant’s resubmitted motion. No hearing resulted. On February 29, 2012, the judge endorsed the motion for reconsideration as “[djenied.” The Commonwealth has appealed.

Discussion. 1. Procedure: vacation of the guilty pleas without an evidentiary hearing and findings. The vehicle for the proposed withdrawal of a guilty plea is a motion for a new trial under Mass.R.Crim.P. 30(b), as appearing in 435 Mass. 1501 (2001). Commonwealth v. Furr, 454 Mass. 101, 106 (2009). Commonwealth v. Diaz, 75 Mass. App. Ct. 347, 350 (2009). Under *738rule 30(b), the judge “may grant a new trial at any time if it appears that justice may not have been done.” The rule authorizes the exercise of sound discretion. Commonwealth v. Berrios, 447 Mass. 701, 708 (2006), cert. denied, 550 U.S. 907 (2007). Commonwealth v. Williams, 71 Mass. App. Ct. 348, 353 (2008). The policy favoring the finality of just convictions imposes a “rigorous standard” upon the discretionary allowance of motions under rule 30(b). Commonwealth v. Berrios, supra. See Commonwealth v. Lopez, 426 Mass. 657, 661-663 (1998). Allowance must rest upon “substantial reasons.” Commonwealth v. Tucceri, 412 Mass. 401, 406 (1992). See Commonwealth v. Amirault, 424 Mass. 618, 637 (1997). The standard of review asks whether the motion judge committed an error of law or abuse of discretion. See, e.g., Commonwealth v. Grace, 397 Mass. 303, 307 (1986); Commonwealth v. Walker, 443 Mass. 213, 224-225 (2005).

The motion judge’s discretion includes the decision whether to conduct an evidentiary hearing. Commonwealth v. Shuman, 445 Mass. 268, 278 (2005). The guideline for its exercise is “whether a substantial issue necessitating a hearing” has arisen from the submitted affidavit material. Ibid, (citation omitted). If so, the established “better practice” has been to conduct an evidentiary hearing. Reporters’ Notes to Rule 30(c)(3), Mass. Ann. Laws Court Rules, Rules of Criminal Procedure, at 1662 (LexisNexis 2013-2014). See Commonwealth v. Stewart, 383 Mass. 253, 260 (1981); Commonwealth v. Companonio, 420 Mass. 1003, 1003 (1995), S.C. 445 Mass. 39 (2005); Commonwealth v. Saarela, 15 Mass. App. Ct. 403, 406-407 (1983). These authorities were available to the judge at the time of both his rulings in February of 2012.

The judge should have held an evidentiary hearing before he ruled on the defendant’s motion because evidence clearly conflicted with the defendant’s claim of ineffective assistance of counsel. The defendant’s affidavit contradicted (1) the green sheet certifications signed by the judge and the defendant’s plea counsel; (2) the defendant’s own green sheet acknowledgment; and (3) the plea counsel’s affidavit. The contents of the resubmitted motion, indicated by the cover sheet, also show that the judge received those materials. Despite the conflicting evidence, *739the judge allowed the defendant’s motion to vacate his pleas without the benefit of an evidentiary hearing and without the opportunity for opposition by the Commonwealth. The omission was an abuse of discretion depriving the Commonwealth of a fair opportunity to contest the motion and preventing the determination of material facts.2

Further, as a matter of law, the omission of an evidentiary hearing inevitably caused a violation of rule 30(b)’s requirement that the judge “shall make such findings of fact as are necessary to resolve the defendant’s allegations of error of law” (emphasis supplied). It similarly precluded any written explanation in support of the final two rulings. Recorded reasoning serves multiple valuable purposes. It guides the decision maker to the merits, assures the parties of a deliberative result, enables informed appellate review, and promotes public knowledge of, and confidence in, the rationality of judicial work. See In the Matter of the Enforcement of a Subpoena, 463 Mass. 162, 174 n.6 (2012), quoting from S.J.C. Rule 3:09, Commentary to Canon 3(B)(9), as appearing in 455 Mass. 1301 (2009) (court, in discussion of the judicial deliberative privilege, “encourage[d] judges ‘to explain the basis for their decisions on the record. ... By helping litigants to understand the basis for decisions in cases, the judge also promotes public understanding of judicial proceedings’ ”).3 The ultimate source of respect for a judicial order is the quality of its reasoning. An unreasoned *740order forfeits that resource. Here the omission of findings and reasoning violates the standards of rule 30(b) and (c)(3), and places the one-word ruling under suspicion of arbitrariness.

2. Merits: applicability of Padilla to defendant’s claim. By briefing and oral argument in this court, the defendant has contended that the District Court judge has implicitly applied the rule of Padilla, 559 U.S. at 374-375, to his motion: that the failure of his counsel in 2005 to advise him of the risk of deportation resulting from a guilty plea denied him the effective assistance of counsel guaranteed by the Sixth Amendment to the United States Constitution. As recounted, a one-word ruling deprives us of the findings and reasoning necessary for any intelligent review of the merits of the claim and of the decision. Since the briefing and the oral argument of this case on February 11, 2013, two substantial decisions have shifted the doctrinal ground beneath a Padilla claim in Massachusetts. The Commonwealth has commendably acknowledged them by subsequent letters in accordance with Mass.R.A.P. 16(1), as amended, 386 Mass. 1247 (1982). Because today’s order of remand may lead to a decision of the Padilla merits by the judge, or an alternate judge, we summarize the general effect of these intervening precedents: Chaidez v. United States, 133 S. Ct. 1103 (2013), and Commonwealth v. Sylvain, 466 Mass. 422 (2013).

In Commonwealth v. Clarke, 460 Mass. 30, 44-45 (2011), approximately seven months before the contested rulings here, the Supreme Judicial Court concluded that the 2010 Padilla decision would apply retroactively to collateral attacks upon criminal pleas submitted from April 1, 1997 (the effective date of Federal legislation exposing such plea bargainers to immigration sanctions), onward.4

*741Soon after argument and submission of this appeal in February of 2013, the United States Supreme Court in Chaidez v. United States, supra at 1105, 1111, characterized the Padilla holding of 2010 as a “new rule” available only prospectively for collateral attack upon pleas of guilt submitted in Federal District Court.5

Seven months later, in Commonwealth v. Sylvain, supra at the Supreme Judicial Court concluded that a noncitizen defendant pleading guilty to a sanctionable aggravated felony in a Massachusetts trial court could challenge a final prz-Padilla conviction by a motion to withdraw the plea.6 The holding rests upon two independent grounds. The first is that the State, as a separate sovereign, has the option to extend the remedial benefit of a newly announced Federal constitutional right retroactively within its own courts even though the Supreme Court has declined to extend the right retroactively to convictions final in its District Courts. Id. at 434-435. See Danforth v. Minnesota, 552 U.S. 264, 266, 275, 280, 282, 289 (2008) (seven-justice majority).7 As a second ground, the Supreme Judicial Court stated that art. 12 of the Massachusetts Declaration of Rights implicitly demands “that defense counsel provide defendants with accurate advice concerning the deportation consequences *742of a guilty plea or conviction at trial.” Commonwealth v. Sylvain, supra at 436. It regarded that State constitutional right as implicit and existent as of the time of Sylvain’s challenged plea in 2007. Id. at 436-437.

Conclusion. For these reasons, we vacate the order allowing the defendant’s motion pursuant to rule 30(b) to withdraw his guilty pleas. We remand the case to the District Court for an evidentiary hearing. The judge hearing the motion shall make specific subsidiary and ultimate findings of fact and provide separate reasoning and conclusions of law.8

So ordered.